Polytone Mini Teeny Brute Reverb Issue

[misterc57]

Reverb switching on/off is not working. It is always on regardless of footswitch toggling.

It has a stereo jack switch for the foot pedal. Controls reverb and distortion.

With no plug inserted the reverb portion of the jack is at 1R to ground. Reverb effect works great and no background noise.

Amp OFF: With stereo footswitch plug inserted I toggle from 1R and 1.5M to ground.
Amp ON: With stereo footswitch plug inserted I toggle from 1R and 750K to ground.

Both switch positions reverb is on. When at 1R no background noise. When at 750K there is also a slight background hum (sounds like a fridge running)

Have tried two different footswitches.

Not clear on the attached schematic where the reverb switch is.

[glebert]

It is using OP5, which appears to be a LDR (light dependant resistor) to pass or block the reverb return signal. This is very similar to the tremolo on a Twin Reverb or similar (although those use neon bulbs instead of LED light sources). You should be able to measure the resistance across OP5 and see if it varies with the footswitch. You can also verify if the photocell driver current changes with footswitch changes. Depending on how OP5 is configured you might even be able to see the light turn on (I turn off the lights in the room if I am debugging a tremolo LDR).

[Helmholtz]

Can you switch the reverb off with one end of C50 lifted?
If so, the cap has a short and the OP5 LED will be always on.
If not, the LDR (of OP5) must have gone low resistance.

[misterc57]

I see OP5 on the schematic but not C50.

[Helmholtz]

I see OP5 on the schematic but not C50.

It's close to the footswitch wihin the reverb photo cell drive frame (10µ/35V).

[misterc57]

Anyone have a board layout diagram?

I am not seeing any LDR. Perhaps I have the wrong preamp schematic.

The brown wire on the lower right side of the board is from the footswitch.

Thank you

[misterc57]

Also worth mentioning there is a big pop sound (even makes the speaker push a lot of air) when toggling the reverb footswitch.

[Helmholtz]

Can't help without correct schematic.
Maybe identify ICs and look up datasheets.

[glebert]

See if this schematic is closer.

polytone_mini_brute_series_i_iii_treeny_brute_preamp_sch.pdf

[misterc57]

I think the attached may be the proper schematic with the RC4739 ICs

Polytone Brute Series Preamp Schematic[1].pdf

[Helmholtz]

Ok, so no optos but switching via Jfet (Q1).
I guess the Jfet is shorted.
Drain - source resistance should be high with reverb off.

[misterc57]

Drain - source resistance should be high with reverb off.

D to S resistance toggles from 43R to 57R

It is a J113 on the board

[Helmholtz]

D to S resistance toggles from 43R to 57R

No wonder the reverb is always on.

[misterc57]

Also toggling causes the gate to jump from 0.6mV to -5V

[Helmholtz]

Replace the Fet.