More PET 2001-8 fun

[falter]

One nice thing about having this 4016 is I don't mind messing around with it. So I borrowed its screen for my 2001-8, which has never worked. I know for sure the -8's screen isn't working -- but I figured I'd play with the ROMs/RAMs again with the 4016's screen connected to see if I got anywhere.

It appears the machine doesn't acknowledge the ROMs at all -- if I pull all of them out and power it on, I get exactly the same garbage screen I get if they're all in. Is there a particular order the machine addresses ROMs, so I could try and nail down which may be the culprit?

I ordered the PETVet... if I've read correctly, that'll let me choose specific ROMs to isolate so I can nail down which is my troublemaker?

 

[MikeS]

...It appears the machine doesn't acknowledge the ROMs at all -- if I pull all of them out and power it on, I get exactly the same garbage screen I get if they're all in. Is there a particular order the machine addresses ROMs, so I could try and nail down which may be the culprit?

No, it doesn't work that way, alas. The garbage screen just means that the computer is not running its kernel firmware correctly and not talking to the display, which could be caused by any number of problems; since it's not talking to the display it can't really tell you much.

I ordered the PETVet... if I've read correctly, that'll let me choose specific ROMs to isolate so I can nail down which is my troublemaker?

Yup.

I assume the ROMs in question are 6540s? You could make an adapter to read them in another working PET or similar computer, but that's not exactly trivial.

 

[falter]

No, it doesn't work that way, alas. The garbage screen just means that the computer is not running its kernel firmware correctly and not talking to the display, which could be caused by any number of problems; since it's not talking to the display it can't really tell you much.

Drat. But it *could* be a/the ROM(s) right? If it were something other than RAM & ROMs, could the PETVet still get around it anyway?

Yup.

I assume the ROMs in question are 6540s? You could make an adapter to read them in another working PET or similar computer, but that's not exactly trivial.

Yes in the -8 they are all 6540s. I had noticed upon changing the RAM chips around (rotating different ones into the video memory) that every now and again I could produce a screen that changed or was changing. But it never got much further than maybe 1-screen of garbage switching over to a second screen of garbage. I figure all the memory chips can't be bad, so that's what has me thinking it's ROMs.

Now if only I had the skill to repair the monitor's video board.. because even if I get the computer itself working.. I'm still deadended by a bad screen.

 

[dave_m]

If it were something other than RAM & ROMs, could the PETVet still get around it anyway?

There's a very good chance the program is not running due to a bad ROM, however if it is a bad I/O chip, bad video RAM or something like a shorted address line, PETVet will not fix it. But with known good RAM and ROM, it is a lot easier to fix whatever else may be wrong.

Now if only I had the skill to repair the monitor's video board.. because even if I get the computer itself working.. I'm still deadended by a bad screen.

Look for an old time TV repair guy. Since the video board schematic is on line, he might be able to fix it.

 

[dave_m]

I ordered the PETVet... if I've read correctly, that'll let me choose specific ROMs to isolate so I can nail down which is my troublemaker?

Well, not out of the box. It come preprogramed with three selectable PET ROM options:

1. BASIC 2 40 columns with no CRT Controller (for PET 2001, PET 2001N and 3000 series)
2. BASIC 4 40 columns with no CRT Controller (for PET 2001, PET 2001N and 3000 series)
3. BASIC 4 80 columns with CRTC (PET 8032)

All ROM and 32K RAM space will be mapped to the PETVet.

To change configurations, it is a matter of editing a php script file (called convertroms.php) to say map the all ROM space to the PETVet except the ROM you want to test on the PET and then assemble a new petvetupdate.bin file to download to the PETVet module. You can use PETVet's built-in boot loader program using RS-232 and a PC terminal program or if you are familiar with AVR controller chips, any AVR programmer that uses the standard ISP interface.

I could help with the needed updates files but it would be up to you to get the download to work. I have done it using an Atmel programmer on a blank ATMEGA644A chip (did not want to risk stepping on the working chip on the PETVet until I learned how to do it properly).

 

[MikeS]

Drat. But it *could* be a/the ROM(s) right?

Absolutely; bad ROMs, bad RAM and poor sockets are by far the most common issues with the old PETs.

You don't say where you are; any chance there's someone nearby with a similar machine? The Toronto PET User's Group (TPUG) just put on their annual World of Commodore show this weekend and I helped someone there find the problems with his 2001-8 PET; it sure simplified matters having spare known-good ROM and RAM chips - turned out to be 3 bad ROMs and several bad RAMs.

ISTR that an adapter to read 6540s in an EPROM programmer was discussed somewhere not long ago although I'll be darned if I can remember where; the issue (aside from pinout differences) is that the 6540s need a clock signal, so they can't be read like a 'normal' SRAM/(E)(P)ROM. Many people with 2001s using 6540s also have a programmer or even another CBM/PET with a spare socket using the 'standard' 2316/2332, so it might be worth revisiting even with the availability of other options today.

That's what I had to do waaaay back when I only had one 2001-8 with a bad ROM and an AIM-65 that I could use to read the PET ROMs; even still have the rat's nest in question:

 

[falter]

Absolutely; bad ROMs, bad RAM and poor sockets are by far the most common issues with the old PETs.

You don't say where you are; any chance there's someone nearby with a similar machine? The Toronto PET User's Group (TPUG) just put on their annual World of Commodore show this weekend and I helped someone there find the problems with his 2001-8 PET; it sure simplified matters having spare known-good ROM and RAM chips - turned out to be 3 bad ROMs and several bad RAMs.

ISTR that an adapter to read 6540s in an EPROM programmer was discussed somewhere not long ago although I'll be darned if I can remember where; the issue (aside from pinout differences) is that the 6540s need a clock signal, so they can't be read like a 'normal' SRAM/(E)(P)ROM. Many people with 2001s using 6540s also have a programmer or even another CBM/PET with a spare socket using the 'standard' 2316/2332, so it might be worth revisiting even with the availability of other options today.

That's what I had to do waaaay back when I only had one 2001-8 with a bad ROM and an AIM-65 that I could use to read the PET ROMs; even still have the rat's nest in question:

View attachment 16270

Wow. Yeah I have a lot to learn here. The only part of this that really intimidates me is the monitor, and that's just fear of getting zapped or blowing it up. I'm north of Vancouver, BC -- don't know a lot of PET people or vintage computer people around here unfortunately. But I'm hopeful the PETVet will at least tell me if my problems are relatively simple (ROM, socket, etc) or something more complicated elsewhere on the board. But even then, I still have to find someone to repair the monitor board. Have to sweet talk the local TV guy in to taking it on.

 

[MikeS]

Wow. Yeah I have a lot to learn here. The only part of this that really intimidates me is the monitor, and that's just fear of getting zapped or blowing it up. I'm north of Vancouver, BC -- don't know a lot of PET people or vintage computer people around here unfortunately. But I'm hopeful the PETVet will at least tell me if my problems are relatively simple (ROM, socket, etc) or something more complicated elsewhere on the board. But even then, I still have to find someone to repair the monitor board. Have to sweet talk the local TV guy in to taking it on.

Yeah; hopefully with Dave's help the PETVet will find your problem; I've got one as well but have not had occasion to use it yet.

Monitors, especially a small monochrome like yours, aren't really quite as dangerous or mysterious as they are reputed to be, but unless it's something really obvious like a bad power issue or visibly blown component you do really need a 'scope.