Quoted from uncivil_engineer:The crystal is banged up, but I still have nice wave at IC1, pins 38 and 39.
I pulled the rom chip, and I never see any pulses at pin 20 of IC17. Using the table above, I see that I am supposed to be getting inputs on pins 1,2,3 and outputs on 4,5,6, and 7. I can see the inputs cycling just fine, but I have no outputs at all on that side of the chip. I can't pull the chip til tomorrow, but am now thinking this is the problem.
Regarding the data buffer at IC9: I did pull that chip for testing. Per my little green box, the chip is fine. I am getting nothing on the inputs or outputs from that chip.
To be clear, you are seeing pulsing on pins 1, 2 and 3 of IC15 (74139) but nothing on pins 4,5,6,7?
How about IC15 pins 13, 14, 15 (inputs) and 9,10,11,12 (outputs)?
Verify good ground at IC15 pin 8 and VCC at pin 16.
If you have good power and the inputs are pusling but no outputs are pulsing, I think you found your culprit.
Benton, PA
