SOLVED PCE Duo - No Sound (Post Recap)

[xodaraP]

Hi everyone,
I have recently been working on a PCE Duo that was very badly bathed in capacitor electrolyte. I have done several other repairs and the system is now working nicely, with no sound

I have beeped out all traces on the top board in the sound section, all op amps have been replaced, volume wheel has been replaced (corroded) and when I power on the console I can hear the thump/crack of the amp starting up. I am getting no sound from HuCard or CD-ROM. At one point I got a single note when I powered on the console, but that has now stopped as well

So far I have tried

- Replacing all op amps
- Beeping out full sound section from diagram provided by Retrosix
- Beeping out all traces in board section behind AV jack
- Confirming voltages are present at all ICs

I pulled out my oscilloscope to try and see if I could determine if there is sound being output from pin 2 on the HuCard slot - but all I am seeing is noise, this could well be an issue with my scope or probes, but I am not able to get any useful information from there

For reference the voltages at op amp IC506 are

Pin 1 - 6v
Pin 2 - 5v
Pin 3 - 7.41v
Pin 4 - GND
Pin 5 - 7.41v
Pin 6 - 5v
Pin 7 - 6v
Pin 8 - 7.41v

I  am wondering if perhaps the multiplexer/mixer or volume balancer are bad. But I?m not sure how to go about testing these

[Keith Courage]

Op amp voltage should only be 3.8V tops. Check that the ground pins for the op amps are getting a good connection and same for any capacitors in the area. Also check continuity of the Via holes.

[xodaraP]

Hi Keith, thank you for your help. Should all pins be 3.8v? The power pin is directly connected to the 7v rail so I thought that would be correct

[Keith Courage]

No all pins shouldn't be 3.8V but two of them should be. Which Op amp is directly connected to the 7V rail? There should be a resistor in line somewhere to cut down the voltage.

Maybe a resistor is shorting from old cap leakage being under it. This happens quite often.

[xodaraP]

Hi Keith,
These numbers are from IC506 - the op amp directly behind the AV jack

[Keith Courage]

Sorry, I completely forgot about 7V on one of the op amp pins as it's extremely rare for the 7V rail to have any issues. So that is correct for pin 8

based on this diagram. Make sure that both pin 3 and 5 have 3.8V. Also make sure that pin 4 is a good ground. Check this for all op amps behind the Heat sinks. There are three 4558 op amps in this area. 

[xodaraP]

I have double checked the ground to multiple other ground pins on the board and to the metal shield where the power supply ground is soldered on

I?ll see what else is on the rail for pins 3 and 5 since they are matching the Vcc voltage - could be they are shorted to the power line?

[Keith Courage]

There are resistors on the board that are supposed to cut down the voltage from 7 to 3.8.

One common spot for these resistors to fail or get old capacitor leakage under them which causes shorts, is right above the 22 microfarad capacitor that's next to the 5205 Oki chip.

Another common spot is just underneath the board of the 4558 op amp near the AV jack. the 22 microfarad capacitor right next to the 4558 op-amp that is near the AV jack should also read 3.8 volts on the positive side.

[xodaraP]

This one is now solved!

Thank you very much Keith Courage, very happy to finally have this fully working after a lot of hours put into it

For reference it wasn't the voltage dividing resistors or capacitor in my case. It was R324 on the bottom of the board. I have beeped it out 100x before but I noticed today that the via had light going through it, but it wasn't completely clear. I cleaned it out, and no longer had a connection to ground. I think it was just barely holding on so the multimeter registered it, but it wasn't an effective ground for the system itself.

I looked up R324 and found this post from Keith on another PCE Fans thread and it described the issue I was having exactly

I just had to fix this problem on a DUO. There was a shorted via at R324. This area can be seen from underneath the board near the AV jack op amp. I was getting 7V on most of the 4558 op amp pins as well as the 10UF caps. Hope this helps.

Patched the via for R324 and immediately had 3.7v on the op amp and sound was coming through loud and clear on both HuCard and CD