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cosam
February 17th, 2009, 02:08 AM
With all the talk about PETs lately, I've been keeping an eye out for one. I managed to pick up a non-runner last night for a pretty tidy price, from a collector who was having a clear-out. It has a 2001 case and 9" screen, but the main board appears to be from a 3/4000 series machine with a full complement of RAM.

It doesn't do much just yet, though. It has the common "garbage screen" when switched on, but only for a second or so, then... total darkness. Reseating everything that's socketed didn't help any, so I whipped the board out for closer inspection. There were some pretty gruesome-looking repairs on the back which I'd like to fix properly before continuing the diagnosis. Looks like someone had set to with a pretty beefy soldering iron and half a bucket full of flux! Most of that is cleaned up now and I'm making a start on repairing some cut and lifted (or even completely absent!) PCB traces.

Should at least keep me busy for a while ;-) I've whipped up a quick web page (http://www.cosam.org/computers/cbm/pet/index.html) with some pictures, on which I'll be documenting my progress.

pavery
February 19th, 2009, 02:22 PM
Hey, congrats on your PET acquisition, cosam.

As I was involved with faultfinding Tezza's PET (http://www.classic-computers.org.nz/blog/2009-01-27-repairing%20a%20commodore%20pet.htm) - I'll be interested to see what you find.


It doesn't do much just yet, though. It has the common "garbage screen" when switched on, but only for a second or so, then... total darkness.

As far as I know, it is normal for the garbage screen to be displayed for the first second or so while initialisation occurs, then the screen clears and Basic is presented.

Two possibles come to mind: Perhaps you have a ROM problem (like we did) and Basic isn't coming up successfully. Or perhaps the machine is just shutting down after a second or so - perhaps Power Supply is failing.

I'll certainly be following your progress with interest - please keep us posted.

Oh, what is the exact Model Number stated on the rear of the case?

Philip

tezza
February 19th, 2009, 07:08 PM
Cool! Seems to be exactly the same model I've got, the one Philip (pavery) helped me fix. I see what you mean about the soldering..ewww..

Anyway, I'll follow with interest. At least is doesn't seem to be hanging at the initial boot up.

Tez

tezza
February 19th, 2009, 07:17 PM
P.S. from the number of ROM chips it looks like you've got BASIC 4.0 in there. (assuming all the ROMs are for BASIC of course).

Tez

cosam
February 20th, 2009, 01:09 AM
I finished up the PCB repair last night and reconnected the board to see if it'd made any difference. Garbage... blank... BASIC! Not what I was expecting, but something I did has apparently breathed life into the old girl. Most annoyingly, however, I've no idea what the original problem was! So the logic is working, although it's revealed a problem with the display (more later).


As far as I know, it is normal for the garbage screen to be displayed for the first second or so while initialisation occurs, then the screen clears and Basic is presented.
Yep, you're right. I vaguely remember reading that elsewhere too, so I wasn't too worried about the garbage part - more so about the blank screen afterwards!


Two possibles come to mind: Perhaps you have a ROM problem (like we did) and Basic isn't coming up successfully. Or perhaps the machine is just shutting down after a second or so - perhaps Power Supply is failing.
The power supply is always the first thing I check ;-) After reading about Tezza's machine, I suspected a bad ROM too. Whilst waiting for some supplies for the PCB repair I dumped them all and checked them against some known-good images. My el-cheapo EPROM programmer didn't take them directly, but their pinout is similar to a 27xx so I made a quick-and-dirty adapter out of a couple of IC sockets. It's a BASIC 2 ROM set, BTW. That extra chip is an expansion ROM called "Command-O" which AFAICT implements a few extra commands after initialising with some SYS-number or other.


Oh, what is the exact Model Number stated on the rear of the case?
I'll have another look for you, but I've not found a model number anywhere yet. I'm also running out of places to look ;-) Best I have right now is 2001-N, and I can confirm it has 32K of RAM.

As for the display problem: right now it works fine just after powering up, but soon after that the image starts to wobble. The shaking gets worse the longer it's left on and stays bad after a power cycle, so I'm guessing something is playing up when it gets warm. Analogue electronics aren't exactly my forte, but I'll have a look at it tonight.

carlsson
February 20th, 2009, 01:15 AM
Apparently what it says on the rear and on the front can be two quite different things. Then if the motherboard was updated or replaced you may find it act like a third model. 2001N-32 sounds as good as anything else. As Nils on the now disabled PET Alive forum mentioned, CBM boards were designed so a newer board can (always?) replace an older board, at least as long as jumpers are set correctly.

tezza
February 20th, 2009, 01:22 AM
As for the display problem: right now it works fine just after powering up, but soon after that the image starts to wobble. The shaking gets worse the longer it's left on and stays bad after a power cycle, so I'm guessing something is playing up when it gets warm. Analogue electronics aren't exactly my forte, but I'll have a look at it tonight.

That's a shame. At least it goes! Hopefully it will be an easy fix.

Tez

cosam
February 20th, 2009, 01:51 PM
Well... I finally got set up to take some measurements as described in the service manual. The video board is a bit of a pain to get due to all the wires going to the CRT. Unfortunately I only got to the third test point before my oscilloscope (an old Tek 2465) started to fizzle and let out a big puff of exceedingly aromatic smoke. Looks like the PSU is fried, so I'll need to get that fixed before I can get back to the PET. So much for my weekend project! :-\

patscc
February 20th, 2009, 01:59 PM
Hey, hey, hey....a 2465 ain't that old. Aromatic smoke, did a cap in the PSU blow, by any chance ?
patscc

cosam
February 20th, 2009, 02:23 PM
Hey, hey, hey....a 2465 ain't that old.
Heh, I guess it's not - the PET itself predates it by a few years, after all!


Aromatic smoke, did a cap in the PSU blow, by any chance ?
That'd be my guess too. Nothing quite like that sickly smell of boiling electrolyte ;-) In any case I don't think it's too serious as I still had a nice trace even when it started doing its chimney impersonation.

cosam
February 21st, 2009, 04:37 PM
Got the scope fixed today - it was indeed a bad cap which had left a nice trail of what could easily have passed for burnt Marmite on the PCB. At least it was easy to replace, once I finally got to the thing...

So I got back to the video board and it looks like at least one of the three signals coming from the main board isn't quite right. I put the monitor back together and started probing under the hood instead. Unfortunately, while I was tinkering away something went "pop" and I'm back to the blank screen again! Not knowing what was wrong last time, I'm not really any the wiser than I was on day one. Looks like I've got work to do after all ;-)

I've started working my way through the schematics again. CPU and its busses seem fine, but video RAM seems decidedly inactive. All I've found so far is that the video data bus is all zeros, apart from SD5 being stuck high, which seems a little odd. A10 on the character ROM (labeled GRAPHIC) is also stuck high. I think I'd better check the "regular" RAM before drawing any conclusions, though.

MikeS
February 21st, 2009, 11:55 PM
It won't affect the PET working, but if that's a Command-O ROM (CO-40N) I think it belongs in UD3 (9000H), the right-hand socket and SYS 36864 to enable (unless that's some non-standard version).

Does anyone know if there's a binary image and/or manual for the Command-O ROM on the Web somewhere? If not and you haven't already, could you dump it for us while you're at it?

Thanks,

mike

cosam
February 22nd, 2009, 09:16 AM
It won't affect the PET working, but if that's a Command-O ROM (CO-40N) I think it belongs in UD3 (9000H), the right-hand socket and SYS 36864 to enable (unless that's some non-standard version).

