PC Engine Duo crackling sound issue after recap

[YoshiKnuuckles]

Hello.

I recently replaced all the capacitors in a PC Engine duo. The old SMD Electro's had puked all over the board, so I stripped them out, cleaned the board with rubbing alcohol, and recapped them. Now I'm having an annoying intermittent crackling sound issue that has me stumped. 

I checked all my work and it's all ok. Some friends told me to check the audio OP amps and a few other IC's. I've done so and they are all working, but there is a bit of noise on some of their output signals. I'm also not entirely sure what I should be looking for here.

I can't see it completely, nor am I completely sure as I only have my old analog 30 Mhz scope(fixed and calibrated with what I had access to at school,) and my Milwaukee DMM to check them with.  If I can't figure it out with what I have at hand here, I can take it to school and check it with an army of old Tektronics DSO's and Fluke 87-V's on Tuesday.

So my questions: How do I tell if an OP amp has gone bad? Could the intermittent crackling be caused by some electrolyte left over on the board? I've also been wondering for awhile what the recommend way to clean electrolyte off of PCB's is.

I'm still rather new, I've only been doing this console modding and repair thing for a few months now, but most of my time is eaten by college right now, so I don't have all that much experience yet. Any advice or clues on how to proceed is greatly appreciated. I hope I'm not asking too much here.

[danadak]

Crackling, there was a time when OpAmps had significant issues
with popcorn noise, but I thought that was in our past.

Troubleshooting, start at the output stage working backward, and use
scope normal sweep, and turn trigger slowly until you get a sweep. At
that point you can tell if its pulse noise or just broadband noise.  Each
stage backwards requires increasing vertical gain of course.

Also examine power supply rails.

In the scheme of things maybe 1 or more of the new caps has issues.
As you are probing look at DC levels, part of finding bad caps.

Regards, Dana.

[mikerj]

Assuming the PC Engine DUO has a standard potentiometer style volume control, then it's quite common for these to go bad with age and wear and cause crackling.  Does moving the volume control cause extra crackling?  If so then Deoxit cleaner can help, but if the carbon track is badly worn you'll probably need a replacement.

Another possibility is corrosion from leaked capacitor electrolyte.  This can leak into vias and corrode the copper away, and won't necessarily be obvious to see.

[wraper]

The old SMD Electro's had puked all over the board, so I stripped them out, cleaned the board with rubbing alcohol

Alcohol is not a good choice for such cases. Board needs to be washed in water to completely clean the electrolyte. In such cases, I usually wash it with a dish cleaning fluid and scrub it with a brush. Then rinse the board well, blow the water away with a compressed air and let it dry well using some heater.

[YoshiKnuuckles]

Thanks for the tips guys.

@mikerj I'm pretty sure it's not the potentiometer volume control. Crackles randomly from both the AV jack and the Stereo headphone jack.

@wraper I had a feeling I should have ran it under water before I recapped it. A friend of mine recommended some electronics contact cleaner from WD-40, so I'm just waiting for that to show up at this point so I can spray it under the IC's before I continue probing it. Might as well eliminate the possibility of it coming from leaked electrolyte left under the chips first.

A few people on another forum recommended that I run the board through a dishwasher, but I don't exactly have one that I'd feel comfortable doing that in. I'm hoping the electrical contact cleaner I have coming does the trick. If not I'm going to have to start checking a ton of vias. The caps were leaking pretty bad, so a corroded trace is a likely issue.

I appreciate the tips! I'll let you know how it turns out.

[FrankE]

Dishwasher jets are probably too powerful.
Try the WD40 decarbonising the VR.
I agree with detergent. IPA will just hold residues in suspension, surfactants will lift them and let them slide off. Scrub with a plain soft flat toothbrush.
My kitchen dish draining rack was frequently populated with ATX boards.

If you have a UV blacklight you could try inspecting the board under it thoroughly covering UVEPROMs. It might not be of diagnostic value so don't buy one specially.

[YoshiKnuuckles]

Hmm. I was thinking of cleaning it with the contact cleaner I have coming, rinsing it under the tap, blasting it with some compressed air, and hitting it with the contact cleaner again and letting it dry. Think that will do the trick?

I also had some trouble getting high enough resolution on the signals I was probing yesterday on my old Leader LBO-520 30 Mhz analog scope. Tempted to use this as an "excuse" to justify buying a Rigol DS1054Z...

[FrankE]

Contact cleaner is for cleaning contacts and should be used sparingly on contacts to decarbonise them.
It can do a great job of desquamating your skin.

I suggest you read the MSDS
http://www.wd40specialist.co.uk/the-specialist-range/fast-drying-contact-cleaner/

Better to take a conservative approach with solvents and your boards.

[YoshiKnuuckles]

Ok then, what should I use to clean this board then?

I should probably note that the majority of that MSDS makes little sense to me. I've yet to take a chemistry course. Ever.