X6800 Compact XVI voltage audio problem

[repoMan]

I'm checking right now my main Compact setup that has been decaying along these last months and now, the audio, specially PCM, sounds low and distorted. I expected a full recap and battery replacement; but to my surprise, the machine has a battery holder already mounted (with CR2032 without diode ) and a full cap replacement done with through-hole capacitors, ugly!

Following the the Compact audio repair guide I've replaced the related electrolytics:  C27, C31, C23, C28 (in my PCB it's named C29), C24, C2 and C40.

So, as there're no changes so far, I've checked next op-amp voltages and measuring in both of them pin 4 I can read 5V and pin 11 reads 0V, must they have +12V and -12V?.
The MC3403 fools me, seems that it doesn't have a real GND, I mean a 0V pin. To take the above readings I have used a GND pad in the motherboard...is it correct?.

In the meanwhile, I've also checked two different original PSUs with the same result, and measured in the motherboard connector, main -12V input seems fine. Also, the three sound sources of the machine sounds the same, internal, headphones and back audio out.

[repoMan]

Most of my questions are answered in this great thread that was the origin of the audio faq. So my measuring method was correct and I get the same voltages as RobIvy64, not 12v/-12V at all.

As I suppose the -12V rail is OK, the critical spots have continuity and I've replaced caps, I'm going to fully desolder the audio area looking for more broken traces.

I'm testing the sound with Bomberman, being a fave of mine I'm used to its sound and music. If the title screen jingle sounds looow and distorted, are we talking of FM or PCM audio?. The fact is ingame explosions (and other bleep sounds) seems to sound more or less fine...