CBM3032 garbage screen (yes, another one...)

[Hugo Holden]

Before to post here, I read that some PET sockets have bad contacts and I set out to change all the sockets beginning by the UC7 one. The result was a disaster and I had to restore the tracks I broke.


Thank you

One thing about a lot of these vintage IC sockets, that you might not know about: In many cases the plastic shoud will lift off the socket claws that are soldered to the pcb. You need to lever the shroud off, but you don't want a metal tool like a screwdriver to engage the surface of the pcb. So it is best to use some cardboard between the flat blade screwdriver and the pcb surface, so that the pcb is not damaged or marked. The thing you want to protect is the pcb, not the sockets or IC's.

Then you can remove the IC socket pins one by one and there is a lot less trauma to the pcb.

Most vintage sockets are like this, though modern dual wipe and machine pin ones are not.

For modern dual wipe socket removal you can remove the pins one by one by soldering a pull wire to the top,then heat the pcb pad and pin from below, and you can take them out one by one that way. But you don't pull on them until the solder is fully melted.

Machine pin sockets are another problem, they do not lend themselves to one by one pin removal, which is why I normally use dual wipe sockets on vintage computer pcbs, just in case they need to be removed one day.

 

[Papalapa]

Well, today it has been a hard day:

I started programming an EPROM to replace UD7 an indeed, the PETTESTER checksums were correct this time so, UD7 was damaged.



Then I removed the PETTESTER EPROM and restored the original EDIT ROM and I got a garbage screen again.

I programmed a new EDIT EPROM and this time finally I got the startup screen so, EDIT ROM was also damaged.

I set the 6520 and 6522 in its sockets and I get also the startup screen but with a character below the "READY" and the keyboard was not working. If I remove UC7 the symbol is not shown but, of course, the keyboard is not working. I have swapped the two 6520 ICs with the same result.



I suspect that the bad job that I have done trying to unsolder the socket damaged some track. As Dave advised, I'll get the multimeter, the diagram and check continuity for the UC7 pins one by one. It's a shame because if it hadn't damaged the tracks, the PET would probably be working now

(Will continue...)

 

[daver2]

Well done for getting this far.

I would put the PETTESTER ROM back in. The 'kbd' values should all indicate 00 instead of f4.

I am just going out now, but we should be able to look at the signals on the PIA. There are PCB tracks on the top of the board (component side). I assume you will make sure these are intact?

You will probably find that the 'rogue' character that is displayed after the READY prompt is what character the firmware thinks has been pressed on the keyboard corresponding to the erroneous 'f4' data that is being returned.

Dave

 

[Papalapa]

It's alive!! It's alive!!



After checking continuity on the 40 PIA pins I have found 7 of them defective. I have had to solder more cables (botched job) but finally the PET is perfectly working . I want to say "thank you" to Dave and Hugo because without his help and patience would not have been possible to repair the PET.

Next steps are more exciting: Replace BASIC 2.0 by BASIC 4.0 and install a cooper band around the transformer to minimize the screen wobbbling.

 

[daver2]

Well done!

Pleasure to help get another PET alive.

Don't forget to download and run the IEEE488 test code from the PET primrose book. Page 181 of http://www.primrosebank.net/computers/pet/documents/PET_and_the_IEEE488_Bus_text.pdf.

Dave