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Recapped Amp Board - High DC Offset SX 434

Not sure I'm up to that...any chance that the bulging filter cap would cause the problem?
 
mikiekimi said:
Not sure I'm up to that...any chance that the bulging filter cap would cause the problem?
Click to expand...
OK, I am always reluctant to use that technique because of the bad potential.
I don't know, which one is it? If it is truly bulging it should be replaced anyway . . .
 
Have you tried using freeze spray to cool components in the suspected circuit? Frequently cooling the defective component will decrease (or sometimes increase) the noise.

A defective filter capacitor would likely cause an issue in both channels, and it would likely be a hum (not white noise).
 
The more I think about it, it has to be something before the tone controls so the filter cap doesn’t make a lot of sense. I just ordered some freeze spray to test the components.
 
merlynski said:
OK, I am always reluctant to use that technique because of the bad potential.
I don't know, which one is it? If it is truly bulging it should be replaced anyway . . .
Click to expand...
The top isn’t firm (like there’s air inside), so I can press the top down a bit like a button, plus I see where the bottom is leaking. I understand how it’s probably unrelated to the noise issue.
 
So I tried the freeze spray - didn't really narrow it down. Mostly because I cannot access (or see) that part of the board when it's running. There are four capacitors that I can see possibly causing the issue (C19, C21, C23, C25). C23 is the only electrolytic with the others being ceramic. From what I've read, ceramic capacitors are pretty solid and don't typically fail - would it be correct to suspect C23 as responsible for the noise? If it's not a capacitor, can a bad resistor cause white noise?

I don't see anything else between the volume (VR2) and tone controls (VR3/VR4) that might cause the issue other than the resistors or capacitors since I just replaced Q5. I re-flowed solder to Q5 and C19 - C25 in that channel just to be sure. Parts list says C23 is a .33uf 25v electrolytic.

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Leaky electrolytic caps can be noisy. That circuit location and value (C23) is a good candidate for a film cap if you can find one that fits. Can you measure the lead spacing? How much room is around it? Picture maybe?
 
I'll have to get back into it tomorrow. Per the parts list, this film capacitor is the replacement for C23...according to the datasheet the lead spacing is 5mm. It's showing as obsolete...

Panasonic ECQ-V1H334JL
 
Here is a 0.33µF/100v with 5mm lead spacing from Mouser: 80-R82EC3330DQ70J, in stock, 0.46 each.
Just a suggestion . . .
 
I tried the film capacitor for C23...no dice. I get sound, but still have white noise on the left channel. Not sure where to go from here...
 
I think I would try spraying the output transistors with freeze spray. I have seen those TO-220 transistors go noisy.

Most noise issues are caused by semiconductors, but other components can also cause noise sometimes.

I find that almost any component causing noise can be affected by heating or cooling. You can spray whole areas of the board while troubleshooting. You can even spray the back side of the PC board in areas (which cools the components through the leads), to see if it effects the noise. Be a little careful if you get condensation on the PC board. If you get much condensation, it is wise to turn the unit off until it dries (as it can upset things and cause high current issues in the power amp circuit). You could also use a hair dryer to heat areas of the board, to see if it will effect the noise. The hair dryer also speeds drying when you get condensation.
 
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AE Bob said:
I think I would try spraying the output transistors with freeze spray. I have seen those TO-220 transistors go noisy.

I find that almost any component causing noise can be affected by heating or cooling. You can spray whole areas of the board while troubleshooting. You can even spray the back side of the PC board in areas (which cools the components through the leads), to see if it effects the noise. Be a little careful if you get condensation on the PC board. If you get much condensation, it is wise to turn the unit off until it dries (as it can cause high current issues in the power amp circuit). You could also use a hair dryer to heat areas of the board, to see if it will effect the noise.
Click to expand...

I replaced the output transistors a while back...I'll try them as well as using the freeze spray from the other side of the board. I actually successfully found a bad diode on another receiver using the freeze spray this morning.
 
I tried freeze spray on the output transistors - no luck. I also sprayed the underside of the board in the area I suspected as well as shotgunning it all over the board with zero effect. I was also able to take a better angle and hit some of the resistors/capacitors in the tone control area and that made . I'll let it sit overnight and try a hair dryer tomorrow.
 
Well I'm at a loss...I tried hot and cold on that area of the board with zero effect, I tested all the resistors in circuit and replaced all noisy transistors. Subbed out some good small caps that I had laying around "just in case" with no effect. I inspected the board for cracks and re-soldered or flowed new solder to just about everything on that part of the board, including all of the circuits involving the volume and tone controls - plus the pots, which I've cleaned at least three times. I also re-routed all wires on the inside, then poked and prodded what felt like everything on the amp board and the problem persists. I think I'm going to let this one sit for a while and let the electrical gremlin responsible have it's way :)
 
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