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Display problems on a Commodore Pet 8032

M

mrmacca

Experienced Member
Joined
Mar 17, 2010
Messages
53
Location
Middle England
Hi All,


This is my first post on the Vintage Computer Forum and I could do with some help or advice. Could you help, (or forward this message on to anyone who could help) diagnose my Commodore Pet 8032 display issue.

I switched it on today for the first time in a while, after the switch on beep I typed in and ran a simple program. After ten seconds random characters appeared.

This got worse resulting in permanently illuminated character positions in the whole screen with two green lines. This fills all rows and characters.

||C||o||m||m||o||d||o||r||e||||B||a||s||i||c||||4||.0||||||||||||||||||||||||||||||||||||
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
||R||e||a||d||y|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||

Without taking a picture this is the best pictorial way I can describe it. :eh:


You can still make out the program running amongst the unwanted illumination but the display now is awful making the Pet unusable really. Shame really the board is from 1980 and I have the top off. Someone’s suggested re seating the character rom but I have no idea which chip it is?

I’d love to fix this computer and I work at a University so any ideas of rectification, how ever complicated I am sure we have the technology to follow or I could get someone at work to interpret anything you suggest if I dont understand ;)

Many thanks for your time, :D

Kind Regards

Alasdair Macdonald
Staffordshire University, UK.
maccatakker69@googlemail.com
 
Do the vertical bars run through the other characters too? That would suggest a stuck bit on the character ROM and reseating may indeed help. From what I can make out, the character ROM is at position UA3 (i.e. the third chip on row "A", which should be marked on the board).
 
My guess is that these are flakey display RAM chips... the 8032 uses 2114 for display RAM, and they are notorious for going bad in a variety of different machines. If they are socketed you can try swapping them out and see if the symptoms change. Should be a couple of them at UC4 and UC5 (per schematics).
 
Hi Guys,

Many thanks for both your replies. I shall get together with a colleague at work hopefully today and see if we can resolve. He used to work on them and being older than me is way more knowledgeable.

Just to clarify, the screen worked fine for only ten seconds then permenantly stayed illuminated incorrectly as above, the bars run the entire length and height filling the screen.

I could take some pictures today hopefully.

Thanks, Alasdair.
 
Alasdair,
I don't think the video RAM can cause these types of vertical lines so the problem must be somewhere from the character generator out to the analog video board. The character generator drives a serial shift register that creates the dot patterns that will have to be checked. Do you have an oscilloscope?
-Dave
 
Gosh we do have an ossiloscope but as you can see I can't even spell it let alone use it. There are plenty of people here that can luckily.

I have taken some pictures this morning so I can show you exactly what I am talking about.
Am I allowed to upload them to here or I could use photbucket links perhaps? :confused:

The board is an has a serial number of 803425 80 Column CPU 8032080 - 1980 - Made in Japan

I can see UDA3 it says MOS on top - I may try removing and re seating this then?
UDA4 and UDA5 are soldered in. A little more tricky to do anything with I fear ?

Thanks for all your replies, really appreciated. Alasdair in the UK>
 
geoffm3 said:
My guess is that these are flakey display RAM chips... the 8032 uses 2114 for display RAM, and they are notorious for going bad in a variety of different machines. If they are socketed you can try swapping them out and see if the symptoms change. Should be a couple of them at UC4 and UC5 (per schematics).
Click to expand...

UC4 and UC5 are soldered in smaller chips Geoff :(
 
mrmacca said:
I have taken some pictures this morning so I can show you exactly what I am talking about.
Am I allowed to upload them to here or I could use photbucket links perhaps? :confused:
Click to expand...
Yes, you can upload pictures as attachments to your post (you will need to "Go Advanced" if you're doing a "Quick Reply" and click on the paper clip icon). Linking to an external image is also fine (Click on the picture frame/tree icon).

I can see UDA3 it says MOS on top - I may try removing and re seating this then?
Click to expand...
Most of the big chips will probably say MOS on them. Does your board look like this one? If so I'd suspect the character ROM is the one on its own in the bottom left corner. You don't necessarily need to remove it completely to reseat it, just enough to loosen it, then push it back home.

If that doesn't help, try putting the scope (actually a simple volt meter would do) on the data pins (9-11 and 13-17) of the ROM to see if any are stuck high.

UDA4 and UDA5 are soldered in. A little more tricky to do anything with I fear ?
Click to expand...
According to these schematics, the video RAM is UC4 and UC5, but it wouldn't surprise me if they were soldered in. I wouldn't worry about that just yet.
 
Well I removed and reseated UA3 to no avail. Still the same. :tellme:

I think the next move is to up load my pictures. I do have a similar board to the one linked, however it looks less complex and there is no expansion cards in it. There is the expansion card sitting next to the machine (not connected to the main board) and I can't remember why it is out? I presume this is unrelated info.

I think pictures may help with the diagnosis. Bear with, thank you, and don't give up on me. :)
 
Last edited:
Hi Guys I have some pics and thought if I may upload a couple?

Note that the pic makes it looks like I have simply increased the brightness on the rear of the screen, I haven't it is unwanted lines which dont go away.
Here goes :

pet1.jpg


board1.jpg
 
Interesting problem. I sat down with pen and paper and figure that to display the text properly then every bit lane of the video ram should be correct, so my original idea is probably not right. The only character that seems wrong is the space. Have you tried inputting in a bunch of text from the keyboard to see if there are any other characters that are incorrect? Might be worth writing a quick BASIC program to output all the different PETSCII characters and see if there are any others that are not right.
 
I can try tommorow, the pets at work and I'm at home. I presume all the keys worked before and after.

Can you see what I mean about lines now.

|||||||||||||||U|P|S|E|T|T|I|N|G|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||


:whaasup:
 
One can only hope. :)

Is there a connector on the back of that board, or does it just have the SIP connector coming off the side?
 
On that little board which has a home made switch attached to it it looks like it should connect into the top of the board. It wont go in the back of the pet (like the old zx81ram pack style) :)

I have a whole inventory of the history of computers here at the uni. We have all the gear but no idea lol :)
 
With perhaps a little help from members of this forum, you will get this fine old computer repaired. I have an 8032 also and can help with comparison waveforms later if needed.

There is a lot of good information on the web about the PET computers. Here are two sites but remember that there are many different models of PETs. Restrict your research to the 8000 series computer with BASIC 4.

http://zimmers.net/anonftp/pub/cbm/pet/index.html

http://6502.org/users/andre/petindex/index.html
 
Yes, the little expansion board is a ROM switcher which fits in a socket on top of the board. I don't think it has anything to do with your problem though. It would be an interesting thing to try out when the computer is otherwise working properly.

One thing to try would be replacing the character ROM with another with the same pinout (one of those on the ROM switcher may fit, but check it's the same pinout first). You will likely get utter garbage on the screen, but if the lines remain it's likely that UA2 (the shift register Dave mentioned) is bad. If the lines disappear, it's probably the character ROM itself. If you can confirm that's the case, I may even have an original replacement ROM for you.
 
Looks like a fine machine. Certainly worth putting some effort into getting it going properly.

Tez
 
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