Broken audio on System 24

[bodgit]

I've got an System 24 (currently with Quiz Ghost Hunter ROMs) that has broken audio, it just pops, sort of in time to the music if I compare how it runs on MAME. I'm trying to work out where/what the fault is.

I've got the System 24 schematics, here's the audio section:

There is a YM2151 OPM that is wired to a YM3012 DAC and then that goes to both an MB3733 amp which drives what I'm (not) hearing on the PCB edge, and also a CXA1034 which looks like it drives a headphone circuit. I'm not sure of the purpose of the R/2R ladder that feeds in between the DAC and the amps.

With my newly built audio probe (thanks @caius) if I probe on the L/R outputs of the DAC (pins 9 & 10) I hear the same popping sounds, and if I probe on the DATA input pin of the DAC (SO output from the YM2151) I hear the music albeit a bit distorted presumably because it's serial data rather than analog audio.

Is it simply the YM3012 that has gone bad here? Is there anything downstream of the DAC that could be affecting its outputs or do I just swap this IC out first and see if things improve?

I haven't recapped this board yet but I do have the caps to do so, however I've tested most of the caps in-circuit and they seem to be ok.

 

[bodgit]

So I ordered a replacement YM3012 DAC and also a TL084 quad op-amp chip as that would likely be the next chip to try swapping out given two of the op-amps are directly connected to the DAC.

Here's the audio output section of the board, the DAC is the rightmost IC of the three:

Judicious use of the Hakko FR-301 and the chip drops out easy enough:

I decided to put a socket in so I only have to solder once:

Replacement YM3012 fitted:

Now the audio is working perfectly, so just a faulty DAC it seems. Now I know the board is good I'll recap it.

I can't recommend enough an audio probe for this sort of problem, it's really handy to probe the various areas of the circuit and listen to it.

I ordered this PCB: https://www.tindie.com/products/caiusarcade/arcade-audio-probe-blank-board/

The amp IC, pots and speaker are probably a required purchase but the remaining components are standard stuff you likely have already. I ordered a cheap probe set from Amazon that had probes, cables and alligator clips, etc. that I butchered to fit: