Heitor,
POST 3000 indicates that the acquisition RAM has failures. The fact that the testing took place indicates the processor, address bus, data bus and ROM should be ok. The next step would be to use the Service ROM to run some tests, especially on the acquisition RAM. No one seems to be able to product a copy of the service RAM so using the manual's troubleshooting by Service ROM is not going to work. Additionally the manual is focused on using a signature analyzer for much of the testing. Most people don't have one, and I assume you don't either. They were popular back in the 80s but you don't see them today. So, I think the way forward is to use a more general troubleshooting process.
Acquisition RAM is on the A18 board so I suggest you start on the "Partial A18 Memory Board Schematic" ACQUISITION MEMORY<14>. The upper right shows the RAM and the address/data decoders. I would start looking there. Perhaps focus your logic analyzer on one of the chips and see where the test stops when the POST 3000 comes up. Could be a bad RAM chip(s). Since both scopes show a POST 3000 error, it could be the RAMs going bad. I would find it hard to believe you would have identical logic failures on both scopes.
I assume you have checked the digital supply voltages, especially for high ripple voltage. There are two +5V power supply systems, one for the microprocessor section at the back of the scope and a second one on the A15 board which powers digital logic in the rest of the scope. Make sure you rock any ICs that are in sockets to make sure they are making contact in the crummy TI sockets. I would avoid touching the A/D converter chip as it is big and if you break it, it might be very hard to find a replacement.
In any case this should get you started.
Regards
Tony
