Sound problem with MV-4F

At first I thought it was one of those problems with old capacitors. So I completely refurbished the capacitors in the boards amplifier section. Sound is pretty good now although it's still not the regular stereo.

The sound test in the hardware diagnostics, the Neo Geo boot screen and the games all only play music and sound effects through the left channel. It doesn't matter if it's the headphone or speaker connection.

However the right channel is not completely dead. These jingling effects of some games are in perfect stereo. For example:

RBS - The sound when you move the cursor in the fighter selection screen or the continue timer.

Bust a Move - The sound when the bubbles "explode"

Money Idol Exchanger - That sound when you throw the coins

Last Blade - The sound when moving the cursor in the fighter selection

Bomberman - The sound when you pick up extras

You also get a very faint right channel beep on the left speaker (ear directly on the speaker) in the hardware test when turning the volume to maximum.

So any ideas what's wrong with my board?

The sound effects are the analog output (pin 27) of the YM2610. The problem must be in the R channel before the YM2610 analog merges into it. Check the traces from the R out of the YM3016. It could also be bad op-amps (in the LM324 next to the YM3016) for the Right channel.

Finally some schematics.

Before I visit my local electronic components shop: The 4558 op-amp does nothing directly for the sound?

The part highlighted in green is what you should check:



The 4558 isn't related to the R channel in particular.

Note that the above schematics are for the NEO-MVH MV4. If you have a NEO-MVH MV4F(S/T), the circuit will be a little different as one of the LM324 is replaced by two 4558 so there will be a few more traces to check.

If all the traces are fine and replacing the LM324 doesn't change anything, it's a bad DAC (YM3016). Run just the bottom board and touch with your finger pin 11 of the YM3016: if it's not dead you should hear a humming noise from the R speaker. A replacement DAC is hard to find.

Just a little update:

Checked traces between the LM324 and the R-output of the YM3016. Visual check seems to be ok.

Continuity check between R-output of the YM3016 and the corresponding input of the LM324: Ok
Check between R-output and ground: Not ok, R-output is shortened against ground.

Removed the LM324 by simply cutting it out. No more continuity between "R" and ground.

I'll buy a new LM324 tomorrow.

Edit: It works. It was the first Op-Amp after the R-output.

Post rised from the grave, I have a MV4F with scratching sounds muxed to normal ones, but only on slot 3 and 4.
On the top board, 2 NEO-G0 located in @E1 for (slot 3 & 4) and @C1 for (slot 1 & 2) are used to mux SDPAD[0..7] and SDRAD[0..7] to CN9 connector (C4 to C19 on top board).
I don't suspect the NEO-253@5E (used to mux SDD[0..7] from the Z80 bus to the 4 slots) as the problem is common to 2 slots.
What could be wrong apart of the NEO-G0 ?
I didn't check the bottom board for the moment.