Scratchy-Static turning Vol/Treb/Mid/Bass Pot on 2nd Ch only of 67' Twin AB763

[nickb]

No, I don't have a scope. I probably should get one and learn to use it.

You can use your meter. Since few meters will measure above 1KHz or so we need to make a simple AC probe, Just make put diode between one side of the speaker and your meter lead with the other lead going to the other terminal of the speaker. Use the meter on DCV. If not steady add a 10nf between the meter leads. Anything more than (say) 500mV with no signal is likely instability.



PS: I didn't spot that the discussion had moved on the real cause when I posted, so this is just for fun.

[nosaj]

You can use your meter. Since few meters will measure above 1KHz or so we need to make a simple AC probe, Just make put diode between one side of the speaker and your meter lead with the other lead going to the other terminal of the speaker. Use the meter on DCV. If not steady add a 10nf between the meter leads. Anything more than (say) 500mV with no signal is likely instability.

[ATTACH=CONFIG]49620[/ATTACH]

PS: I didn't spot that the discussion had moved on the real cause when I posted, so this is just for fun.

So is this a rf demodulator?

nosaj

[nickb]

So is this a rf demodulator?

nosaj

It would work at RF but the point was just to get a cheap meter to detect something over 10Khz. Not perfect due to the diode drop but good enough for the purpose.

[J M Fahey]

Wow! You all narrowed in on the exact culprit! I found the faulty “silver mica” treble cap!

As noted above I confirmed I had DC at the pots. I lifted one leg on all three pots in the tone stack (legs connected to the pots). Sure enough, there was DC on the treble cap. I had replaced original ceramic cap in January with a Silver Mica. I read years ago that this is a good practice. It has been work fine up to a month ago when the scratching occurred....... do you think it just failed?
2.) If not Silver Mica, any other suggestions?

The original Ceramic

Silver Mica was the best option available ... for RF .... because of precision and stability, both traits highly valuable in a precision tuned RF amplifier or oscillator.

In the 1940's somebody thought that since they "were better" they would also be better in Audio.
Not bad for sure, far from that, but not necessarily better in circuits which tolerate 20% variation in their components.

In a Fender Tone Control? .... almost anything works there ... as long as it does not leak.