Assistance with CPS3 mobo with no sound

[tiff_lee]

So I couldn't see any obvious logic chips on the board that would be dividing that crystal oscillator frequency (/3) so I surmised that it was being done by one of the custom chips. I soldered a piece of kynar wire to pin 8 of the exchange switch (PI5C3383Q, which as per above I said was the incoming 14Mhz clock) and probed the board until I found the other end of it .

Good news I found the other end.

Bad news it is coming from the PPU, Pin 91.

So knowing that I would check for continuity between those pins with the board off (pin 8 PI5C3383Q and pin 91 PPU), if it's good then that rules out a broken trace (if the trace is indeed broken check for activity direct at the PPU chances are it will be active and then just run a repair wire and retest). Then fire up the board and check for activity with the logic probe at either pin 8 of exchange IC or at PPU 91 whichever you find easiest, obviously a scope would be better but any activity is going to be better than what you have so far.

If there is activity then investigate the exchange IC more (as per the above post, supply, enable signal etc), if there is no activity then the issue is PPU related whether that be a lack of input clock to the PPU or a defective PPU I have no idea at this time i've not looked any further.

 

[ShootTheCore]

Great job tracking down the source of the clock signal!

 

[tiff_lee]

Yeah just waiting for Julius @mR_CaESaR to pull his finger out and stop blaming his misses for not cracking on using the kitchen table!

 

[mR_CaESaR]

Managed to test some things tonight.

On the quiet board

- There is continuity between the 6D exchange switch pin 8 to PPU pin 91
- Pin 1 = Hi
- Pin 8 = just flashes (same as pin 91 on the ppu)
- Pin 9 = Lo
- Pin 10 = nothing
- Pin 11 = Lo
- Pin 12 = Lo
- Pin 13 = Lo
- Pin 24 = Hi

I guess the next real step is to change the exchange switch?

 

[tiff_lee]

On the quiet board

- Pin 1 = Hi

Pin 1 being Hi is a problem

Pin 1 Low, Pin 13 Low = B1 > D1
Pin 1 Low, Pin 13 High = A1 > D1

I didn't mention it before but from the data sheet there is a third state on the truth table ( I didn't mention it as I didn't think it was the issue, my bad).

Pin 1 High, Pin 13 in any state = OFF, high impedance between input & output pins

So if the exchange switch is in that state a quick and dirty test to get audio working would be short pins 8 & 9 which will effectively put the clock signal to your DAC (since you have now confirmed there is a signal there and it's connected to the PPU).

The real issue now though is to find out why is that switch 'turned off', Pin 1 Bus enable needs to be low and something is holding that pin high.

 

[GoldenGlover]

I’m following this thread with fervor as I am literally in the same boat as you @mR_CaESaR with a CPS3 that works perfectly aside from no audio via JAMMA or RCA. I’ve replaced the DAC & amp chip & even substituted 6D from a parts board I have & still have dead audio. I too also have continuity from pin 91 of the PPU to pin 8 of the relay at 6D.

Could it be the crystal that’s the problem here?

 

[tiff_lee]

Could it be the crystal that’s the problem here?

Depends what else uses that 42.9545mhz clock signal if that was defective would the board even function? the clock from the PPU is a derivative of that main clock, so is the 42 mhz clock going in and the actual 14 mhz clock coming out?
I haven't looked into it too much more as fortunately I don't have this problem but perhaps you could fit an external counter to the 42 mhz oscillator output and derive that 14 mhz clock externally (from the PPU) and then feed that into the DAC clock circuit. I dunno just spit balling.

 

[mR_CaESaR]

Haven't really done much for a while on it, I think I'm well and truly out of my depth.

I spent some time with it with @tiff_lee assisting and tried to get sound by making the switching bus 6d act like it was switched to activate the audio, but I still didn't get any sound. Based on the info on the datasheet, the IC is doing what it's supposed to be doing, the problem is why is the state of the pad the state it is. Unfortunately I haven't really had the chance to look further into it.