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Commodore CBM 8032 video display problem

Juleshsmith

Experienced Member
Joined
May 1, 2012
Messages
53
Location
Melbourne, Victoria, Australia
Hi world. My first time on this forum!

Have just acquired a CBM 8032 (32K) with a video display issue (see pic).

I've read lots of forums about similar issues and know it could be a few things... just need guidance on where to start looking with this one as I really get lost in the technical jargon.

I have only a basic knowledge of electronics, and have real problems understanding schematics. From what I understand it's most likely a video RAM (2114) or memory/ROM issue? I have a second non working 8032 I can use for parts. Could someone explain (in geographical terms) what specific chips I should try and change/swap with my spares (ie which slots to start looking at (for example UC4, UC5 etc)) or any other advice you would recommend given my limited knowledge with vintage computer repairs.

I have basic equipment (and very basic knowledge), but have no idea on how to check if the RAM/ROM chip are faulty (or reset them if they were), so hope to just take the cuplrit chip out and replace with a spare and cross my fingers! Just don't want to make it worse...

The computer beebs when you fire it up, and there is movement on the screen when I press keys. I also get an error beep if I hold the space bar down etc. So I'm pretty confident it's not an issue with the CPU. I've cleaned all the dust off the PCP/tracks and checked for dry joints. I plan to re-seat the seated chips later tonight to see if that helps.

Thx world! Jules (Australia)

Commodore CBM.jpg
 

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Couple of other questions... I'm thinking I should start by re-socketing the main ICs (ie UC4, 5, 6,7 with 18pin socket cradles) to make it easy swapping chips. Are there any other chips (related to this possible problem) I should re-socket whilst I'm at it?

And... what's the easiest was to remove the chips without damgaging the tracks etc. I was thinking about just using de-solder braid on each pin??

Thx!
 
Ok, the first questions I have are.
When you turn it on, does it beep?
Can you type anything and see any change on the screen?

Later,
dabone
 
Hey dabone

Yep, there's a 'sart-up' beep and characters do seem to change (if you look really closely) when I hit the keys. Also get an error(?) beep if I hold the space bar down. So CPU-wise, I think it's looking good!
 
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I would start then by socketing the 2114 rams. (There are 4 of them on a 8032.) These are the display ram, and I just fixed one myself that had 2 bad ones.
These chips are pretty easy to find, and if you have a dead vic-20 lying around it uses them also.

So socket one, swap it with a different (good) one and see if the display changes, if not, socket the next one, test, lather, rinse, repeat.

Schematics are here. Look at sheets 8 and 9.

http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/8032/index.html

Good luck!

Later,
dabone
 
... I'm thinking I should start by re-socketing the main ICs (ie UC4, 5, 6,7 with 18pin socket cradles) to make it easy swapping chips. Are there any other chips (related to this possible problem) I should re-socket whilst I'm at it?

If you are not that familiar with soldering, I would advise you not to umsolder all four. In fact do not unsolder any until you try a 'piggyback' test on each 2112 RAM chip. With luck that might zero in on the bad chip and you can leave the others alone. Let us know if you are not familiar with this simple test.

When you do discover the bad one, to remove it as safely as possible, remove the chip by cutting all the legs off with a small pair of diagonal cutters. With the 'body' of the chip removed, use solder braid to remove each leg one at a time without overheating the printed circuit board. After all legs are unsolderd from the board, use more solder braid to clean up the 18 feedthrough holes of excess solder. Then you can install a socket. This way you should not overheat the board amd possibly damage the circuit traces. Good luck with your repair of a classic PET.
 
Couple of other questions... I'm thinking I should start by re-socketing the main ICs (ie UC4, 5, 6,7 with 18pin socket cradles) to make it easy swapping chips. Are there any other chips (related to this possible problem) I should re-socket whilst I'm at it?

And... what's the easiest was to remove the chips without damgaging the tracks etc. I was thinking about just using de-solder braid on each pin??

Thx!
Yup, those are the suspects all right; as to the easiest way it depends on your skill level, but Dave made some good suggestions.

I can't tell from the pic if those are all actually valid characters; if not, then I'd add the character generator at UA3 to the list of suspects.
 
Awesome advice guys!! Totally makes sense, easy to follow with my 'limited' skills. Glad I waited...

Mike, about 20% of the characters on the top half of the screen are recognisable, but the rest aren't familiar characters, so will def check UA3.

Could someone explain how to do the ' piggyback' technique that Dave's talking about ("In fact do not unsolder any until you try a 'piggyback' test on each 2112 RAM chip") I presume you 'connect'/ hold a second 2112 on the opposite side of the PCP to suspect 2112... Is it as simple as that, or do I have to disconnect part of the suspect 2112???

