Random Characters Issue on a COMMOODRE PET 2001

[sbo]

Hi,

I am trying to to restore a PET 2001, the one with the real keybaord.

My problem is that when I trun it on, it keeps displaying always the same (apparently) random characters on screen :

See the pictures for reference :

Any help appreciated. :wink:

Thank you.

Stéphane.

 

[cosam]

Well, you're certainly not the only one to have this problem - have a browse through the forum and you'll find several threads on this exact problem. We have some good PET guys here so most of them have a happy end!

There are many possible causes, but bad ROMs seems to be among the most common. Reseating them (and any other socketed chips, while you're there) is a good start, but ideally you'd want to dump them and check against known-good images. Other causes would be bad RAM, CPU, etc... the list goes on. A chips that's (very) hot to the touch a few of minutes after powering up is one of the tell-tale signs of stuff like that. It's probably not a power supply issue, but it's worth checking the voltages as a matter of course.

If you start poking around, there's a lot of useful info here.

 

[sbo]

Still garbage on sceen

Still garbage on sceen

Hi Cosam,

Thank you for your kind reply.

Unfortunately, I have tried some of the tips I could find on this forum already, like :

- Play with the socketed chips (cleaning)
- Try to identify tricky chips
- Check the power supply

It seems to be all good.

So I have poor hope that I will be able to get my PET come back to life ...
I should try to find a new motherboard or new chips I suppose ...

What do you think ?

 

[sbo]

A few points I would like to mention :

1. The garbage dos not change unless I remove and replug the chips 2114. If I do not touch anything, the screen shows exactly the same data at every boot.

2. All MOS chips are hot while computer is ON ; I could not determine if one was more hot than the others.

3. Motherboard is very clean ; all components seem in very good condition.

3. I received the computer with a mods on the power supply : the ON/OFF button had been doubled with another one on the side of the left computer ; probably to make it easier for the user to turn it ON/OFF. I have canceled that system.

4. I also had a DD DSK board on top of the motherboard ; I guess this is a Disk Driver Controler ; right ? It was half plugged in ; I took it off for the testing.

5. Finally, there was kind of a RESET button with a cable linking to a socket. It wasn't installed on the mainboard, but may have been plugged in the past ...

I hope this will help.

Stephane.

 

[sbo]

One last point ; I have noticed on this picture from a CBM 2001 that there seem to have an EEPROM I do not have on my PET 2001 :

Could it be missing on my 2001 or is it just a different motherboard ? (They look very similar to me on the picture...)

Thank you.

 

[carlsson]

Your PET 2001N should not need an extra floppy disk controller. It would communicate with e.g. a 2040 drive via the IEEE bus. Perhaps the extra board is for some custom, dumb floppy drive.

As for the extra chips, those are "expansion slots" so quite possibly yours is not meant to have them populated.

Good luck with trouble shooting. I can't promise, but I might have one spare 2001/3000 series board that might be in working condition, minus a few of the big chips that you would need to move over. PM me if you get stuck and want to discuss details in obtaining a such board if I still have it.

 

[sbo]

Thank you Carlsson,

I'd be more than interested in such a MB as i have tried almost everything I could to get mine work, without success. (I'll get in touch as you kindly offered me too in PM).

Just to give it a last chance, this is the board that was set on when I received the PET :

With this card on it :

I do not htink this is connected with my issue. But before I give up, I prefer to be as exhaustive as possible.

Stephane.

 

[carlsson]

Correct me if I'm wrong, but that board looks very similar to those Think-something 64K memory expansions for the PET 8032. Perhaps this also is a memory expansion, but smaller amount of memory? The small cartridge looking board would be a "personality" card with some custom chips, yet not identified exactly what they contain or do.

 

[sbo]

You must be right.
But I couldn't tell ; for now, I'm still stuck with those random characters sticking on screen.:crazy:

I think I've tried everything but nothing had any effect.

I think the PET does not even boot up.
It reezes immediatly showing those characters...

Thanks.

Sté^phane.

 

[Moonferret]

Stephane, I have a PET in pretty much the same state as yours. When you power up the machine, does the contents of the screen change at all after a second or two? If so, I believe you might be seeing the machine try to initialise the display and print the welcome message. This would indicate that the display RAM may be at fault.