It's actually labeled COM40N, but putting it in UD3 certainly makes sense as I did indeed read somewhere that SYS 36864 would enable it. Good catch ;-)


Does anyone know if there's a binary image and/or manual for the Command-O ROM on the Web somewhere? If not and you haven't already, could you dump it for us while you're at it?
I've just had all the ROMs out again to check they're still OK. Only trouble is I'm not getting consistent reads. Some parts are being repeated as if some address bits are getting stuck, but the addresses repeated are different from read to read. I'm guessing it's down to how I'm reading them, whether that be due to the adapter I made or that I'm trying to read them too fast. When I get that nailed and start getting consistent dumps, I'll upload an image.

As for a manual, so far I've only found some photos of a few pages which were taken for an eBay auction. If there's a known-good image or a full manual out there, I'd definitely like to see them!

MikeS
February 22nd, 2009, 11:06 AM
I do have the Command-O manual somewhere (such as it is) so if it isn't already out there somewhere I could look for it and scan it when I find the time (which is even harder to find ;-).

As to the ROM itself, mine would be the 8032 version which is not the same.

Meanwhile, good luck getting the PET running; 'pop's are not usually good sounds, but they do often leave visible evidence of their origin.

As to the model numbers, I think they were a little confusing; AFAIK that dynamic RAM 9" screen board was known as a 40xx over here in North America (not to be confused with the FAT40, which was actually a version of the 8032 board), and in Europe as a 30xx, and also as a 2001N everywhere ;-)

dave_m
February 22nd, 2009, 06:11 PM
Well... I finally got set up to take some measurements as described in the service manual. The video board is a bit of a pain to get due to all the wires going to the CRT. Unfortunately I only got to the third test point before my oscilloscope (an old Tek 2465) started to fizzle and let out a big puff of exceedingly aromatic smoke. Looks like the PSU is fried, so I'll need to get that fixed before I can get back to the PET. So much for my weekend project! :-\

Be careful of the high voltage in the video area. As I recall everything is rather exposed. Do you have a schematic of this board? I have one somewhere if you need it.
Regards,
Dave

cosam
February 23rd, 2009, 11:17 AM
Be careful of the high voltage in the video area. As I recall everything is rather exposed.
Definitely good advice, yes - and you recall correctly ;-) I found discharging instructions on the web and switched off before hooking onto different test points if they weren't easily accessible.


Do you have a schematic of this board? I have one somewhere if you need it.
Thanks very much but I already found a full set of those on the web too. The video board schematic even has pretty pictures of what the waveforms should look like at the various test points.

cosam
February 23rd, 2009, 11:30 AM
I finally got some good dumps of most of the ROMs. Two of them check out fine, but the other two (leaving the Command-O to one side for the moment) look like they've had it. They start out looking OK, but later on there are all sorts of random errors.

I guess whichever ROM is responsible for blanking the screen works well enough to clear away the garbage, but not to display anything sensible. That would explain the inactive-looking video RAM, anyway.

So... anyone happen to have a spare 901465-02 and/or a 901465-03 lying around? ;-) It'd be great to replace them with original (or at least period) parts. If not I'll need to see about shoehorning some modern chips in there...

cosam
February 23rd, 2009, 01:01 PM
Looks like I spoke too soon - A10 on my wonderful ROM adapter had an intermittent connection which was the cause of the read errors. I'm relieved to say that all ROMs are intact. No working PET yet, but I suspect whatever's broken is easier to replace than the ROMs...

Now that I'm confident of the accuracy of the dumps, here's an image of the Command-O ROM (http://www.cosam.org/computers/cbm/pet/com40n.bin).

dave_m
February 23rd, 2009, 07:21 PM
Looks like I spoke too soon - A10 on my wonderful ROM adapter had an intermittent connection which was the cause of the read errors. I'm relieved to day that all ROMs are intact. No working PET yet, but I suspect whatever's broken is easier to replace than the ROMs...


Congrats on good ROMs. If ever needed, I have some 2532 EPROMs you can have, but at the moment my Data I/O Model 29B has a bad UniPak and I can not program PROMs. I hope that the UniPak only needs calibration which I am struggling to do, but it may be really broken and need replacement. Here is a link to a photo of the setup.

Data I/O Model 29B (http://home.att.net/~betty.p.mercado/Old_Test_Equipment/Data_IO_29B.JPG)

-Dave

tezza
February 23rd, 2009, 08:42 PM
I always wondered what these EPROM burners looked like!

Tez

MikeS
February 23rd, 2009, 09:51 PM
Now that I'm confident of the accuracy of the dumps, here's an image of the Command-O ROM (http://www.cosam.org/computers/cbm/pet/com40n.bin).
--------
Thanks! There's a lot of stuff out there for the PETs, but not all those odd ROMs and their docs are; good to save those images when we find 'em. Now I just have to make an adapter to install it in my 2001 (and maybe upgrade the standard ROMs while I'm at it). Might even inspire me to look at the stuff that isn't working...

chuckcmagee
February 23rd, 2009, 11:22 PM
Congrats on good ROMs. If ever needed, I have some 2532 EPROMs you can have, but at the moment my Data I/O Model 29B has a bad UniPak and I can not program PROMs. I hope that the UniPak only needs calibration which I am struggling to do, but it may be really broken and need replacement. Here is a link to a photo of the setup.

http://home.att.net/~betty.p.mercado/Old_Test_Equipment/Data_IO_29B.JPG

-Dave

I never did get the Data I/O that I own working. It was close when I got it off of ebay, never got close enough. I expended numerous hours reseating, examining solder joints, all that stuff. Must be time to figure out how the old Cromemco ByteSaver works instead.

dave_m
February 24th, 2009, 02:33 PM
I never did get the Data I/O that I own working. It was close when I got it off of ebay, never got close enough. I expended numerous hours reseating, examining solder joints, all that stuff. Must be time to figure out how the old Cromemco ByteSaver works instead.

Cromemco, wow that's a famous old name. They had some good S-100 hardware. I've bought a lot of stuff on ebay. It's buyer beware there, but most sellers will answer your questions about self test passing and things like that. However usually they know very little about the equipment. One person told me the unit passed self test but would 'hang up' soon after. I took a chance and found out it had 'bouncing' keys that overflowed an input field and the unit would not allow any more input until a clear entry key was pressed. A little time spent with contact cleaner fixed the problem altogether. Here is a photo of some gear that is working nicely for me. I had used this type of test equipment in the 70's & 80's at North American Rockwell when they cost in the thousands, but now can be picked up fairly cheap on ebay. The logic analyzer is useful in troubleshooting old computers and a lot of fun to use, but they are hard to find with all the associated pods and test leads (flying lead sets). A logic analyzer is absolutely useless without them.
Regards, Dave

Lab Setup (http://home.att.net/~betty.p.mercado/Old_Test_Equipment/Lab_Setup1.jpg)

MikeS
February 24th, 2009, 02:46 PM
Yeah, Cromemco made some good stuff in their day; I still have about a dozen Cromemco systems of various models and conditions. To bring this back on topic, before I got my first PC I used a 6502 cross-assembler on a Cromemco to develop software for PETs and Rockwell AIM-65s; I never had a Bytesaver but I built homebrew EPROM programmers for both the PET and the AIM-65 and used those until I got a couple of commercial RS-232 programmers later on. Those were the days...

dave_m
February 24th, 2009, 09:58 PM
To bring this back on topic, before I got my first PC I used a 6502 cross-assembler on a Cromemco to develop software for PETs and Rockwell AIM-65s; I never had a Bytesaver but I built homebrew EPROM programmers for both the PET and the AIM-65 and used those until I got a couple of commercial RS-232 programmers later on. Those were the days...

Yes, those were days of electronic adventure. I was already working for Rockwell when the 6502 family came out. We licensed it from MOS Technology/Commodore. Rockwell sold us the AIM-65 at half price to get us playing with them at home and if we wanted a PET to take home, they paid Commodore and took a little from our weekly pay checks until we paid them back. Super company. We used PETs in the labs as cheap IEEE-488 instrument controllers for our in-house test needs. 'Real' 488 controllers from HP were costing us $10-20,000. Later even the secretaries requested PETs for word processors (remember those terrible dot matrix printouts).