Thx! Jules.
 
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OK, swapped chips at UA3 (character generator) no difference, so pretty confident that's all good. What's interesting is when the chip is out I still get a distinctive 'split screen', eg the top half has a different character set/'look' to the bottom half (as also evident when the chip is in). I've learned that alternate video ROMs (UC4 vs UC5 etc) are responsible for vertical and horizontal/odd-even? If that's the case, is it possible to narrow down the suspect ROM if the top half of the screen is obviously different from the bottom half??? Or is that saying that there's probably more than one suspect?
 
Could someone explain how to do the ' piggyback' technique that Dave's talking about ("In fact do not unsolder any until you try a 'piggyback' test on each 2112 RAM chip") I presume you 'connect'/ hold a second 2112 on the opposite side of the PCP to suspect 2112... Is it as simple as that, or do I have to disconnect part of the suspect 2112???

Thx! Jules.

The piggyback test is a quick & dirty test to try to discover which of the 4 video memory chips may be bad. You already have a screen photo of the present display of the 2000 screen characters (80 columns X 25 rows) for reference.

With power off, you would hold a new 2114 chip on TOP of one of the chips, say UC4 so that each of the 18 pins touch the proper corresponding pin on the soldered part being careful not to short to adjacent pins. If this is hard to do, you might tack solder pin 1 of the piggyback part to pin 1 the soldered part to help keep alignment. With all pins making proper contact, power on the system looking for a noticeable change in the display (hopefully for the better). If so, that part will be a candidate for replacement. If no change, that part may be OK. Power off and try the test on the next part, UC5, UC6, and UC7 in order. Now you may have a better idea of which parts to start replacing one at a time.

From looking at the screen picture, it is hard to tell if the problem is with the odd or even characters, all of them look equally messed up. It is possible that all the 2114's are bad or there is another lurking issue.
 
..if the problem is with the odd or even characters, all of them look equally messed up. It is possible that all the 2114's are bad or there is another lurking issue.
Yeah, I noticed that the two halves of the display memory are quite different; the fact that they're still different with the CG removed kinda points at something other than or in addition to video RAM...

What tools have we got? DVM? Logic probe? Scope?
 
I've got a DVM and scope Mike (but really not good using the scope). If you can tell me where to put the probes and what to look for (preferably via the DVM) that would be awesome!

I'm also concerned it may be something other than the video RAMs. There's a good thread talking about a similar issue

http://www.vintage-computer.com/vcforum/archive/index.php/t-12855.html

In one case it came down to...

"I had a similar problem:

http://pagesperso-orange.fr/ampuma/pet-2.JPG

After some research, the regulator 5 Volt power line in 6502 was down. 1N4005 diode fails.

I replaced these two components and PET to restart:

http://pagesperso-orange.fr/ampuma/pet-3.JPG

If this can help.
"

Just looking under the PCB, noticed the solder joints for UA4-UA7 are looking a little 'thin'... not obviously dry. What's UA4-7 for? Worth re-soldering?

Thx Dave for the piggyback instructions... will give it a go once I get the spares outa the other 8032 or my vic 20!
 
UPDATE 1810hrs Aussie time.

I re-soldered UA4-7 and piggybacked each 2114 with one I took out of my other 8032... alas no real noticeable change at all... Would you expect to see a dramatic change using the piggyback method? I noticed the odd character being different, but I think that was just a random thing that happens regardless of the piggybacked 2114s.

Unfortunately I don't know if the spare 2114 I'm using to test with is actually OK itself, as my other 8032 is DOA... I could take one outa the vic 20 (which is working), but if there are other possible culprits I should look at, I'd rather eliminate them before cannibalising my ol vic!

Any suggestion on 'where to now'?
 
Welcome to the forums! These guys helped me with mine, so you're in the right place.

It's 3AM here. Got woken up by a pretty wicked thunderstorm and couldn't get back to sleep. I didn't actually go look at one of my 8032s, but UA4-UA19 should be system RAM. There should be 16 4116 chips. The piggyback test works on these as well. As far as re-soldering, it can't hurt, but in my opinion, it's never good to re-flow existing solder. It's better to remove what's there and re-solder with brand new solder.

Remember that since your other 8032 is non-working, you can't know that the chips you're using to test aren't also bad. That's not to say I haven't also done it, but it can make troubleshooting even more frustrating. If you've got a source of these chips, it might be a good idea to buy some new ones. Here in the US, I often use Jameco Electronics, where I bought all new 4116 RAM chips for my CBM 4016. There only ended up being 2 bad ones, so now I have spares.
 
Our messages crossed in the mail, so to speak! :p

Another thought. If the VIC uses 2114's, you could put the ones from the 8032 into it, and see if that works. Dunno if they're socketed though.

