Yamaha G100-212 Guitar amp crackling - stumped

[Raccoon1400]

I'm working on a yamaha guitar amp from the 70s. There is a crackling noise in the output. It sounds a lot like transistor noise. It takes a while to start up when I turn the amp on from cold, but once it starts it is pretty frequent. I'm pretty thoroughly stumped now, I seem to have ruled out everything.

The power amp is a cap coupled design, and it runs of a 110VDC supply from one filter cap.

-disconnected input pin to power amp and noise continued
-removed fuses to other parts of the power supply to prove noise wasn't getting into the ground from other circuitry
-Recapped power amp board
-tried swapping all transistors and diodes on power amp board. (though I didn't change all transistors at once)
-swapped input fets on power amp
-removed assembly glue
-tried a different output coupling cap, and a different power supply cap
-reflowed the entire power amp board, and much of the power supply board
-tried adjusting the trimmers on the board to see if that made a difference
-used freeze spray on the power amp board
-thought it could be the connector between the amp and power supply, so I removed pins for ground and power and hard wired them to where they needed to go
-also wired the speaker directly to the output cap to rule out the output jacks

None of this has changed the noise at all.

All that I haven't swapped on the power amp board are the resistors, STV3H, and ceramic/film caps, and trimmers.

 

[jt01]

Did you try cleaning the switches and pots with deoxit or other contact cleaner?

 

[Raccoon1400]

Did you try cleaning the switches and pots with deoxit or other contact cleaner?

Yes. That was done before anything I could even run it well enough to notice the issue. But there aren't any pots or switches in any area the problem could be coming from.

 

[cademan]

You said you tried everything. Well, you haven't tried the resistors. In my experience, it sounds like a noisy resistor as it heats up.

And.....it could also be a noisy transistor.

 

[Raccoon1400]

You said you tried everything. Well, you haven't tried the resistors. In my experience, it sounds like a noisy resistor as it heats up.

How would I find it? Freeze spray?

And.....it could also be a noisy transistor.

There's none left it could be. This was my first idea.

 

[petehall347]

fuse contacts maybe .

 

[Tom B]

If you suspect a noisy resistor or bad solder connection, you can use a wooden stick like a wooden stick cotton swab. fiddle with the resistors, and other components with the wooden end to see if you can produce the sound by messinf with it. also Freeze spray (circuit chiller) works good to troubleshoot noisy resistors, but be careful with that STV bias diode and the other transistors. Don't get them too cold. Just a small momentary hit of the spray will tell if the component is getting noisy as it warms up.

 

[Raccoon1400]

fuse contacts maybe .

I'll have another look at this. Something like this seems most likely at the moment.

 

[Raccoon1400]

Doesn't seem to be the fuse contacts.

I can't seem to get any results with the freeze spray either.

And it kept crackling after I powered it down, as the cap bled down. That should rule a few things out, right?

 

[petehall347]

dry joint or resistor then

 

[Raccoon1400]

dry joint or resistor then

I reflowed the entire amp board.

Any other ways to find a noisy resistor? I could just replace them all but that sort of approach without proof rarely works well for me.

 

[petehall347]

try freeze spray

 

[Raccoon1400]

try freeze spray

That's one of the things I've been trying. It hasn't told me anything so far.

 

[petehall347]

have you tried heat yet ? i have a hair drier for this purpose .

 

[Raccoon1400]

have you tried heat yet ? i have a hair drier for this purpose .

I've got a heat gun I can try. I don't even own a hairdryer...

If I touch the top of a certain cap, I can hear radio through the speaker. Don't know if that means anything.

 

[petehall347]

set on lowest heat if you can . if its like my heat gun solder soon melts . low heat is the key and be methodical .. if noise clears up or gets worse hit a suspect with freeze spray .
just found a bad cap like that today in a tv . damn thing was under a metal casing thats soldered to the board .

 

[previd01]

I would be suspicious of where that black glue was on the board. I have had some instances where it became conductive and contaminated the board. by using an oscilloscope set to dc, probe around the glue areas and see if any dc is floating around. May be nothing, but worth a try.

 

[Raccoon1400]

I would be suspicious of where that black glue was on the board. I have had some instances where it became conductive and contaminated the board. by using an oscilloscope set to dc, probe around the glue areas and see if any dc is floating around. May be nothing, but worth a try.

Already removed all glue.

 

[Raccoon1400]

I think I should reconsider the trimmers... I'm suspicious of that center voltage pot.

 

[Northwinds]

Did you check the guitar jack input itself? Bad cable used?