Of course about 1985 we all discovered the IBM-XT with its amazing 10MB hard drive...
Regards,
Dave

cosam
February 25th, 2009, 11:41 AM
http://home.att.net/~betty.p.mercado/Old_Test_Equipment/Data_IO_29B.JPG
Nice! That puts my "eBay special" to shame. Then again, it did only cost me about $12 US, IIRC!

There's a picture of mine, featuring my go-faster 6332-to-2764 ROM adapter, on a new page I added to my site about verifying the ROMs (http://www.cosam.org/computers/cbm/pet/20090218.html) the first time around. I also added one detailing the PCB repairs (http://www.cosam.org/computers/cbm/pet/20090219.html).

Anyway, I picked up some spare 4116 chips today to try piggybacking the RAM. I got a bit of a funny look in the shop as the guy knew exactly what they were and commented they must be about 20 years old. I replied that the ones I had were even older ;-) After a couple of minutes rummaging he returned with a big box full of the things. They'd probably been there a while because he didn't even have a price for them. He asked what it was for and, surprisingly, he was even familiar with the PET. Turns out that very shop, who haven't sold computers for donkeys years, used to sell PETs when they were new.

So I got back home and switched the PET on for some more troubleshooting. This time, however, there was something really weird on the screen - the BASIC prompt again! And that's before I even opened the bag containing the new RAM chips! I've really no idea what's going on, but I'm glad it's running again. Maybe all that farting around with ROMs has fixed a bad connection somewhere.

I guess I'll get back to the display problem again and hope the dreaded black screen of death doesn't come back. I'm not getting my hopes up, though...

cosam
February 25th, 2009, 12:05 PM
Just plugged the Command-O in too, in UD3. The SYS 36864 command however drops me into some kind of monitor. Not what I was expecting, but quite cool:

http://www.cosam.org/images/pet/sys36864.jpg

MikeS: any luck with that manual? ;-)

Also a small update on the model number: I've found "4032" on (of all places) a hand-written label on the power cord. Would that be it?

MikeS
February 25th, 2009, 12:09 PM
Yeah, Tezza's experience notwithstanding the dynamic RAMs and 6332 ROMs didn't cause problems too often (unlike the old 6540s & 6550s).

Always good to check the power supply connector; a little underrated and sometimes troublesome.

Good luck! We're rootin' for ya!

MikeS
February 25th, 2009, 12:25 PM
Looks like we posted simultaneously ;-)

That sure doesn't look right; AFAIR you get dumped into the monitor when various illegal things happen. I assume that when you peek(36864) you do get the first byte of the ROM (76, I think)?

I did find the manual and it suggests that Command-O needs BASIC4, so you may be out of luck unless you can upgrade. Don't have a scanner available here at the moment, but I'll scan it anyway as soon as I do. I don't have a BASIC2 machine at the moment so I can't try your image right now.

As to the model number, I think 4032, 3032 and 2001N-32 all referred to the same model (9" screen, dynamic RAM PCB), but in different countries and with different keyboards, although the (substantially different) FAT40 was also known as a 4032, at least over here.

MikeS
February 25th, 2009, 01:32 PM
You did remember to move it to the correct socket? SYSing to an empty socket would indeed drop you into the monitor.

Wonder of wonders, I found mine; here's what it looks like on my 8032:

dave_m
February 25th, 2009, 02:35 PM
Nice! That puts my "eBay special" to shame. Then again, it did only cost me about $12 US, IIRC!

Thanks for the picture. Nice gadget and it works which is more than I can say.



After a couple of minutes rummaging he returned with a big box full of the things. They'd probably been there a while because he didn't even have a price for them. He asked what it was for and, surprisingly, he was even familiar with the PET. Turns out that very shop, who haven't sold computers for donkeys years, used to sell PETs when they were new

I wish I had a local place that stocked old parts. Those kind of places are long gone around here. However I found a good mail order place in Texas that stocks old parts. He tells you if they are unused or refurblished from socket pulls. The refurbs I got from him had good straight pins and clean UV windows.

MikeS
February 25th, 2009, 02:51 PM
Yeah, some nice looking equipment on both your benches; nicer than mine fer sure.

Mind you, what I lack in quality I make up for in quantity: I've got 4 'scopes (1 Tek, 2 Philips and 1 Eico (!) ), 4 EPROM programmers, and a PC-based logic analyzer.

Cosam: now that it's sort of working, did you read Tezza's story about transferring software to/from his PET? Time to get it doing something! Have you got a tape or disk drive for it?

dave_m
February 25th, 2009, 03:25 PM
Just plugged the Command-O in too, in UD3. The SYS 36864 command however drops me into some kind of monitor. Not what I was expecting, but quite cool:

http://www.cosam.org/images/pet/sys36864.jpg

MikeS: any luck with that manual? ;-)

Also a small update on the model number: I've found "4032" on (of all places) a hand-written label on the power cord. Would that be it?

I am unfamiliar with a PET with a 9" monitor and 32K RAM. Does yours have the small keyboard and cassette drive of a 2001 or a regular size keyboard?

Regards, Dave

dave_m
February 25th, 2009, 03:40 PM
Mind you, what I lack in quality I make up for in quantity: I've got 4 'scopes (1 Tek, 2 Philips and 1 Eico (!) ), 4 EPROM programmers, and a PC-based logic analyzer.



The name Eico brings back memories! Did you build the scope? I built an Eico VTVM (vacuum tube voltmeter for the youngsters), a HiFi amplifier and FM tuner during my college days in the 60's. But my god, it would have taken me a year to build a scope. The trim pots alone would have put me over the edge.

Regards, Dave

MikeS
February 25th, 2009, 03:46 PM
The chiclet keyboard and internal tape drive were only used on the first version of the 2001, with 4K and later 8K of (static) RAM; upgrade kits were available later to replace the upper case half and keyboard to look like the 4032 we're dealing with here.

http://www.commodore.ca/products/pet/commodore_pet.htm

http://en.wikipedia.org/wiki/Commodore_PET

Note that some references to the 40xx series are 9" models while others are 12"

Yeah, I built the Eico but it's a pretty basic 'scope; not quite as many trimpots as the Tek and Philips. It does use tubes though, as does the fine old Tektronix.

dave_m
February 25th, 2009, 06:24 PM
I always wondered what these EPROM burners looked like!

Tez

PROM programmers like the Data I/O Model 29B were used in the engineering departments to fuse parts for experimental purposes (breadboards) or engineering model designs (prototypes). For production runs, parts would be procured from a distributor pre-programmed to your specification. Or if the run was very large, a ROM mask would be made.

The Model 29B was replaced around 1990 with the UniSite model which was more adapted for the higher speed and smaller size (surface mount) of the newer devices.

Of course this made the 29B obsolete and their resale value dropped like a rock. But for buffs of old hardware, they are perfect for programming the old +5 V dual in-line package (DIP) parts.

One real problem to note is that Data I/O upgraded their firmware often to add capability for new PROMs, so there are many versions (V numbers) of firmware not only in the main frame box, but especially in the UniPak (for PROMs) and LogicPak (for PALs) adapters as well as the in the little Program/Test (P/T) Adapters that go on the LogicPaks.

This was not a problem when the 29B was in production, if the user paid for the update service, they received new firmware (PROMs) for their machine on a regular basis. For a large company like Rockwell, the Data I/O representative would even come by with the mod kit and install them for you. But obviously some users did better than others at keeping their units at the latest version. When the 29B went obsolete, tons of them went on the market, unfortunately all at an unknown version levels.

