[BoogieOogie]

I recently found an old Pioneer VSX-3300 receiver in my basement. It's got some issues with the left and right channels cutting in and out, but otherwise it's fine. I opened it up and took a look, and found these greenish-yellow splotches on the underside of the PCB:
 

 

 
I've done some research and I believe that it's solder flux, but I'm not sure about that (I feel like it could also be oxidization). I'm inexperienced in electronics repair, so I have no point of reference to fall back on. If anyone could confirm what this stuff is and advise me on how to safely clean it off, I'd appreciate it! Thank you kindly.
 
Also, if this is a totally inappropriate topic for the DIY forum, I apologize profusely and effusively for my ham-fisted intrusion. Have a nice day.

 

[wakibaki]

Some of that is flux, but some of it, it's not possible to be sure from the photo. It could be leakage from a cap. My advice, just leave it alone. Fiddling with it will almost certainly not fix the problems. Concentrate on finding the intermittent contacts in the L & R channels and resoldering them.
 
You can remove flux with flux cleaner or isopropyl alcohol, but:
 
1. You need to let the board dry thoroughly afterwards
 
2. You will probably need to scrub hard with an old toothbrush, and you can damage the board or components by doing so.
 
w

 

[BoogieOogie]

Thanks for your quick reply.
 
I do not believe any caps are leaking, which is a relief. The top of the board looks to be in very good (but dusty) shape. I'll do as you suggest and attempt to find the faulty contact visually. If that doesn't work I'll try anhydrous alcohol on the discolorations and if that too does not work then I'll take it to someone who actually knows what they're doing

 
Once again, thank you for your quick reply and your suggestions.

 

[arteom]

Try this: rotate all knobs and flip all switches. You want to get each know/switch about 100 times. So rotate the volume know from min to max, repeat! Many receiver out there will issues with dirty pots or switches. Which will cause the channels to cut in and out. Try this and see if things improve. Focus on volume and balance knobs especially. If things improve you have identified the problem, you can get deoxit faded lube and spray it inside problem potentiometer to try and clean dirty out of it.

Try this out. You honestly would have to be lucky to find an old receiver that does not have problems with dirty pots. Ive seen many old units abandoned befause owners thought it was past spending money for repair, and they only needed pots and switches to be cleaned.

 

[BoogieOogie]

Thanks for the advice. I'll do that and see what happens! I've already determined that jacking up the volume and balancing all of the sound into the afflicted channel can provide a temporary fix, which was making me thing there was a faulty connection on the PCB somewhere but it could just as easily be an issue with the pots.
 
And I definitely won't be giving up on this thing. When it's working, it sounds at least 100x better than the cheapo Sharp component system I use now. Even if I do wind up sinking some cash into it to get it healthy again, it'll be saving me lots more over buying a newer receiver which may not sound as good for my money.
 
Thank you again!