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Random Characters Issue on a COMMOODRE PET 2001

MikeS said:
Waddya think, Dave; want to fly to Paris?
Just pick me up on the way.
Click to expand...

Mike,
Stéphane is doing really good with his first repair, isn't he? It's been a long time since I stayed up to 3:00 AM to fix a computer problem. He really wants to fix that PET.

What do you think about the 'green screen'? Maybe a thermal issue with the character generator ROM? That is a lot of pixels being put on the screen.

Have you ever found a PET with ALL the big chips bad? This is a first for me. He is lucky they are on sockets. Maybe a previous owner plugged them all in backwards?

I wonder what the deal is with the EPROM's? Why do they react differently than the ROM's? Maybe one of the big chips is loading down the bus?

This is an interesting PET problem.
 
dave_m said:
Mike,
Stéphane is doing really good with his first repair, isn't he? It's been a long time since I stayed up to 3:00 AM to fix a computer problem. He really wants to fix that PET.
Click to expand...
Absolutely; I think I vaguely recall a time when I was that motivated, but lately... ;-)

What do you think about the 'green screen'? Maybe a thermal issue with the character generator ROM? That is a lot of pixels being put on the screen.
Click to expand...
Probably not actual pixels; maybe just the monitor scanning at full brightness or loss of video.

Might just be a dirty/noisy brightness pot.

Have you ever found a PET with ALL the big chips bad? This is a first for me. He is lucky they are on sockets. Maybe a previous owner plugged them all in backwards?
Click to expand...
Looks like somebody definitely poked around in there before but at least there's no catastrophic damage (i.e. 12V on the Vcc lines); adds an extra challenge to the long-distance troubleshooting.

I wonder what the deal is with the EPROM's? Why do they react differently than the ROM's? Maybe one of the big chips is loading down the bus?
Click to expand...
Like I said, it wasn't clear; maybe your EPROMs were in there when it worked and it is just a bad ROM on top of everything else.

This is an interesting PET problem.
Click to expand...
Yeah, and we haven't even looked at those interesting accessory boards yet...

Very inspirational; I've got a couple that are spread over the so-called bench and a couple of related projects; this just may get me off my duff.
 
Hi,

At first try this morning, same symptom. With a little difference :
All I have now is a horizontal green line in the middle of the screen.

I have checked the video cable, the plugs, it's all good.

Could it be an isue with the power supply ?

Many thanks again for your great support and kindness (and patience with me ;-))

Stéphane.
 
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sbo said:
Hi,
All I have now is a horizontal green line in the middle of the screen.
Stéphane.
Click to expand...


just one line in the middle and nothing else? I had this too with some of my PETs: the video/crt board was dead! This happened to me several times.. I could fix one board when I soldered a new capacitor onto the board (the biggest capacitor on the board)...
 
Hi Alker,

It might be the same issue !
I could get acces thanks to another PET owner here in France to a CBM 2001.

We tried to plug MY mainboard to HIS PET :
Same symptom ; so I guess we can tell that the issue comes from the mainboard itself ...

So you mean the big capacitor near the power plug on the mainboard ?

Thank you.
 
For the record, I just pulled out three 2001/3000 series motherboards from the basement. I will apply a round of chip lifting this afternoon and see if they work OK. I have two potential takers in form of Stéphane and a fellow from Italy with whom I've had PM exchange. His PET shows similar symptoms to yours, except that swapping 6502 etc from a floppy drive made absolutely no difference.
 
Now I have made an inventory of my last PET items to "get rid of". To start with, consider this photograph:


Both these boards boot into Basic with 32K free. I successfully ran this short test program:

5 FOR I=1 TO 10
10 PRINT"HELLO"
20 NEXT

and PRINT FRE(0) returns amount of free memory as expected.

As you see, neither board includes the four big chips: 6502, 6522, two 6520. Any taker will need their own supply of those chips. The ones I used are known good, so yours better be too.

The board to the right also is missing a couple of ROM chips: 901465-03, 901447-24, 901465-23. I have a broken 901465-03 that yields the infamous screen of garbage characters but it no good to you. Those 2532 EPROMs are good to have, perhaps some of the taker's existing ROM chips turn out to be good as well.

Apart from those two boards, I have a third one that boots up with broken garbage no matter which ROM set and big chips I plug in. Obviously there is more than one way to kill a PET. I didn't bother to troubleshoot it further, but it is possible that e.g. some 2114 RAM chips still could be good and worth desoldering if anyone doesn't have a source for those. This board thus completely lacks socketed chips.

My fourth board is a 4000/8000 series board with a few big chips soldered to it. I tested it previously and found it didn't boot as it should. I don't remember exactly what was the fault, but possibly some of the soldered chips need to be replaced. It is out of scope for now, but I thought I'd mention it here as well.

Finally I have a loose motherboard from a 3040 floppy drive, DOS version 1. I have reused 6502 and 6522, but it still has the RIOT and ROM associated with the drive. Otherwise unknown functionality, I'm afraid.

As for other items, I have a few spare keyboard mechanisms, both 3000/4000 and 8000 series. I have promised to send one to a gentleman in Germany (I will get back to your PM shortly) but have a few more to spare. Of course I also have a bunch of IEEE cables of different kinds, some books in Swedish and a few other items, but not much worth mentioning.

