Sound crackling after PC Engine Duo recap

[YoshiKnuckles]

Hello,

I recently finished recapping a PC Engine Duo and have encountered a crackling sound issue. I've been pointed towards a few of the OP Amps, and other IC's related to the audio circuitry. I only have a basic DMM here, and an old analog scope. I have access to a slew of DSO's and Fluke 87-V's at school though.

What should I check moving forward? I'd like to fix it if possible.

Background: I'm currently going to school for Electronics Engineering Technology, yet I'm still early in my studies.

[Duo_R]

The board might need a dishwasher if old capacitor fluid got on the MB. It's hard to see but I had this happen to me once.


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[YoshiKnuckles]

It did have electrolyte leaked all over. I cleaned it up with some 91% rubbing alcohol before recapping it. Are you saying I should give cleaning the board again a try?

[Duo_R]

Id recommend dishwashing it, if you search an old post I had this exact same issue happen. I tried cleaning too but eventually the dishwash method worked. You want to use something very subtle like either just water only or 7th Generation dishwash soap. The crackling is shorts caused be leaked fluid and it's in places you cant see.

Also Google PCB dishwash for more info.

[BlueBMW]

Gentle dishwasher cycle with minimal detergent.  No heat dry.

After washing, preheat your oven to its lowest temp.  Put the board inside, turn off the oven and leave the board in there overnight.  Works like a champ!

[Duo_R]

Yes good tips Beemer!


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[Duo_R]

I also just put my board in the garage and set it on its side and had a box fan dry it off for a day.


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[BlueBMW]

Box fan works good too!

[thesteve]

odds are you have an open via by the 10uf caps behind the heatsinks

[YoshiKnuckles]

Thanks thesteve! I was going to check for that after I gave the board another good cleaning to rule out any electrolyte causing issues under IC's.

Is there a good way to check for damaged vias?

[grendelrt]

I had 2 different sound issues after my recap,  both were due to bad lines, via etc going to the ground pin on 2 different 4558 op amps.  Running a new ground on both solved it.

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[Duo_R]

Bust out the ohmeter and just checking out all the traces in the audio circuit

Thanks thesteve! I was going to check for that after I gave the board another good cleaning to rule out any electrolyte causing issues under IC's.

Is there a good way to check for damaged vias?

[Keith Courage]

With every complete capacitor change I typically remove 2 op amps, clean under them really well, and then reinstall. The one near the AV jack and the other near the M5205 chip are the ones known for getting old cap goo under them.

[YoshiKnuckles]

Alright, I re-cleaned the board and rinsed it off really well, leaving it to hang and air dry overnight.

Today when I got home from my Lab class, I plugged it in to see what I'd get. The result? What was seemingly no audio output. However, after cranking up my PVM to max I was able to barely hear it, and the same outcome with headphones and the headphone jack.

My guesses at this point are either a dead via, or a dead op amp. I'll be taking it to school with me tomorrow to check it out under the microscope in the electronics lab, and see if that gives me any hints. Any other things I should check for?

[Keith Courage]

Check to see if the op amp near the AV jack is getting around 3.85 volts on the power input pins. If not then there is a chance there is a bad trace or that the amp is bad. Also, try testing the ground trace as well