Basically this ADM-3A needs some help. I am not sure if this is the same ADM-3 I wrote about in 2008, or if I gave that one to VCFed for their museum. At one time I had three. This is the only ADM-3A I have now, so I decided to bring it to the VCFed Workshop this past weekend, June 12th (13th).
Here is the basic data path to test.
74LS157 --> 2102 RAM --> 74LS175 --> 74LS166 --> character ROMs --> signals sent to screen hardware.
Logic probe showed signals going to the display:
L6 and K6 74LS157 - OK
2102 RAM testing - moving around no effect. Removing entirely screen showed ?? characters (correctly)
L13 and K13 74LS175 - OK
All LS166's OK
I went ahead and tested all the TTL remaining TTL chips and found there were three bad chips on the motherboard.
K9 74293 (up counter)
J2 7442 (BCD to DECIMAL)
B11 74293 (up counter)
There are three 74293's, one was still good. The rest of the chips were at least good enough to pass my tester's test. Hopefully replacing these chips will restore service to the unit, but further study is necessary. At this point I can't say there is anything wrong with the RAM chips, the symptom of all zeros does not point to a RAM problem. I also have no way yet to test the UART (TR1602B). Hopefully it's OK.
New pics (dated 6/13/16)
Oh, and I found the keyboard option switch cover...nice.