MVS 2-Slot Low Left Audio Problem

[chaoticjelly]

Ok so I got this MVS 2-Slot for repair. The problem was that one of the colour was missing, it had already been diagnosed on this very board that was caused by a possibly faulty 74LS273 behind the Jamma connector. So I replaced this chip and the colour was working great again.

I then proceeded to remove the old rechargeable battery, did the 470 ohm resistor removal, and fit a CR2032 in socket.

I then removed and replaced ALL of the electrolytic capacitors with brand new ones.

There is a problem with the sound which was present before replacing the caps, the music on games will go all distorted and not sound quite right.

For example, Blazing Star will be okay, and then when you have chosen your character and it plays some music, it's distorted, the music is then distorted throughout the game. The same happens with my other cart. Samurai Shodown, when you get into the game, the music is all distorted.

The Neo Geo chime when you boot up is fine.

When I do a sound test, "RIGHT" sounds fine.. nice and loud.. "LEFT" is very very weak, unless you turn the volume on the TV up really high, you can't hear it at all. "CENTER" is fine.. nice and loud.

I've tried checking the output pins of the YM3016 with my Oscilloscope, but tbh I am not really sure what to set the scope to, as I haven't had it long (Tektronix 2246 100MHz).

My logic probe does not have audio output, so that's not a possibility.

The only thing I can see from inspecting the board is that a trace is broken near a 470 Ohm capacitor near to the Jamma edge. Then there is a piece of kynar patching to ground I think.. no idea what is going on there, but when I looked up on the service manual I think that Jamma pin was something to do with key mech or ???

Any ideas gratefully received. Once solved this will be a cracking 2-Slot

 

[channelmaniac]

Check the inputs to the HA13001 IC. Compare the amplitude of the inputs to the chip. If they are pretty equal in amplitude then check the outputs and troubleshoot from there.

If the inputs are way off then check the inputs and outputs of the op-amp ICs to see where the problem is.

This is where your oscilloscope comes in handy.

 

[chaoticjelly]

Check the inputs to the HA13001 IC. Compare the amplitude of the inputs to the chip. If they are pretty equal in amplitude then check the outputs and troubleshoot from there.

If the inputs are way off then check the inputs and outputs of the op-amp ICs to see where the problem is.

This is where your oscilloscope comes in handy.

Ok I've checked both the inputs and the outputs of the HA13001.

Both channels waveforms are equal in height, I presume that means they are the same amplitude?

 

[Xian Xi]

Wouldn't it make sense that it's possibly a cap since caps are used as filters on audio lines when coupled with a resistor, if the sound is getting distorted it could be because the "high pass filter" is disabled since the cap is not connected?

That was a question, not a statement.

 

[channelmaniac]

If the waveforms are equal in height going in, what do they look like coming out?

Are you hooking this up to a cabinet that's wired for stereo sound? If not, you won't get proper audio.

If they look good coming out then you can check the final couple of caps between the amp and the JAMMA connector. Those 470uf @16v caps are always a pain. Are you sure you installed them the correct way?

BTW: Those 470uf @ 16v caps after the amp are "DC Blocking Caps" in that they allow the audio signal to pass but block the DC component from hitting the speaker as that would keep the cone pushed either way out or sucked way in depending on the orientation of the wires to the speaker terminals.

 

[chaoticjelly]

Wouldn't it make sense that it's possibly a cap since caps are used as filters on audio lines when coupled with a resistor, if the sound is getting distorted it could be because the "high pass filter" is disabled since the cap is not connected?

That was a question, not a statement.

It's fixed

You're right, I'd discounted the fact that it could be that cap since the trace coming from it to the JAMMA edge already seemed to be repaired..

I'd checked and it wasn't going to the correct pin, but I thought it was too obvious to be the source of the problem

Changed the patch to the other side of the break... the sound is perfect now.. games, soundtest, everything.

Thanks guys!