Hey guys -
Need a second opinoin on this, as I lost half of my hair out while trying to debug this board and getting mixed results.
Summary of everything done:
It's from a Bally Nitro Ground Shaker. Issue was reported as the sound 'suddenly dropping out'.
Owner did a cap kit.
Amp is good, as when powered up, you can hear a soft hum, and sot crackle as you adjust volume.
TP4 matches what schematics state - 0VDC w no sound, ~2.5VDC with.
TP6 has between 2.5 and 3.0 VDC, 0 VAC regardless of whether sound is playing or not.
TP5 has 0VDC with one CPU chip (original), and with replacement shows 2.5VDC.
Tried four different LY-3-8910 and replacement chips. (1 8910, 3 YM2149F which is a drop-in on this board, since pin 26 is floating)
U8 (LM3900) was replaced.
C3, C2 replaced. Old C2, C3 and U8 had pin U8p3 floating. Afterwards, I cannot remember if I was getting a high or low reading.
U10 (RAM) reported as bad with a memory tester. Replaced with known good working 6810. No change.
On U1, DA0 - DA7 lines, according to logic probe, are always low.
On U1, lines coming from MPU *do* pulse.
On U3 and U2, R/W line is always high. (Read). Logic probe never reports it going low.
U2 (PIA 6820) was replaced with two known working ones. No change.
U3 (CPU) Address lines do pulse.
What am I missing? With TP6 never having AC voltage on it, it tells me it's the General Instruments / Yamaha chip. But 4 of them giving the same issue? .. Hard to believe. With it's data lines stuck low - that points to the PIA. But that has been replaced and same issue. And even WITH that, the VMA line does alter depending on whether the game is playing music or not.
Next steps are appreciated. I'm.. stumped, and feel like it's an idiot thing I'm missing.