If sbo wants to buy a motherboard from me instead of trying to troubleshoot the one he already got, you can send me a PM to discuss the details. As you got a set of fresh EPROMs, theoretically you should be fine with the board that lacks most chips.

Edit: The total costs including shipping and payment fees will be 34 Euros within Europe. That consists of PET board 4 Euros, postage 28.50 Euros and fees 1.50 Euros...
 
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hi, no I meant the bigger capacitor in the CRT board back inside the monitor.. but when you checked with a second board and the motherboard is the problem than there is another problem. Did you test your friends pet board in your pet? does it display the start message? so you'd be sure that the power supply, crt and monitor are working.
 
sbo said:
We tried to plug MY mainboard to HIS PET :
Same symptom ; so I guess we can tell that the issue comes from the mainboard itself ...
Click to expand...

Stephane,
Your mainboard seems to be developing more intermittent problems. You may need an oscilloscope to check out the video control problems.

However there is one signal called 'Video On' that is connected to both the 6520 and the 6522 so there is a small possibility that the new chips will correct that problem also.

But it may be time to consider a replacement mainboard.
-Dave
 
@Anders : Many thanks for your offer! I have just sent you a private message to confirm my interest for the first mainboard.
@Mike & dave : This is what I am going to do :
1) I will wait to receive the 2x 6520 ships and then I will pursuit my investigations on my actual mainboard; (I start to appreciate this little game and I would be proud to finally manage to fix it ;-))
2) In the meantime, I will give a try to Anders' mainboard. This will help for the investigations.

Really guys if I manage to get my PET back to life this will definitely be thanks to all of you !

Thanks again.

Stéphane.
 
sbo said:
@Mike & dave : This is what I am going to do :
1) I will wait to receive the 2x 6520 ships and then I will pursuit my investigations on my actual mainboard; (I start to appreciate this little game and I would be proud to finally manage to fix it ;-))
2) In the meantime, I will give a try to Anders' mainboard. This will help for the investigations.
Click to expand...

Stéphane,
This is a good plan of action. This is the order I would try:

1. Gather all the needed chips, CPU, 6520 (2), 6522 and ROM/EPROM.
2. Populate first in the new mainboard from Anders. This is prudent because it is a tested good board and will minimize unknown conditions.
3. After getting a set of chips to work, use that set in the original mainboard. If it works, you are done. If it does not, you know there are other problems with your mainboard and can proceed to troubleshoot the new problem if you wish.

-Dave
 
Yes, I've found diagnosing faults is SOOO much easier if you have a working unit for comparison.

Makes me think I should get duplicates of all my classic machines, just for diagnostic purposes! :)

Tez
 
Haha! I just realized Basic 2 consists of just four chips, so the 901465-23 (or 901465-19) should not be there at all. Instead I found a custom Basic extention, (C) 1979 PAICS located at $B000 on one of the two loose boards. Time to dump, analyze and upload.
 
Yes, I think I may already have the instructions elsewhere. I have a small bunch of ROMs, disk images and docs to complete and upload to Zimmers. I just need to go through what may already be uploaded.
 
carlsson said:
Haha! I just realized Basic 2 consists of just four chips, so the 901465-23 (or 901465-19) should not be there at all. Instead I found a custom Basic extention, (C) 1979 PAICS located at $B000 on one of the two loose boards. Time to dump, analyze and upload.
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Tsk, tsk; you're not following this thread very carefully - see posts # 5 and 10 ;-)

I've got a few accessory ROMs and docs as well, but last time I looked I think they were all already out there. I just wanted to double-check but, as happens more and more often around here, although I had that box in my hands yesterday today I can't find it anywhere... gotta get organized... ;-)
 
Stephane: as you may have read in another thread, we've just realized that on that particular model a 2532 will not work in the Exxx (D8 ) socket, so your original ROMs may in fact be perfectly OK. Try replacing the other three with Dave's EPROMs and leave your original 901447-24 in D8.
 
OK back to work ;-)

@Mike : Thank you for your explanation ; I must say I had totally missed the point.
I am going to give it a try tonight and I will let you know the result.

Thanks. Stéphane.
 
sbo said:
OK back to work ;-)

@Mike : Thank you for your explanation ; I must say I had totally missed the point.
I am going to give it a try tonight and I will let you know the result.

Thanks. Stéphane.
Click to expand...
It may not fix the problem, but if the symptoms are exactly the same with your ROMs and Dave's (except D8 ) then your ROMs are probably OK.
 
dave_m said:
Stéphane,
This is a good plan of action. This is the order I would try:

1. Gather all the needed chips, CPU, 6520 (2), 6522 and ROM/EPROM.
2. Populate first in the new mainboard from Anders. This is prudent because it is a tested good board and will minimize unknown conditions.
3. After getting a set of chips to work, use that set in the original mainboard. If it works, you are done. If it does not, you know there are other problems with your mainboard and can proceed to troubleshoot the new problem if you wish.

-Dave
Click to expand...

OK Dave ! I will follow the plan ; while waiting for the ships, I have in project to restore the metallic case of my PET ; following a previous post, I plan to give it a better external aspect.
(This will help me wait during the debugging ;-) )

On that part, I have a lot to do to get it pretty ;-).

@Mike : OK ; understood for the ROMs testing.

Stéphane.
 
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