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Pet problem

Gary C

Veteran Member
Joined
May 26, 2018
Messages
2,258
Location
Lancashire, UK
I was going to post needing help to fix my PET but at the last second it failed in such a way that I could diagnose what appears to be the fault.

Thought I would still post it as it may help others


So, its a 2001-N dynamic PET and it is crashing

Using PET tester V4 it runs and throws errors at various times.

Often it stops at the second (page 0 & 1) tests showing ggggg..... for the first 256 bytes then bbbbbb....... for the second

Sometimes it runs through to the ROM test and the countdown starts. Often it will then crash but sometimes the ROM checksums change at random.

That was the symptoms, but then I managed to get it to cycle on the first memory test and noted that I had no CAS0 signal even though it was clearly trying again and again to test the memory. Following CAS back to UG7, the signal from the MUX UH4 was missing and from there the Gate to pin 15 of UH4 was missing. Back through UH5, bank select was also dead and back even further the address line BA15 to UB2 was dead. Back even further, it was obvious that while AB15 was active from the CPU, BA15 wasn't coming though the buffer UB3 !

Swapped UB3 and now it works.

well, it runs but is occasionally showing a RAM error during the MARCH C test but thats probably a simple chip swap.

Fantastic tool is PETTESTER, cheers :)
 
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