CBM 610 troubleshooting

Hello,

In October 2012, my Swedish localized CBM 610 (upgraded to 256K RAM) 
suddenly died in the middle of operation. When I power it on, the LED lights 
but I don't get a video signal. Any IEEE drive connected to the bus keeps 
spinning forever. In case it'd boot to Basic anyway, I tried CTRL+G to check 
for any bell sound but nothing.

While I don't have much of skills with troubleshooting, at least I have some 
spare parts and tools. Yesterday I took it apart and found that the Kernel 
EPROM adapter had a leg partly missing and replaced with solder. I found a 
proper 2764 -> 2364 adapter and replaced it, but no difference. All three 
EPROMs (Kernel, Basic High & Low) read out fine with my EPROM programmer and 
match the already dumped binaries.

The next step was to look at the PLA. On this machine the original chip was 
purposedly (?) misaligned by one pin, meaning the top pins are in the 
position of the second row of pins and the lowest pins not connected. I 
don't know if this was due to the RAM upgrade but the machine used to work 
fine like this. In any case, I plugged in my SuperPLA Multi configured for a 
610, but no difference. I never tried the 710 setting.

Out of the other chips on the motherboard that are socketed, remains the 
character EPROM, 6509 and 6526. I happen to have a working 710 minus 
keyboard, but I haven't tried to swap CPU between the two.

I saw there are schematics and a service manual on Zimmers et. al. Are there 
other documents of troubleshooting elsewhere? Would it be a good case to 
swap CPU, at least put the one from the 610 into the 710 to see if the 
latter still boots?

I suppose I could measure voltages here and there, if anyone likes to guide 
me through what to measure and where. However I don't own anything like an 
oscilloscope.

Best regards

Anders Carlsson


       Message was sent through the cbm-hackers mailing list