For instance even to program very old parts like the PET EPROM (2532) or the 2732/2764 type EPROMs, one needs a UniPak of V07 or better. A Select Code EF will display the V level of the UniPak. Iím not sure of the last version of the UniPak, at least V018 or more. Even more problematic is for newer parts say like UV erasable AMD C22V10 PAL. For that, one needs a fairly late version (V009) of the 303A-011 P/T Adapter. If you get one on ebay and it turns out to be a V007, you are out of luck.

Be even more careful of older Data I/O products like the Model 19 or 29A as they will invariably have old firmware.

This is probably more than you ever wanted to know about the 29B, but it might save someone a problem.

MikeS
February 25th, 2009, 11:10 PM
Since we're exchanging pictures, here's my B&C 1409 programmer, my two 2001 PETs (one with the keyboard upgrade), my PET EPROM programmer, a speech board and music interface, and a ROM expander. That particular 2001 PCB went down to the South pole BTW... ;-)

MikeS
February 25th, 2009, 11:15 PM
Quadra ROM expander (since one usually had three or four different ones)

Terry Yager
February 25th, 2009, 11:40 PM
Mike,

I just know that John (barythrin) won't think of it, but any guidance you could provide as to how to use the ROM expander would be greatly appreciated.

--T

tezza
February 26th, 2009, 12:06 AM
Good to see photos of the gear Mike.

Tez

MikeS
February 26th, 2009, 12:25 AM
Mike,

I just know that John (barythrin) won't think of it, but any guidance you could provide as to how to use the ROM expander would be greatly appreciated.

--T
-------------
A bit of a misnomer I suppose; not really an 'expander' but a 'selector'. The problem was that there were all sorts of accessory ROMs (I've got a half dozen); some were actual software or components of software (like the Command-O ROM we're talking about here) and some were just copy-protection 'dongles'. Since there were only two sockets available (and most went into the 9000 socket anyway) you'd have to swap ROMs when you went from using WordPro to programming with Command-O (for example), so just like the C64 and VIC20 cartridge expanders presently being discussed on another thread you had these (EP)ROM expanders for the 24-pin PETs. Most of them (like the Quadra ROM pictured) would have a DIP or rotary switch to select the desired ROM; if you didn't feel like going 'under the hood' then you'd run a little cable to a switch somewhere on the side or back (but no serious user ever screwed down the top anyway). AFAIK there were no software-selectable versions; at least I never saw any...

carlsson
February 26th, 2009, 02:43 AM
Oh yes, it reminds me my 3032 has a custom Basic extention waiting to be dumped.

cosam
February 26th, 2009, 12:31 PM
I did find the manual and it suggests that Command-O needs BASIC4, so you may be out of luck unless you can upgrade. Don't have a scanner available here at the moment, but I'll scan it anyway as soon as I do. I don't have a BASIC2 machine at the moment so I can't try your image right now.
Needing BASIC 4 would definitely explain it. Makes you wonder what the chip was doing in the machine in the first place, although I've come across much stranger things!


You did remember to move it to the correct socket? SYSing to an empty socket would indeed drop you into the monitor.
It's in UD3 right now which is meant to be 0x9000. I suppose I could always put it back into UD5 (where I originally found it) and see if that helps. It'd be a nice bonus to get it working, but I'd settle for bog-standard BASIC 2 at the moment!


Cosam: now that it's sort of working, did you read Tezza's story about transferring software to/from his PET? Time to get it doing something! Have you got a tape or disk drive for it?
I'm afraid it's all a bit temperamental at the moment. Even when it's running, the goofy display doesn't really allow for much tinkering. I'll definitely be trying out Tezza's techniques when it starts behaving itself. I even have a proper "PET model" 1530 which I picked up with a C64, although I'm not sure how easy it is to find even blank cassettes these days.

dave_m
February 26th, 2009, 12:32 PM
Since we're exchanging pictures, here's my B&C 1409 programmer, my two 2001 PETs (one with the keyboard upgrade), my PET EPROM programmer, a speech board and music interface, and a ROM expander. That particular 2001 PCB went down to the South pole BTW... ;-)

Nice PROM burners. Iím intrigued by the PET burner. One connector to the user port is for control but is the one to the cassette port used to obtain +5V power? Is the transformer on top of the unit used to generate a small AC voltage and then regulated to get the 25V programming voltage? You must have to be very careful with the PROM socket. It should have used a Textool zero insertion force (ZIF) socket.

cosam
February 26th, 2009, 12:47 PM
I wish I had a local place that stocked old parts. Those kind of places are long gone around here. However I found a good mail order place in Texas that stocks old parts. He tells you if they are unused or refurblished from socket pulls. The refurbs I got from him had good straight pins and clean UV windows.
Yeah, I'm really spoilt here - I have that shop and another like it within walking distance! Between them they have pretty much anything I've needed up to now and, if they don't, they're knowledgeable enough to look out possible substitutes to get you going. A lot of it is quite obviously salvaged, but they also have plenty of old stock. The 4116s I picked up, for example, all have date codes from the early '80s and there are plenty of equally old 74 series chips right there on the shelves too. I get the feeling neither of them ever throw anything away, which is of course perfect when you need vintage parts! The best thing is that both shops are very well established and also have a healthy trade in more modern parts and gadgets, so hopefully they're here to stay.

tezza
February 26th, 2009, 02:28 PM
Pity about the wavy screen. It seems to be common in older PETS (well...vintage computers generally) and was really pleased to find my own 3032 video rock solid.

I'm in awe of the gear you have there..not that I would know how to use it! :)

It's also great you can get parts so readily. Not the case here...often they need to come from overseas!

Tez

MikeS
February 26th, 2009, 07:12 PM
Nice PROM burners. Iím intrigued by the PET burner. One connector to the user port is for control but is the one to the cassette port used to obtain +5V power? Is the transformer on top of the unit used to generate a small AC voltage and then regulated to get the 25V programming voltage? You must have to be very careful with the PROM socket. It should have used a Textool zero insertion force (ZIF) socket.
-----------
Yes, yes and yes; I did use ZIF sockets but instead of dedicating them I just plugged them into the 'real' sockets wherever and whenever I needed them.

Some more PET/AIM related pictures: The homebrew AIM65 EPROM burner, a couple of ROM emulators or RAM expanders (one commercial, one homebrew), a PET composite video interface, and a monitor extension cable to make working on the mainboard or monitor *much* more convenient; also a little beeper to let me know that the 8050 disk drive had an error (I always forgot to notice the error LED).

I also have an MTU graphic video board for KIM/SYM/AIM and the PET; now that you guys have reawakened my interest and got me digging through all those PET & AIM boxes I just may try to get it working again...

MikeS
February 26th, 2009, 07:22 PM
This isn't really relevant to you guys with the 3032/4032s, but for Anders and any other owners of the 'old' 2001s:

I've got a couple of incomplete 6540 ROM sets where one or more has failed; I'm assuming that there must be others out there who have either replaced them with standard ROMS in an adapter or just scrapped the system, so, does anybody have some that they don't need (or, conversely, need any that I or someone else may have?) Maybe what's needed is a 6540 ROM/RAM registry somewhere (or is there already one somewhere?)

carlsson
February 26th, 2009, 10:46 PM
I don't own any chicklet 2001, but whenever I get the chance to go parts looking, I could keep an eye for 6540's. Since the oldest machine I've seen at the place I found all those PETs was a 2001N, I'm not sure there would be any parts predating it.

cosam
March 5th, 2009, 11:20 AM
The PET seems to be in a good mood today. It's running quite reliably after replacing a couple of capacitors which were actually completely missing from the board (C25 and C64, IIRC) and resoldering the pins of those IC sockets from a previous repair. Still no idea what was wrong with the board as everything I've checked so far looks fine.