No change at all would probably point to a fault elsewhere, preventing the machine getting to the point of screen initialisation.

 

[MikeS]

One last point ; I have noticed on this picture from a CBM 2001 that there seem to have an EEPROM I do not have on my PET 2001 :

Could it be missing on my 2001 or is it just a different motherboard ? (They look very similar to me on the picture...)

Thank you.

Same board, different version of BASIC; version 4 used one more ROM (D5, Bxxx).

 

[MikeS]

You must be right.
But I couldn't tell ; for now, I'm still stuck with those random characters sticking on screen.:crazy:

I think I've tried everything but nothing had any effect.

I think the PET does not even boot up.
It reezes immediatly showing those characters...

Thanks.

Sté^phane.

Could be tricky; sometimes an add-on board required some minor modifications to the main board and then it would not work properly if the add-on board had a problem or was missing altogether.

Where are you? Maybe someone close with some good test equipment could have a look at it for you.

 

[barythrin]

You don't happen to have another 6502 processor laying around do you? From the little amount of troubleshooting I've watched folks do on PETs the processor clears the memory then reads the ROM to load basic. It could be turning on and the processor is never doing it's part. Alternatively you could check the voltage pins on the processor to make sure it's getting the rights volts it needs to. Hmm.. well I thought of course 6502.org would have the pinouts but dang if I'm too tired to find them. Here's some troubleshooting tips for the PET, Here's a nice blown up version of the 6502 pinouts

 

[sbo]

Hi Moonferret,

In my case, there is absolutely no change at all. The only way to notice changes is to unseat the 2114 chips (two) and then seat them again. Then characters appear in a different manner, but still no improvement.
So I think I am in the second case, where the machine does not even initialise.

Thanks.

Stéphane.

 

[sbo]

Thank you guys for your kind answers.
I was thinking again about tiny modifications that could have been brought to the MotherBoard to adapt the optional card.

So just to be sure, I post theese pictures so you may be able to confirm me that you have the same mods on your own MB ; that could help a lot !

Thank you for your support

1) I have those shortcuts on the reverse side of the mainboard ; I have tried the empiric way by taking off those small wires. As a result, I had nothing on screen. anymore. So I soldered them back : random characters are back. :sad:

This is a detail of the chips on the mainboard :

How could I preventively fix any issue that could be linked with a power issue ? Which components might need to be replaced ?

Stéphane.

 

[sbo]

Meet my PET

Meet my PET

This is the mysterious card from closer :

 

[Moonferret]

That expansion board looks like it has some extra memory. What are the markings on the gold chips? 4116? Also, the markings on the large 40pin chip might identify what the board is for.

From what I can see in the photos, it looks like the expansion board is just using the existing expansion headers so I'd guess that there are no mods on the mainboard. I'd be tempted to remove the expansion board for now and try to get the machine working as standard spec. If the gold topped chips are 4116's it might be worth pulling one them and piggybacking it onto the RAM chips on the main board to see if you can pinpoint a duff chip. This is the method I've just used to find & repair the fault on my second PET.

 

[sbo]

Hi Moonferret,

Gold topped chips : 16pins / MOSTEK 7928 MK4115P 41

Sorry for this newbie question, but what "piggybacking" means ?

I have removed the extension board : same diagnostic with OR without it.

All is written on the 40 pins is "B1".

 

[sbo]

I suspect the guy who sold me the computer may have played already with the chips.
Do they need to be organized in a specific way ? If so, could you confirm that this is correct on my mainboard ? I have :

^^ BACK ^^

MPS 6520 / MPC6821P / MPS 6522 /MPS 6502

Is this correct ? Is there a specific order by the way ?

MPS901447 / MPS901465/ MPS901465 / MPS901465

MPS901447

Thanks for supporting.

Stéphane.

 

[dave_m]

Do they need to be organized in a specific way ? If so, could you confirm that this is correct on my mainboard ? I have :

^^ BACK ^^

MPS 6520 / MPC6821P / MPS 6522 /MPS 6502

This is correct except that a 6821 PIA was substituted for a 6520 PIA which should be OK.

MPS901447 / MPS901465/ MPS901465 / MPS901465 MPS901447

the dash number is an important part of the part number for these ROMs. Can you list them?