9815S repair

[artag]

I'm trying to repair my 9815S. It used to work but is now inclined to cash on 9 out of 10 startups. Investigation so far seems to indicate some RAM failure but perhaps possibly a data bus clash (ie a an address decoding problem). It could be both - I suspect that one ram chip is being enabled sometimes when it should not be.

I'm working from the machine-drawn schematics that are available. I should probably also check these against Tony's hand-drawn schematics to see if they agree. I have worked out most of the differences due to it being an S model, which has 8 x 1kx4 2114 RAM chips and 2 8kx8 mask roms.

In doing this I am adding to the information available so I expect the outcome (as well as a working 9815) to be a re-engineered i/o (or rom) pcb design with some simple debug tests in rom. I am also attempting to turn the scanned partial rom listing from the patent document into searchable / assemblable text and read out the existing roms to provide the missing sections. I may also build a copy of Curious Marc's crowbar protection circuit adapted for the 9815.

Is there anyone out there with a 9815 and an interest in this who would help me ? Some electronics would be useful but even such completely separate parts as cross-checking the OCR'd listing against the scans would be extremely helpful.