Fender Silverface champ noise issue

[beedoola]

My guitar pots are crazy with the amp but not with other amps.

The Can cap tests fine and I've replaced the electrolytic capacitors. The fiberboard was pretty dirty so I removed all the components, cleaned the board and reassembled. I replaced the 2W 1k and 10k resistors.

Anyone have any suggestions as to what I should test. Could it be a tube issue? I swapped the 12AX7 with another and no change.

[The Dude]

Was the noise there before you cleaned the board? What did you clean the board with? I'm wondering if the board has become conductive. If the noise wasn't there previously, take a hair dryer to the board and dry it out as best you can.

[beedoola]

Noise was not there but someone had added capacitors from the input jacks to the board and another resistor to ground and 2.2m and increased the input resistor to 2.2m.

I removed these mods and that’s when the noise came, so it looks like those mods were done to deal with the noise issue.

[vintagekiki]

Is there a 1M resistor at input jack 1.
Look at the schematic.
https://schematicheaven.net/fenderamps/champ_aa764_schem.pdf
If there is oxide on the chassis below the input jacks clean it.

[beedoola]

I put a 1M resistor back on the input Jack when I rebuilt it.

[Helmholtz]

What kind of noise: hum, hiss, crackling?

My guitar pots are crazy with the amp but not with other amps.

Please elaborate.

[beedoola]

Also, I used rubbing alcohol with a toothbrush to clean the board and then rinsed with water. I left the board sit outside for a few days before reassembling.

noise is that my guitar pots are scratchy and hitting certain notes makes some noise that comes through the speaker, like the vibration of that note frequency effects something.

I haven’t tried yet to do this with the amp outside the chassis - to see if it’s the vibration of the cab causing something in the chassis to vibrate.

[Helmholtz]

Insert a spare phone plug into input jack and see if you can measure any DCV between tip contact and ground.

[eschertron]

I'd second the hair dyer procedure. "airing out" for a few days may not be sufficient to drive any residual moisture away. When using a hair dryer, the board/components will get warm to the touch but not hot enough to melt the solder

10 minutes with the hair dryer on MAX is where I'd start. If that doesn't help you haven't lost anything but ten minutes...

[Helmholtz]

Just make sure the caps don't get hotter than 80°C to 100°C.

[eschertron]

I'd hope my hair doesn't get to 100C

[vintagekiki]

My guitar pots are crazy with the amp but not with other amps.

Read informatively

https://music-electronics-forum.com/forum/amplification/guitar-amps/vintage-amps/50709-62-kay-503a-volume-squeal
I'm getting a crazy annoying and loud squeal when I turn the volume knob up

#12
Most likely to oxidize the ground from input 1.
Clean the chassis below input 1 and replace jack.

#14
Success!! I took the jack off, clean the chassis, used a little emory cloth on the part that makes contact with the tip of the jack and emory cloth'd the inside of the jack. Then used contact cleaner on the parts and Voila!

Now all 3 of the jacks work, pots are all cleaned, filter caps replaced with fresh stock and a few smaller caps. Amp sounds great at 10, tremolo is very sweet too.

#13
About repairing potentiometers

[Helmholtz]

I'd hope my hair doesn't get to 100C

I measured up to 120°C with a hair dryer at close distance.
Distance makes the difference.

[beedoola]

Measured DC voltage at the input jacks and there is nothing.

I measured some of the voltages and compared to the original layout.

For instance, point "A" coming off the can cap and 10K 1W resistor shows "330v" on the layout but I'm getting 394v.

on Pin 6 of the 12AX7 the layout shows 205v and I'm getting 258v.

Pin 3 and 4 of the 6V6 show 350v and I'm getting 420v

The AC voltage going from the transformer to the 5Y3 shows 320AC and I'm getting 357.

The voltage coming off Pin 3 of the 12AX7 is about right - 1.8v

Pin 8 of the 6v6 shows 18v on the layout and I'm getting 27v.

Any of these too out of the ordinary?

[Enzo]

You are looking at the AA764 schematic, but I can see a 1972 date stamped inside the chassis in your photo. My schematic for the 1972 version matches your voltages pretty closely. There were many CHamp models over the years, they are all different. Always use the correct drawings.

Champ.pdf