No joy on the display front, though. The vertical signal coming in from to motherboard checks out, but horizontal and video in look a bit odd. Then again, they look equally odd when the machine's just switched on (and the display is fine) so in all likelihood they are probably OK. I've hit a bit of a dead end troubleshooting the video board/monitor and the distortion/wobbliness are still present. I took some snaps of it today:

About 1 minute after power on (http://www.cosam.org/images/pet/display1.jpg)
A minute or two later (http://www.cosam.org/images/pet/display2.jpg)
About 15 minutes later (http://www.cosam.org/images/pet/display3.jpg)

I had it running longer during testing and the BASIC banner and prompt had shrunk down to almost nothing and was flickering around, almost at the center of the screen. What's also weird is that column 23-or-so is about half the width of all the others, and the last few characters on each line are wrapped at the right.

What the photos don't capture is the waving/pulsating, so I shot a quick video of it (now on youtube (http://www.youtube.com/watch?v=3GDIN2wlCW8)). It's not too clear, but you can just about make out three "ghost" images rotating around each other. I'm not really at home in CRT stuff so maybe someone could tell me what I'm looking at ;-)

dave_m
March 5th, 2009, 01:04 PM
Looks like a classic case of "pincushion" distortion. Perhaps an adjustment pot is way off or the CRT has been magetized. If magnetized, somehow you will have to deguass it. Maybe someone here knows how to do this. Does anyone know what pots to tweek?

dave_m
March 5th, 2009, 01:18 PM
Looking at the utube clip, there is a lot more going on than pincushion and it may be thermally related. In the old days one would spray some 'instant cold' stuff from a can on the components until the problem part was isolated. This is a really tough problem.

cosam
March 5th, 2009, 01:30 PM
Yep, pincushion it is, but "progressive pincushioning" is a new one for me ;-) There are a grand total of two pots on the board - one for brightness, the other for vertical size - and both of those do exactly what you'd expect and nothing more. I got the image re-centred, but that's done by twiddling two metal rings on the CRT's neck.

Magnetisation may be something, though. I know nothing about it, but the fact that the image corrects itself after cooling off doesn't really lend itself to any "permanent" cause. I'd love to be proven wrong!

After reading up a bit on CRTs, I'm wondering if it isn't just something simple like the power supply. The video board has its own rectifier, regulator and smoothing caps, after all. I reckon I'll take a bit of freeze spray to it and see if I can zero in on the culprit...

cosam
March 5th, 2009, 01:38 PM
Ah - looks like we've got another case of simultaneous posting ;-)

In the old days one would spray some 'instant cold' stuff from a can on the components until the problem part was isolated. This is a really tough problem.
But hey - this machine is after all from the old days, so it's fitting to use contemporary techniques to fix the thing!

MikeS
March 5th, 2009, 02:12 PM
Not much to those displays; the schematic and test point scope traces are all on the Zimmers site, so if you've got a scope it shouldn't be too hard to diagnose.
http://zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/index.html

tezza
March 5th, 2009, 02:45 PM
I'm no techie and can't provide anything except moral support.

However, stick with it! I've read somewhere that faulty power supplies can indeed cause displays to wave and bend.

Tez

cosam
March 6th, 2009, 02:55 AM
Not much to those displays; the schematic and test point scope traces are all on the Zimmers site, so if you've got a scope it shouldn't be too hard to diagnose.
I thought it'd be pretty easy too - I found the video schematic with waveforms before I even picked the PET up and figured it'd be a quick diagnosis with all those test points. All the points on the vertical circuit correlate to the waveforms on the schematic. The horizontal and video in signals sort of resemble the illustrations, but look like they're missing cycles or have a couple of extra peaks here and there. As far as I can see those missing and/or extra signals are also present when the image is stable just after powering up, so I'm really none the wiser... The timescale on which the image deteriorates (and takes to recover) also makes it kinda hard to test.

As everything seems to point to a thermal problem, I think a few blasts of freeze spray will reveal the culprit quicker than the probing, waiting and probing again cycle.

patscc
March 6th, 2009, 06:09 AM
The CRT doesn't have a shadow mask, so there's nothing to magnetize.
patscc

patscc
March 6th, 2009, 06:20 AM
It's not just pincushion, a classic pincushion fault will also have a vertical 'bowing in' component.
Can OP hook a volt meter to pin 3 of the flyback (which should be the collector of Q702, if I've got the right schematic),. Or even better, a scope. See if there's a 60-Hz sinusoidal component that increases in amplitude, and if the overall voltage decreases.
Have you checked if the output of the 7812 is stable over time, and if there is increasing ripple on it ?
patscc

cosam
March 6th, 2009, 12:27 PM
Can OP hook a volt meter to pin 3 of the flyback (which should be the collector of Q702, if I've got the right schematic),. Or even better, a scope. See if there's a 60-Hz sinusoidal component that increases in amplitude, and if the overall voltage decreases.
Have you checked if the output of the 7812 is stable over time, and if there is increasing ripple on it ?

It's actually the earlier model video board (schematic here (http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/video-1.gif)) but if I compare the two I think we're talking about test point 13. Unfortunately all that stuff is of course tucked away in a corner and a bit of a pain to get at...

However - I did leave the scope on the +12V line for a few minutes and you can literally watch it go wrong. Check this out:

Nice and flat to start with... (http://www.cosam.org/images/pet/scope1.jpg)
Looks like the bottom half of a sine wave is getting through. (http://www.cosam.org/images/pet/scope2.jpg)
After that the trace just went bonkers! (http://www.cosam.org/images/pet/scope3.jpg)
And voltage starts to drop off rapidly. (http://www.cosam.org/images/pet/scope4.jpg)

I feel more than a little daft for not checking this before - I guess I'd convinced myself it was something more complicated!

patscc
March 6th, 2009, 12:41 PM
I especially like the last trace...give your PET less coffee...
patscc

cosam
March 6th, 2009, 12:46 PM
I especially like the last trace...give your PET less coffee...
Heh - it does look pretty energetic, yes! Any suggestions on what I should replace while I'm at it? If I need to remove the video board, I'd rather just do it just the once...

patscc
March 6th, 2009, 12:51 PM
The schematic I have has the 12V coming from a 7812 fed by a seperate bridge rectifier. I'd check the caps & rectifier. If you have a 12 VDC source handy, you can just disconnect the out of the 7812 and feed in 12V, and see if the ripple clears up. I have to run, do you have a link to more schematics ?
patscc

cosam
March 6th, 2009, 01:19 PM
The schematic I have has the 12V coming from a 7812 fed by a seperate bridge rectifier. I'd check the caps & rectifier. If you have a 12 VDC source handy, you can just disconnect the out of the 7812 and feed in 12V, and see if the ripple clears up.
I have a replacement for C3, so I was planning to stick that on in parallel and see if it helps any. I don't have a replacement for C1, but will pick one up tomorrow if necessary. Rectifier appears OK but I'll give it a proper look tonight. An external 12V is a great idea - I'm sure I have an AT power supply around here which would do the job.


I have to run, do you have a link to more schematics ?
Not sure what you're looking for - the one I linked to is the only video board schematic I have, which came from here (http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/index.html). The rest of the machine is later, so I've been working from these (http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/index.html).

cosam
March 7th, 2009, 07:57 AM
OK - looks like we're finally getting somewhere. Replacing the caps each side of the regulator didn't help but a new 7812 has cured the ripple and with it the wavy screen. Still can't believe I managed to miss that, especially having said earlier in this very thread that power was always the first thing I checked! So many thanks for the tip, patscc - I owe you a beer ;-)

Only thing left now is that skinny column and the wrap-around on the right-hand side. I did manage to get a trace of test point 13 (http://www.cosam.org/images/pet/scope5.jpg) while the video board was out. Going by the illustrations, I'm pretty sure that big "M" bit (to use the technical term) is correct, but those two little peaks preceding it shouldn't be there. Could they correspond to the narrow column and horizontal wrapping?

