MAMEWorld Forums - Hardware - The rare moment when I'm posting for help....(MCR Stack)

I used to be pretty good with these MCR stacks, but I have a Tron that is kicking my behind right now. If anyone has an idea, I'm all ears.

THE ISSUES:

No sound, sound board interface error, P1 Up and P1 Start do not work.

WHAT I'VE DONE:

---Socketed &/or replaced with new the following chips---
(in no particular order)
A12 (Z80)
A11 (74ls245)
A13 (74hc08)
A4 (74hc273)
A5 (74hc374)
A6 (CMD6116)
B14 (74hc174)
B13 (74hc138)
B12 (74hc138)
B11 (74ls670)
B9 (74ls670)
B5 (74ls244)
B4 (74ls244)
B2 (74ls244)
C13 (7427)
C11 (74hc04)
D3 (LM3900N)
E11 (74hc161)
E2 (74ls244)
F2 (74ls244)
SW1 & SW3 replaced

Ribbon cables are brand new SCSI like cables.

both AY-3-8910's have been traded for known good.
The z80ctc has been swapped on the cpu board with no effect.

(some of the above were previously replaced for issue not pertaining to the issues I'm currently chasing)

WHAT I KNOW:

Line BuD2 is shared by both P1 Start, and P1 Up. Most of the chips replaced are in an attempt to get that line to pull low past F2 & B2 (74ls244). Something seems to be keeping the line high, but I can't seem to nail it down.

Checked the inline resistor packs for the inputs, as well as the diode. All seem good. Joystick and start button are OK.

Game plays fine otherwise.

Any thoughts? I've been playing with this board for three days, and I haven't made an ounce of progress. Appreciate any advice offered.