I bought a Sega Apollo 13 a few weeks ago and I noticed that the CPU/Sound board had been changed by the previous owner to a more recent revision. The original board was a 520-5136-00 REV D (with 3 amps) but the one in my pin is REV E (with 2 amps) which corresponds to part number 520-5136-16. As a result, I had no sound coming out of my right backbox speaker. I then decided to add the 3rd amp components to get the stereo output, as described at the link below:
It worked, well kind of. When I perform the individual left speaker test from the diagnostics menu, I also hear sound coming out of the right speaker and vice-versa.
This definitely does not appear to be normal behavior (or is it?) and I have not found what might be causing it. Any help would be greatly appreciated.
PS: I also changed the R106 and R110 resistors as mentioned in the Stern service bulletin for this particular board (which I assume comes from a Monopoly pin)