BTW: the collector of Q14 isn't actually connected to anything, although according to the parts layout this is apparently correct for the TIP55A fitted(?)

dave_m
March 7th, 2009, 10:25 AM
Only thing left now is that skinny column and the wrap-around on the right-hand side. I did manage to get a trace of test point 13 (http://www.cosam.org/images/pet/scope5.jpg) while the video board was out. Going by the illustrations, I'm pretty sure that big "M" bit (to use the technical term) is correct, but those two little peaks preceding it shouldn't be there. Could they correspond to the narrow column and horizontal wrapping?

BTW: the collector of Q14 isn't actually connected to anything, although according to the parts layout this is apparently correct for the TIP55A fitted(?)

Comparing your signal with TP13, the overall period of 64 uS is correct, but the positive period should be 24 uS, so to me should not the the positive time period incorporate all the pulses you see big and skinny?

And how about the signal amplitude? It does not looks correct. Are you using a 10X probe? The schematic seems to indicate 0.5V peak, is that correct?

You are doing a great job.

dave_m
March 7th, 2009, 10:36 AM
Looking at the documentation again, the Horizontal Drive input to the video board is TTL level (~5V), is that correct? So either the source is bad or something on the video board or elsewhere is loading it down in a weird way.

patscc
March 7th, 2009, 11:20 AM
Cosam, could we get a pic of the screen how it looks now ?
The peaks could be some ringing, the band you describe sounds like there's some sort of phase shift going on in the hor. circuit. Does the board look like it's been repaired before ?
patscc

cosam
March 7th, 2009, 11:26 AM
Comparing your signal with TP13, the overall period of 64 uS is correct, but the positive period should be 24 uS, so to me should not the the positive time period incorporate all the pulses you see big and skinny?

And how about the signal amplitude? It does not looks correct. Are you using a 10X probe? The schematic seems to indicate 0.5V peak, is that correct?

You are doing a great job.
Are we looking at the same docs? My video board is an early version (schematic here (http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/video-1.gif)). Unfortunately the illustrated waveforms are not very clear and have no scales. I believe TP13 on this board corresponds to TP17 on the newer 2001N version. Just to confuse matters, both TP13 and TP17 on the new schematic have a period of 64uS ;-)

I think, however, the positive period (the big "M") should be about 10uS. The two smaller peaks before it fall outside of that, so I don't think they're meant to be there. Now just to figure out where they're coming from...


Looking at the documentation again, the Horizontal Drive input to the video board is TTL level (~5V), is that correct? So either the source is bad or something on the video board or elsewhere is loading it down in a weird way.
Yep, input is TTL and TP3 on the old schematic (coincidentally, TP13 on the new one!) looks to be a bit busier on the scope (http://www.cosam.org/images/pet/scope6.jpg) than it ought to be. I don't really get this part at all, as if it's that different I wouldn't expect the effect at the CRT to be so subtle.

cosam
March 7th, 2009, 11:40 AM
Cosam, could we get a pic of the screen how it looks now ?
The screen looks pretty much like it did before here (http://www.cosam.org/images/pet/display1.jpg), but then stable and without the bowing in at the top. The narrow column is the 31st from the left, if I'm counting correctly. The last 3 or 4 characters on each line also "double back" on their predecessors.


The peaks could be some ringing, the band you describe sounds like there's some sort of phase shift going on in the hor. circuit. Does the board look like it's been repaired before ?
Oh, there's definitely been someone in there before me, yes. Some transistors have been replaced but I think only Q11 in the horizontal circuit - I'll whip it out again and check. I resoldered all the repair-looking pins just in case, but no joy.

patscc
March 7th, 2009, 12:00 PM
Can you check to see if the caps & parts in the hori. drive & output stage are the same as in the schematic ?
Is C28, the 10uF non-polarized, actually non-polarized ?
Oh, can you connect your scope to 11 and see what you get ?
patscc

cosam
March 7th, 2009, 12:25 PM
Can you check to see if the caps & parts in the hori. drive & output stage are the same as in the schematic ?
Is C28, the 10uF non-polarized, actually non-polarized ?
The replacement Q11 is the correct part and everything else looks to be original. C28 is the real deal and even still glued in place. The flyback could be a replacement, but it's neater than the rest so probably just manually soldered at the factory.


Oh, can you connect your scope to 11 and see what you get ?
Glad to: http://www.cosam.org/images/pet/scope7.jpg. Hope you can make more out of it than I could!

patscc
March 7th, 2009, 12:30 PM
So Q12 is the actual PNP it's supposed to be, and Q13 is correct as well ?
Waveform at 3 ? (Sorry I'm bugging you with all the different measurements, I unfortunately can't just pop 'round and take them myself)
patscc

cosam
March 7th, 2009, 12:44 PM
So Q12 is the actual PNP it's supposed to be, and Q13 is correct as well ?
Yep, both are correct and look to be the original parts.


Waveform at 3 ? (Sorry I'm bugging you with all the different measurements, I unfortunately can't just pop 'round and take them myself)
Here's TP3 (http://www.cosam.org/images/pet/scope6.jpg) (and are you kidding? I really appreciate you taking the time to look at them!)

dave_m
March 7th, 2009, 01:25 PM
Here's TP3 (http://www.cosam.org/images/pet/scope6.jpg) (and are you kidding? I really appreciate you taking the time to look at them!)

cosam, TP3 doesn't look at all like the picture in the docs. Start checking the Horizontal Drive circuit in the main logic board. Maybe the 12 V regulator was all that was wrong with the video board. The Horizontal Drive circuit should not be at a high level so long. Look at the signals called Display On and Display Off. They are at JK flipflop C5 pins 5 & 6.

cosam
March 7th, 2009, 02:41 PM
cosam, TP3 doesn't look at all like the picture in the docs. Start checking the Horizontal Drive circuit in the main logic board. Maybe the 12 V regulator was all that was wrong with the video board. The Horizontal Drive circuit should not be at a high level so long. Look at the signals called Display On and Display Off. They are at JK flipflop C5 pins 5 & 6.
Ah yes - garbage in, garbage out, as they say. At least we're back in the realm of things I understand.

But: wow - way to hit the nail on the head! That flipflop is on one of a row of three replacement ICs on the main board. Pins 4 and 6 are meant to be connected, but they weren't, leaving one of the inputs floating. A bit of wire-wrap wire across them and I've got full-size characters again. I'll have to recentre the image on the CRT, but this is looking good!

On the downside, it looks like I owe you both a beer now ;-)

patscc
March 7th, 2009, 03:00 PM
Hey, no fair ! The schematic I've got doesn't show the flip-flop. Where do y'all get the good stuff from ?
Nice catch. Glad you found the problem.
patscc

cosam
March 7th, 2009, 03:21 PM
Hey, no fair ! The schematic I've got doesn't show the flip-flop. Where do y'all get the good stuff from ?
Heh - the real stinker is that the culprit was literally just one component away from the header on the video board schematic you were most probably looking at. Physically of course a different board and a different page! Don't get too upset - you deserve all credit for catching the 12V ripple problem ;-)

I'm not usually one for posting images inline, but I just had to share this one with you:

http://www.cosam.org/images/pet/fixed.jpg

Looks like my rusty BASIC is at least good enough for writing test programs ;-) There's still a very slight hint of a image wobble, but I'm more than pleased with the results. Thanks to all of you who put you time and effort into troubleshooting this thing - very much appreciated indeed.

Now I need to go dig up Tezza's thread to see about getting some software onto this thing...

tezza
March 7th, 2009, 03:37 PM
Great stuff!

Even though I didn't understand most of the electronics involved (hopefully I will someday), it's great to see people helping each other in bringing a classic micro back to life. I'll bet you feel very satisfied.


