Audio jumpers have been verified (shorted on S11, S14, S16), A13 is the original EPR-12617 EEPROM and A11 is the original MPR-12616 Mask ROM.

As best I can tell, the problem is in the digital side of the audio generation circuit. When I increase the volume at the pot, the audible hum over the connected speakers increases.

I have no analog out on Pin 12 of the YM3012 DAC, but I am seeing a steady pulse from Pin 21 of the YM2151 received on Pin 4 of the DAC. The Z80 is alive and functioning (there's activity on the Address and Data lines at the Z80), but I've swapped it with a known working Z80B and no change. I've verified clock for the Z80 and YM2151 (5 and 4MHz, respectively).

When I check the Address and Data lines at the A11 and A13 ROMs, I have pulses on some of the Address lines but the Data lines are stuck either High or Low. Here's the relevant bits from my logic probing:

(pulsing means I saw a pulse but both the high/low indicators were lit on my logic probe)

z80

1 - a11 - low

2 - a12 - low

3 - a13 - low

4 - a14 - pulsing

5 - a15 - low

6 - clk - pulsing (clock measured at 5MHz)

7 - d4 - high

8 - d3 - low

9 - d5 - low, pulsing

10 - d6 - no signal

11 - +5V - high

12 - d2 - high

13 - d7 - low, pulsing

14 - d0 - low, pulsing

15 - d1 - high

16 - /int - low

17 - /nmi - high

18 - /halt - low

19 - /mreq - pulsing

20 - /irq - high

21 - /rd - pulsing

22 - /wr - high

23 - /busak - high

24 - /wait - high

25 - /busrq - high

26 - /reset - high

27 - /m1 - pulsing

28 - /refresh - pulsing

29 - gnd - low

30 - a0 - pulsing

31 - a1 - pulsing

32 - a2 - pulsing

33 - a3 - pulsing

34 - a4 - pulsing

35 - a5 - pulsing

36 - a6 - pulsing

37 - a7 - pulsing

38 - a8 - pulsing

39 - a9 - low

40 - a10 - low

A11 ROM

32 - vcc - high

31 - /pgm - high

30 - a17 - low

29 - a14 - low

28 - a13 - low

27 - a8 - pulsing

26 - a9 - low

25 - a11 - low

24 - /oe - low

23 - a10 - low

22 - /ce - high

21 - d7 - low

20 - d6 - low

19 - d5 - low

18 - d4 - high

17 - d3 - low

16 - gnd - low

15 - d2 - high

14 - d1 - high

13 - d0 - low

12 - a0 - pulsing

11 - a1 - pulsing

10 - a2 - pulsing

9 - a3 - pulsing

8 - a4 - pulsing

7 - a5 - pulsing

6 - a6 - pulsing

5 - a7 - pulsing

4 - a12 - low

3 - a15 - low

2 - a16 - low

1 - vpp - high

A13 ROM

28 - vcc - high

27 - a14 - pulsing

26 - a13 - low

25 - a8 - pulsing

24 - a9 - low

23 - a11 - low

22 - /oe - pulsing

21 - a10 - low

20 - /ce - pulsing

19 - d7 - low

18 - d6 - low

17 - d5 - low

16 - d4 - high

15 - d3 - low

14 - gnd - low

13 - d2 - high

12 - d1 - high

11 - d0 - low

10 - a0 - pulsing

9 - a1 - pulsing

8 - a2 - pulsing

7 - a3 - pulsing

6 - a4 - pulsing

5 - a5 - pulsing

4 - a6 - pulsing

3 - a7 - pulsing

2 - a12 - low

1 - vpp - high

Not sure what else to check at this point. I've validated continuity for the Z80, YM2151, YM3012, A11 and A13 ROMs, as well as reflowed the pins on the 315-5195 since the Address and Data lines are connected there.

Any ideas of what to check next (I've already been stuck in a loop checking the SRAM at D14, the LS273 at E19, and the CK2605) would be greatly appreciated. If there's any more information y'all want, just ask.

Thanks for any help you can offer!