IREM M72 Image Fight No sound issue (solved)

[neonmegadrive]

Hey guys,

So, I've purchased many different working M72 boards at a great price recently. While they work they also show heavy signs of age. Normally, my process when cleaning these boards is, teardown, clean, recap and rebuild with a Semload case. However, This time around my Image Fight, after rebuilding, booted with only SOME sounds playing. Specifically those sounds would be just the blasters, ship boosting and a few other enemy sounds. No music at all and many other sound effects missing.

I tried reseating everything, checking for physical shorts (stray debris) and even tried another known working set of Image Fight IC's. Still same result. I then tried the middle board (M72-A-C/D) from my X-multiply in place of the Image Fight board and everything started working / sounding fine. So that narrows down the issue a bit to the middle board. I do recall having a bit of trouble when removing a cap underneath the heatsink. So thinking I may have pulled a trace there. I've redone all the caps very carefully and inspected all the traces in that area. Now there is NO sound at all.

I've scoured the web and found a few folks with similar problems but I'm still having a bit of trouble. I do not have a logic probe yet and have limited knowledge with that tool. Any help would certainly be appreciated.

I know @caius is an M72 expert, if you have some time to spare, I will be so grateful. Thanks in advance!

 

[CoolFox]

Sounds like one of the outputs of a pre-amp isn't doing anything. I would start at the pre-amp.....or since it sounds like you cleaned the board and might have pulled a trace, I would check the Quad flat pack chips.

 

[neonmegadrive]

I did find that my oblivious self has accidentally squished these two leads here, so I expanded them back to normal. Now I have some sound back but still no music and other missing sfx just as before. Back to square one.

Thanks @CoolFox, will investigate the pre-amp.

 

[ForestOfDreams]

I had a similar problem that replacing the audio amp with new old stock and a full recap fixed completely.
You'll want chipquik on the amp if it is your issue, and the pads are extremely delicate and easily pulled.
My issue was the audio would come and go completely.

 

[frsj8112]

@neonmegadrive
Look at my repair log of Data East Robocop here: https://jammarcade.net/repair-log-3-robocop/

Different hardware of course, but the procedure of troubleshooting is similar.

Look at the schematics of R-type and the sound section.

Make your own audio probe, so that you can ”listen” for audio in the analog section.
If there’s no sound there, then you need to go further up into the digital section of the audio circuit

 

[neonmegadrive]

@frsj8112 Thanks for the response. I've fixed this recently despite buying many tools and parts, it turned out that I had a capacitor that was DOA, forgot to mark this thread solved. Embarrassed to say I didn't detect this sooner, but it is what it is.

Thanks for the advice, I will book mark the link.