Fender M-80 112 non chorus channel bleed

[jvm]

Correct , g1. Audio signal fed through input jack and open plug inserted into PA input jack

[jvm]

Helmholtz, when switching between channels, R27 to ground reads open with drive ch selected and 52 ohms when clean ch selected

[Helmholtz]

Helmholtz, when switching between channels, R27 to ground reads open with drive ch selected and 52 ohms when clean ch selected

You should have 0R between U5C pins 4 and 3 in clean mode. Looks like bad relais contacts.

Try to directly short R27 to ground.

[g1]

You should have 0R between U5C pins 4 and 3 in clean mode. Looks like bad relais contacts.

Try to directly short R27 to ground.

U5 is a 4053 IC, not a relay. Should 0 ohms still be expected there? He did say he replaced it (with MC14053).

[Helmholtz]

U5 is a 4053 IC, not a relay. Should 0 ohms still be expected there? He did say he replaced it (with MC14053).

Thanks, missed that.

Let's see if directly connecting the lower end of R27 to ground makes a difference.

Up to now results are quite confusing.

[jvm]

Grounding R27 made no difference. Note that only the drive channel gain control has any effect on the dirty signal when in clean mode. When drive ch selected, all controls work as they should

[Helmholtz]

Is the point between C35/R63 connected to the power amp input via a shielded wire? If so, check if the shield is connected to ground (Ohmmeter).
Generally check all wire shields for ground contact.

[jvm]

Helmholtz, no shielded wires in the amp at all. It is a single pcb amp

[Helmholtz]

Can you still hear some drive signal if you lift one end of C35?

Does it help to wire a 470p cap between power amp input (U6, pin 3) and ground?

[jvm]

Helmholtz, lifting C35 reduced the level but the signal is still present. Adding a 470pf to ground at U6 pin3 had no effect at all.

[Helmholtz]

lifting C35 reduced the level but the signal is still present.

Wow, that's weird.
Lifting C35 should completely isolate the power amp from the preamp.


I'm running out of ideas.

Maybe a conductive board?

Do you have a scope?

[jvm]

Helmholtz, I have a scope but I'm not very experienced using it.

[Helmholtz]

Helmholtz, I have a scope but I'm not very experienced using it.

Did you use the scope for signal tracing (post #6)?

Do you have a signal source?

[jvm]

I used a audio signal tracer and cd player input. My scope has a sine wave generator.

[Helmholtz]

Ok then try to do the signal tracing with the scope (AC coupling). Feed a 100mV/1kHz signal to the amp's input.
Especially see if there's any signal riding on power supply rails.