Sound crackling after PC Engine Duo recap

[YoshiKnuckles]

Checked the Op Amp this morning, 1.4V on the output on both sides, 3.8ish volts on the normal input, 4V on the inverting input.

Power rail shows +7.8 volts, ground rail is showing -0.2 mV for some reason... Hmm.

[thesteve]

so the amp is working and something is pulling the inverting input out of range

[YoshiKnuckles]

Hmm, so what should I check? I think it might be time to crack out my Electronics Troubleshooting textbook and do some reading.

[thesteve]

yes, look up simple OP-AMP circuits
check your grounds, check your input couplings, check your resistors between out and -in
you need to figure out whats pulling the - inputs up

[YoshiKnuckles]

Will do. Won't be covering them in class till next spring I think though. =/ Have to get through all the Circuit Analysis and such first.

Guess it's time to turn to The Art of Electronics and my Electronics Troubleshooting textbook from the 90's and actually learn some stuff.

[Slypty]

Not sure if this will help point you in a better direction, but the fix my repair guy did for this exact same problem involved cleaning under some of the slightly raised chips in that area, where some acid had leaked underneath.  I'm not techy, so I can't say which one's to look for, but the fix did involve a small broken trace underneath a chip, due to prior leaky caps.

[YoshiKnuckles]

That's the general conclusion I've come to. I've looked the PCB over under a microscope I have access to at school, and I didn't see anything jumping out at me. Have a feeling the dead trace/via causing my issue is under one of the op amp chips. I think I know exactly which one too. Need to poke around with the meter a bit more and confirm it before sucking off chips though.

I haven't looked at it for a week or so now, will have to give it another go sooner or later.

Going to college for Electronics Engineering Technology is a time killer. =x

[Vimtoman]

Clean the audio and PSU section with isopropyl alcohol. Push and pull a headphone jack into the headphone socket with some isopropyl alcohol a few times.

[YoshiKnuckles]

Fixed it! 

Decided to probe around the board a bit more yesterday and found 5 volts on the negative input pin on the back of the board. Sure enough, the via connecting that pin to the ground plane was corroded away!

So I ran a jumper from ground on the next chip over to the op amp, and it's back to normal. =) The damaged via was right by a capacitor that had leaked horrendously before the recap, so it makes sense now. 

The pin is back at ground, and the jumper has 1.96mA flowing through it to ground, so I'm not worried about it at all.

[YoshiKnuckles]

Update: Went to tune in the disc drive, managed to get the disc spinning, read, and playing, but no audio from my PVM speaker.

I do however faintly hear audio from the headphone jack. Based on this, I'm assuming I have another amplification-related problem.

I've checked the DAC with my Rigol DS1054Z oscilloscope, the audio is present, I've traced it from there to the 4560 Op AMP on the top of the board, yet I'm having trouble finding the next spot in it's path.

I'm learning a lot of stuff working on this Duo.


Edit: Figured it out! Had a via out that kept the 5V rail from making it to the pin. (I traced it out first) Soldered a jumper wire from the 5V rail to pin 8, and she's working perfectly again. =)
Edit: After checking another thread, I checked a few caps in the audio section of the board and found that C162 has no voltage on the positive side. I think I found part of the problem. =)