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PET 4032 restoration

Right, back to the memory fault.

I've written a short proggie that pokes a random number into upper RAM at consecutive addresses. If on PEEK I get a different value, I conclude the RAM has a problem. However, I'm looking for patterns, so I wrote a short "int to binary" subroutine and ran it.

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Looks like bit 6 is misbehaving. On he left hand column is the random byte as POKEd, and on the right hand side is the PEEKed value.

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According to this lovely schematic, upper RAM bit 6 is the 4116 DRAM at UA6.
 
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..and here it is..

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It's a MOSTEK IC, like the original, but probably newer. Anyway, my Mickey Mouse memory test has detected no problems. But does the PETDisk work now?


Nope, still getting a "device not ready" error. :(
 
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So.. back to the IEEE interface. KC9UDX suggests the VIA may be in trouble, but I didn''t see how it is connected to the IEEE interface. Then I noticed a few lines such as /NDAC IN that are connected to the VIA shown here.

Drat. Now I have to remove another huge chip for testing.
 
OK... I pulled the VIA and fitted a socket, then populated it with another VIA from a BBC Micro. I am still getting the "device not found" error on my PETdisk.

I have also reloaded the PETdisk firmware, just in case the Atmel chip has got confused. I guess it loaded OK, because the BIN file was wiped from the card straight afterwards.

Interestingly, when I powered up, I didn't get a chirp either, but there is the usual amount of RAM free and the keyboard is fine. But neither tape interface is powering the drive now. I think I may have fried a transistor - Q1 and / or Q2 - by accidentally earthing one of its pins (the brass post nearest to cassette port 1 is the most likely culprit), because neither port will start the motor up; or maybe the 9v supply is out.

:(
 
Checked 9v, all good. Traced the circuit to a burned out track that I'd already repaired once. I think this explained why it was burned out in the first place...

So, I re-repaired it and the tape interfaces both look OK. Phew.

Now I am back to the IEEE-488 interface. It seems to be dead. I have replaced all the major chips on the board as well as the line transceivers. Not sure what next?
 
JonB said:
Now I am back to the IEEE-488 interface. It seems to be dead. I have replaced all the major chips on the board as well as the line transceivers. Not sure what next?
Click to expand...

If you can load programs from the cassette, try this BASIC program. It checks the individual IEEE control lines (DAV, NRFD etc.) and verifies that they are seen at the receivers. Have nothing attached to the IEEE connector. If it catches an error, it will STOP and wait for you to either scope the signal or issue a CONTINUE to the next test. I try to find the program to test the data lines.

ieeetest.prg
 
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JonB said:
Checked 9v, all good. Traced the circuit to a burned out track that I'd already repaired once. I think this explained why it was burned out in the first place...

So, I re-repaired it and the tape interfaces both look OK. Phew.

Now I am back to the IEEE-488 interface. It seems to be dead. I have replaced all the major chips on the board as well as the line transceivers. Not sure what next?
Click to expand...

Sounds like you need a fuse!
 
It shorts on the mounting post, so I insulated the PCB at that point. My fault, I'm running it without screwing the board down, so it moves about a little (enough to earth the unregulated 9v line).

Any advice on troubleshooting the IEEE?
 
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