Excellent! Well done to everyone involved! That's what these forums are all about.

Tez

cosam
March 7th, 2009, 04:23 PM
Even though I didn't understand most of the electronics involved (hopefully I will someday), it's great to see people helping each other in bringing a classic micro back to life. I'll bet you feel very satisfied.
It was almost an anticlimax when it finally worked, but the grin-factor is sinking in, yes ;-) Now that the causes are known, maybe you should have a browse through the schematics? You might understand more than you think!

pavery
March 8th, 2009, 01:08 PM
Hey, well done cosam on your PET fix. I've been following this thread since the first post and really appreciate you detailing your fault-finding work and the technical input by others (as no doubt you did too), as it gives good insight to this aspect of vintage computing - hardware repair to component level.

Your website (http://www.cosam.org/computers/cbm/pet/) covering the fault & fix process is great too. Certainly a worthy resource for future PET repairers.


That flipflop is on one of a row of three replacement ICs on the main board. Pins 4 and 6 are meant to be connected, but they weren't, leaving one of the inputs floating.

How come pins 4 & 6 weren't connected? Were the PCB traces gone?

(An observation of mine: An increasing number of the faults we are encountering today in this vintage hardware is the result of a *previous* repair, or rather an *attempted* repair). IE, it's not always old components that are to blame, but increasingly so the action of some humanoid!

Philip

cosam
March 8th, 2009, 02:16 PM
Hey, well done cosam on your PET fix. I've been following this thread since the first post and really appreciate you detailing your fault-finding work and the technical input by others (as no doubt you did too), as it gives good insight to this aspect of vintage computing - hardware repair to component level.

Your website (http://www.cosam.org/computers/cbm/pet/) covering the fault & fix process is great too. Certainly a worthy resource for future PET repairers.
Thanks, Philip. I try to document as much as I can about what I do with any of these old machines, especially I don't find a remedy just from documentation and a bit of Googling. If it can help even one person get their classic computer(s) running again, it's time well spent as far as I'm concerned. And indeed: top marks to those who contributed with their knowledge, time and experience. I'd probably have got there alone eventually, but it would have definitely been a long way down the line!


How come pins 4 & 6 weren't connected? Were the PCB traces gone?
It's not too clear why the pins weren't connected. That area is a bit of a mess, even after tidying it up. I suspect removing the original IC probably broke a track - something I've done myself by being a bit heavy-handed with the desoldering pump. Going by the number of "solder bridges" on the board, I wouldn't be surprised if it was in fact my reflowing that uncovered the problem.


(An observation of mine: An increasing number of the faults we are encountering today in this vintage hardware is the result of a *previous* repair, or rather an *attempted* repair). IE, it's not always old components that are to blame, but increasingly so the action of some humanoid!
You're not wrong... I'm doing my best to avoid being one of "them" myself!

Anyway, today I had the nice simple job of cleaning out (read: completely dismantling) the keyboard. Just when you can finally actually use the thing, sticking or otherwise temperamental keys seem to appear from nowhere. Now the keyboard puts the rest of the machine to shame, so I suppose I'll have to give the case a scrub up too! As if I haven't revealed enough problems by solving others on this project already! ;-)

cosam
March 10th, 2009, 02:46 PM
I've not quite figured it out yet, but I found an interesting problem whilst trying to load programs from tape today. I did get the PET to save a program to tape and read it back properly, which didn't work before. But then a couple of times, I'd issue a load command, press play and, whilst searching for the program, the PET hung and the tape drive stopped. After power cycling all seemed fine, so I carried on. However, after one hard reset, the PET started writing characters to the screen on its own and after that I ended up with that all-too-familiar blank screen.

Seeing those characters appear from nowhere, I thought there might be a problem not far from the keyboard interface. Sure enough, whilst poking around with the 'scope, I found that /IRQ was being held low, apparently by the PIA responsible for the keyboard and tape interface. Popping this out solved the blank screen and got my BASIC prompt back, although I of course couldn't use the keyboard. After moving the other PIA (for the IEEE-488 interface, which I'm not using right now anyway) to UC7 (the keyboard/tape PIA's socket) everything seemed to work again.

Now I'm wondering if a dud PIA was the original cause of the occasional blank screen I was getting, or whether it's just now gone bad due to to some (possibly tape drive-related) problem. I'm rather reluctant to start experimenting with my one remaining PIA if the latter is the case, especially seeing as I just had the tape stop again during a load.

Is it under any circumstances normal for the tape to stop like it did, or should I be looking for a potentially PIA-frying problem on the main board?

dave_m
March 10th, 2009, 03:27 PM
The cassette interface was always very touchy. It would often not find a file especially if written from another PET but those were alignment problems in the reader. I do not remember it 'hanging up' and certainly the screen never went blank. Hopefully it is just a bad PIA. There is still a source of PIA chips from Western Design Center in Arizona. But the bad news is that while the cost of a chip is only $7.50 US, they ship overseas using US Global Priority at a flat rate of $25 and a special handling fee of $15. Quite a surcharge for a little chip. Here is the website info on the 65C21. They claim it is totally compatible with the MOS Technology or Rockwell N-Mos chip.

http://www.wdesignc.com/wdc/w65c21s-chip.cfm

cosam
March 10th, 2009, 03:47 PM
The cassette interface was always very touchy. It would often not find a file especially if written from another PET but those were alignment problems in the reader. I do not remember it 'hanging up' and certainly the screen never went blank.
Yes, I should probably point out that the "problem" tape was one I made myself on a different tape deck. I have however made working tapes in exactly the same way on this deck before. I can understand a bad tape (whether it be down to alignment or anything else) not being readable, but crashing the machine?

Probably also good to clarify that the blank screen did not occur directly after the stopped tape episode. When the tape stops, the screen is as you'd expect, but RUN/STOP does nothing. Only after a power cycle did I get the stream of characters on the screen, and it took yet another hard rest to get to the blank screen.

I'm still wondering whether the PIA was the cause of my original blank screens, but the funny thing with those was that if you left the machine off for long enough, it'd start right back up as if nothing was ever wrong. I guess it could've just been on its last legs and getting it to read and write tapes finally finished it off.


Hopefully it is just a bad PIA. There is still a source of PIA chips from Western Design Center in Arizona. But the bad news is that while the cost of a chip is only $7.50 US, they ship overseas using US Global Priority at a flat rate of $25 and a special handling fee of $15. Quite a surcharge for a little chip. Here is the website info on the 65C21. They claim it is totally compatible with the MOS Technology or Rockwell N-Mos chip.

http://www.wdesignc.com/wdc/w65c21s-chip.cfm
Thanks for that - at least I know if all else fails I can get a replacement, even if it does end up costing an arm and a leg...

dave_m
March 10th, 2009, 08:21 PM
Checking around the WDC website, it turns out they have distributors in Austria, Germany and Denmark among other countries.

http://www.wdesignc.com/wdc/distributors.cfm

MikeS
March 10th, 2009, 08:26 PM
Surely someone will have one that's closer and cheaper; the 6821 is also compatible.

Time to get acquainted with some of your fellow Dutch PET enthusiasts; there are a few. And there's Anders; he's got everything and knows everyone;-)

Good luck!

carlsson
March 10th, 2009, 10:56 PM
Yes and no. I may be able to scour (?) a couple of parts but more importantly know the prominent C= people around the world. In this case, the first name that pops into my head would be Ruud Baltissen (http://www.baltissen.org/).

cosam
March 11th, 2009, 03:57 AM
Thanks all for the leads on getting a new chip - looks like I'll be able to get my hands on one without too much trouble.

