Champ 12 Torres mod and OD channel problem

[Marc]

I am having a bit of trouble with a Torres mod for a champ 12.
It came to me not functional and with a request to change the overdrive pots to lower values (500K and 100K) in the hopes that it would make the OD channel more usable.


the attached schematic shows previous mods ( Ray Ivers recommended) that were done to the amp which worked quite well.

Once the OD pot change was complete the channel switching would make the OD channel full on, no gain or volume control.

The schematic shows clean voltage at the switching LEDs to be 1.68V and OD to be 3.28V. I get the clean voltage but the OD voltage is 21.4V? the 390 resistors that feed the voltage to the LDRs read 390 ohms. I thought maybe one of the dual vactek optoisolators was bad since the OD panel led did not light. changed it and the led still doesn't light and the od channel is still full on. I subbed in an LED for the LDR 2 to see if the voltage was working and the led lit and the panel led lit too.

Another odd thing is the tone stack has almost no effect. Clean volume works in clean position but not in OD. So I'd surmise that LDR1 is functioning.

Any ideas would be appreciated as I am stumped at the moment.....

thanks,
Marc

[Gunny]

The Torres mod has to do with cathode bypass cap values and I know from experience that it works well.
Perhaps the different value of R26, overdrive, won't allow the LDR to work in it's normal range?? Can you change the circuit back, even temporarily, to confirm that?

[Marc]

Hey Gunny,
thanks for the response.
I did try the old pots and they made no difference.

this is actually a different Torres mod that involves changing the pots, no caps are changed in this mod. the intent is to load the OD signal down with lower value pots to tame it.


Marc

[Marc]

well, I found the problem earlier this week.
It makes perfect sense, the indicator led for the od channel had a bad trace to ground so the circuit wouldn't engage the optoisolater.

the changed pots in the od channel work pretty well to tame the od into a more usable range....

Marc