I have to go to bed. Gotta be up in 3 hours. Good luck.
 
Thx Kevin. I lived in the USA for a few years, saw some amazing electrical storms unlike anything in Australia!

Yeh, I don't think the vic 20s 2114s are socketed... I'm getting better at de-soldering non socketed chips, but at the cost of breaking my first one! So I'm still hesitant to take it out of the vic unless I'm pretty confident that is the issue.

Can you get a garbage screen with faulty 4116-es??

At the very least, I'm re-discovering electronics (I actually did study it for a few years but quickly learnt it wasn't one of my strengths!)

I tried swapping the 6502 but no difference. Not sure which is the "1N4005 diode"... there's one next to where the power supply connects to the PCB, that's fine.

I'll keep learning...
 
Can you get a garbage screen with faulty 4116-es??

I am not sure, but I would think that could cause it. We'll let the veterans weigh in on this question.

I wouldn't worry about the 1N4005 diode. You can easily check all 3 power supply voltages with your DVM on any of the RAM chips. Use the right side of one of the little capacitors in front for your ground, C30 or C38, then measure UA4, pin 1 is -5v, pin 8 is 12v and pin 9 is 5v. If those are all good, within say 5%, your power supply components should be ok.
 
Kevin, perfect instructions! The -5v, 12v and 5v lines are fine! I feel like a real technician now!!! Ok, so what should I test next??? (If you can explain everything like that, that really helps!)

This seems to be similar to my problem...

http://www.classic-computers.org.nz/blog/2011-07-13-pet-fixes-part-1.htm

Should I check each chip (eg the 6520) for power loss? if so, do I test them the same way Kevin explained (eg use right side of C30 for ground, test pins by refering to schematics for voltage specs)? Are there specific chips I should check (and ones I shouldn't worry about)?

And, could UD6-UD10 also be candidates?

Loving the learning curb!!!

J.
 
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I'm just trying to pay back some of the awesome help the guys here have given me! However, I joined just a few weeks ago when I started working on my 4016, so I'm just a newbie too. I have electronics background though, which helps, and I spent some time fixing C64s and Amigas, mostly through chip swapping rather than heavy duty circuit tracing.

Anyway, I too read Tezza's repair adventure, which was a good read. The real value I got from it was that those old sockets are crap, and they don't make good contact with the chips. If I had your 8032, I would replace the 4 video RAMs first, putting them in sockets, and then replace the Character ROM socket. On mine, after I replaced the 2 bad RAMs (system RAM, not video RAM), it finally gave me a beep and characters on the screen, but they were garbled. When I put the Character ROM (or I guess we're calling it Character Generator) from my working 8032 into it, I got different characters garbled, and I knew the socket was bad. Replaced that, and my PET was working perfectly!

Oh, and I should have mentioned this first, but CLEAN THAT BOARD! I use a natural bristle paint brush to get the loose stuff, then Stoner Flux Remover (every time I grab the can, I say "Whoa man, this stuff is pretty goooood!") and a toothbrush for the hard-to get filth. It's not cheap, but it does a great job. Maybe the guys can chip in on what they do to clean old PCBs. EDIT: I just re-read your posts and saw that you did dust the board off. Sorry.

Regarding cleaning, corrosion and such, the piggyback test will not work if the new chip can't make contact with the pins on the chip being tested. So if the pins are black and corroded, that test is useless.

And, could UD6-UD10 also be candidates?

I don't know. I'd like the veterans to weigh in on this. During my 4016 adventure, we focused on UD6 and UD7, but mine wasn't booting at all, so those might be just fine on yours. Can someone provide the information on what each of the ROMs does? In fact, I would love to have a "theory of operation" document on the PET. I've got C64 manuals that step you through the process of operation. Is there such a thing out there for any of the PET series?

Oh, and finally...it's "learning curve" :p
 
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Can you get a garbage screen with faulty 4116-es??

With a PET board using the CRTC chip (6545), at power on, the screen come up with garbage for a second or two. You can
usually see this when the CRT is already warmed up such as after a reboot. Then the CRTC is initalized and the program will clear
the screen and post the power on Commodore message. Your main RAM is mostly good as you get a beep and the system reponds
to keystrokes. So the fact that the screen is stuck in garbage may mean the video RAM is bad, or the system can not write to that area of memory, or perhaps something is preventing a proper refresh of the CRT screen.

But the fact that the screen is similar even if the character generator ROM is removed is trying to tell us something. With it removed, you should get 2000 consistent characters on the screen not garbage. Perhaps the problem may be downstream of the character generator? What could be causing the video data stream to behave this way? Curiouser and curiouser.

Hang in there. We'll get to the bottom of this.
-Dave
 
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