Fixed: PCE Duo - sound quickly dies, op amps at 11v and 22uF caps at 5v

[HuDuo]

I tried to get all the cliff notes info into the title to get to the point.

So I have a black PC Engine Duo, it has been fully recapped (console5 Kit), its current problem is the audio amp circuit seems to be at an oddly high voltage in general. All the op-amps show 11v on the VCC pins and as much as 11v even on the op-amp Inputs and outputs. All the 22uF caps in the same area are around 5.7v to almost 6v. I understand the voltages should be more like half those values? (7v op-amps, 3.5v caps)

The audible symptom is (tested with a Hucard), the audio plays for about 1 single note of music then goes completely dead.

I read elsewhere by Keith Courage that oddly high voltage on the caps can mean a failed op-amp chip? Should I replace all the op amps with new ones? Or is there a problem with the cap kit install and some 11v line is bridging with the op-amp power circuit?

Lastly, I’m fairly certain the audio wasn’t working properly before the cap kit but honestly, the audio is always not working on these broken Duos before the recap so at this point I don’t pay that much attention.

[Keith Courage]

Is this the same for all 3 of the 4558 Audio op amps? There is one near the AV jack and two others on the bottom part of the board near the heat sinks.

Yes the higher voltage can be a bad op amp but also a bad ground for one or more of the capacitors can do the exact same thing. Also, check for shorts in the via holes near the op amps as more often then not, the problem is a bad connection through a via hole. I only change out an op amp once it seems apparent that all connections through the board are good.

You can repair shorted vias by soldering an extra capacitor through the holes.

[HuDuo]

Yes, the high voltage issue was affecting all three audio op-amps.

After much work looking over the board and adding patch wires and “cut capacitor legs through corroded vias” fixes, I finally fixed the audio on this Duo! CDs, Hucards, AV Port and headphone jack all work great now. The final fix was R323 and R324, specifically I removed both resistors, cleaned up under them and added a patch wire from R324 to ground.

I went back after and did a final voltage check and there wasn’t a big change, but still a bit. Audio op-amps show about 10v on input voltage and the 22uF caps are around 5v now. I won’t look a gift horse in the mouth though, the audio works again so I’m happy.

Also fixed the issue of “CD spindle spins immediately on power-on” but I did that before making this thread by reading old posts and finding a certain Keith Courage mentioning a corroded via by C149 causing the issue. 

In total, all the fixes to this board:

C149 (positive side, corroded via)
C135 (positive side, via was partially corroded but worked, fixed for future-proofing)
Big via near C149 (fixed CD spindle)
R321 replaced (cap leakage under it)
R322 replaced (cap leakage plus the corroded via connecting to IC503 ground)
R323 replaced (cap leakage under it)
R324 replaced (cap leakage plus the corroded via connecting to ground)
C616 (positive side connecting to IC505)
R639 to IC505 (corroded via)
C305 (both solder pads were corroded on component side of PCB)

IC503 replaced (pin 5 and pad were badly corroded, pad fell off board)
IC521 replaced (found cap leakage underneath and a leg fell off)
IC506 replaced (maybe didn’t need replacing but some legs were really weak and broke off easily)

Job well done I’d say. Pretty scary for a PC Engine Duo like this that externally was in barely used and nearly scratch-free condition, all case screws had that distinct “virgin snap” too. Even the CD drive works perfectly after fixing that via by C149, didn’t need to do anything else to it.

Thanks so much for your help Keith Courage, you’re still the Duo repair wizard! 

PCB pics:



https://imgur.com/a/mgi451I

[Necromancer]

Another system saved to play more Bonk.  Good job!