What I'm more concerned about right now is whether my main board (or tape unit) is some kind of PIA assassin. Looking at the tape interface, I see that the motor is switched by the PIA. Supposedly any number of hang/crash situations (including /IRQ being forced low by the PIA) could cause that signal to drop and therefore the motor to stop. I almost daren't test the thing for fear I may be slowly roasting my last remaining PIA... Any inspired theories or am I just being overly cautious?

dave_m
March 11th, 2009, 06:54 AM
Well, 30 year old transistors will lose some 'beta' (gain) so the PIA outs may not source or sink current as well as they used too, but perhaps the problem has more to do with old motors/capacitors and dirty read/write heads in the tape transport. I would start with that and not worry too much about blowing the PIA. To be safe, monitor the PIA outputs to the tape drive while they are active to see if there are large inductive spikes being induced on them somehow. I would hope that the PIA signals are buffered in the tape transport, but I do not know.

cosam
March 11th, 2009, 03:35 PM
I suppressed my irrational fear of blowing stuff up and to had another go at experimenting with the tape drive. Not surprisingly, the PET has a preference for it own tapes (i.e. those written from the machine itself) and reads them flawlessly. I did get a "foreign" tape to load, although by no means reliably. Had a couple of hangs, but the tape kept rolling this time.

I noticed a slight "step" in the spool of tape after it came out of the deck I used to write it, so it could just be down to an alignment issue. It's is a vertical affair, so there's more possibility of the tape wandering over the heads without gravity's assistance to keep it in place, as is the case with the C2N. I don't know if that fraction of a millimeter would make any difference - maybe I'm just on the threshold. In any case, with the vertically oriented drive laid down on its back, I managed to write a working tape. I'm sure I'll be able to reproduce the hang and troubleshoot that properly some other time.

On the parts front: I think I might have to add a replacement VIA to the wish list, too. Anyone else notice the PET was stuck on lower case? I wouldn't mind, but it meant I couldn't get any of those lovely PETSCII graphics. I checked the character ROM and it's fine - even went so far as to write a script to print out the characters at each location. It would appear the VIA is pulling GRAPHIC high, regardless of POKEs to 59468. PEEKing that same address shows that the POKE has "taken" so, as there's nothing between the VIA and A10 of the character ROM, I reckon the VIAs had it, partially at least. I read a small resistance between VCC and CA2 - not sure if that's normal. For now I've tied GRAPHIC to ground as a workaround, but it'd be good to fix it properly.

Vlim
March 12th, 2009, 06:54 AM
Cosam,

Been reading your contributions with much interest.

I have a spare PET without keyboard lying around, probably a 4032 or a 4016 with the early metal housing (it's boxed and stored at the moment). Perhaps that will help?

cosam
March 12th, 2009, 08:02 AM
I have a spare PET without keyboard lying around, probably a 4032 or a 4016 with the early metal housing (it's boxed and stored at the moment). Perhaps that will help?
Hmm... what did you have in mind? I'd definitely be interested in some parts if they're up for grabs. Or are you looking to get rid of the whole machine? OTOH: Maybe you should just try to source a keyboard for it ;-)

PS. This is probably getting a bit borderline re. public/private posts. Drop me a PM if you see fit (Dutch is fine).

cosam
March 21st, 2009, 03:55 PM
Well, I picked up Vlim's spare 4016 earlier this week. Unfortunately all the bits I need appear to be soldered in, so it'll take a bit of work to transplant them.

Even better though: he's also set me up with a 8050 dual disk drive. No cable to test it with yet (hence the post in the Wanted section) but it has quite literally given me the green light, so it's at least passing its POST. If it wasn't for it needing a good clean-up, it would've almost been boring ;-) More about that here (http://www.cosam.org/computers/cbm/8050/index.html).

MikeS
March 21st, 2009, 07:03 PM
Congratulations and good luck! They're much more fun with a disk drive.

That's an excellent article by the way, as are your others; is English a second language for you (if you don't mind the question)?

(Loved the 'scuzzy drive' pun BTW! ;-)

tezza
March 22nd, 2009, 02:27 AM
Good stuff.

I'm keeping my eye out for a PET drive down here also.

Tez

cosam
March 22nd, 2009, 04:07 AM
Congratulations and good luck! They're much more fun with a disk drive.
Yeah, the tape drive is fine if you just want to run a particular program, but I'm looking forward to the extra speed and convenience of disks. I really don't know how I managed to put up with such long loading times back in the "old" days!


That's an excellent article by the way, as are your others; is English a second language for you (if you don't mind the question)?

Thanks :-) English is in fact my mother tongue although, after ten years living over here, it's starting to deteriorate!

cosam
March 22nd, 2009, 01:11 PM
Had a very productive hour or so this evening, desoldering the VIA and a PIA from the 4016 board. At 40 pins a pop, they weren't the easiest ICs I've ever removed. Good news is they emerged unscathed and after cleaning up and straightening the pins, they're a good fit in the PET's sockets.

The PIA went in first. No blank screen, so I think it's OK. Won't know for sure until I try out that disk drive. For a moment I thought I'd replaced the original VIA with one even worse, but the blank screen I got had a much simpler cause this time: apparently these things work better with the power cable attached to the main board... ;-)

So I now have upper case as default, without my little grounding wire. A choice POKE to 59468 switches between the two character sets, so it looks like we're finally there. Again though, the VIA has to prove its real worth by working with the disk drive...

Vlim
March 23rd, 2009, 04:41 AM
Good to see that it worked out.

Any idea what the piggyback board in the donor-PET was yet?

cosam
March 23rd, 2009, 04:49 AM
Good to see that it worked out.
Yep - thanks again for the supplies!


Any idea what the piggyback board in the donor-PET was yet?
Had a look yesterday when taking it apart. It appears to be a 64K memory board. Looks like it connects to the main board where the original CPU would be, the CPU then being inserted in the expansion board.

tezza
March 23rd, 2009, 09:57 AM
Good one.

Desoldering a 40 pin IC is a heavy task. In a fit of masochism I did this once (http://classic-computers.org.nz/blog/2009-02-14-blowing%20up%20and%20then%20fixing%20a%20c-64.htm)to obtain an IC to repair a C-64.

Tez

MikeS
March 23rd, 2009, 12:19 PM
Well, at least you guys had a good reason! I couldn't connect a terminal to one of my AIM65s a couple of weeks ago and it sure looked like a bad 74LS38, a good suspect since it's directly connected to RS-232 lines and prone to ground loops, etc. Went to my local junk shop but he didn't have any, so I had to pull one off another board (as well as the one in the AIM65 of course). You guessed it, after all that pulling and replacing it turned out that I had never had this one on line and so it just needed a jumper installed... D'oh - another dent in my forehead.

But if it ever does go, at least now it's in a socket...

cosam
March 26th, 2009, 02:55 PM
Managed to get the disk drive working this evening. I'm not very good at waiting, so I went ahead and ordered the edge connector needed to make my own IEEE-488 cable. If/when I get a proper cable I can always use the connector to make a cbmlink cable for the user port. Anyway, it arrived in the post today and after a bit of soldering on one end and crimping on the other, I was all set.

First disk format didn't quite turn out right - loading the directory listing I got nothing but a 0 and a double quote in reverse video. Formatted again and it looks like we're in business! I wrote the obligatory "hello world" program and it saved it OK. Weird thing is, of I load it back (even after a "new") the listing still contains the directory information. That can't be right, can it?

Also had a bit of "machine gunning" at the start of the format. I say machine gunning; it was more like a loud fart really ;-) During some operations the centre LED starts switching from green to red pretty rapidly. Either of those things anything to worry about?

Oh, and it looks like drive 1 (I'm assuming that'd be device 9?) doesn't work yet. The format command exits immediately without any kind of error message, but if you try a LOAD the PET reports it's found the program and tries to load it regardless! No activity at the drive though... something to look into.

Update: Nevermind that last bit - both drives are of course device 8 but you need to specify the drive number!