Hi friends,
Well, there are good news. The Simmons woke up again !
The main problem was the IC3 (74155) which -I understand- demux some address busses and controls the four RAM chips thru the OR gates of 7432 (IC15).
It was a NOS 74155 but obviously was dead. I found it testing the signals on the /OE pins coming from the OR gates. First I changed that 7432 with no result. Then I replaced the IC3 and the machine recovered it coherence.
Since IC2 came from the same bunch of IC3, I've replaced both with a pair of new 74LS155.
About RAM chips, I bought four UM6116-3 which works perfectly. Obviously I was wrong, when the machine woke up few years ago, the four RAMs should be installed for sure.
However, if I place right now the four old HM6116 chips, the machine doesn't work. I think there are at least one HM6116 dead. Maybe it died on the last years.
btw, the machine works fine with both CPU's: R6502 or R65C02, but not with the original SY6502. The original VIA SY6522 neither works, but it is understandable since the corrosion affected it directly.
It remains to solve some minor failures like one non-working trigger output (1 from 8 ones), dirty tempo pots, and several push buttons with erratic contact... but I can handle all of that problems.
In addition, I will make a new CPU board from the scratch, since there are several traces I replaced by wires, and other ones which are in very bad condition. I'm attaching photos of a "before and after" of the corrosion cleaning works.
I wish to thanks to all of you for your unvaluable help. I would never have advanced with this repair without your help.
A lot of thanks for your time and knowledges. You helped me to recover a machine that had been dead for 10 years !
