Mesa dual rectifier (trem-o-verb) repair various issues

[diydidi]

Any Mesa experts out there?
I recently received this amp for repair. It had been sent to various techs without any luck.
The general complaint is that the red channel is much softer than the orange(clean) channel.
Also the footswitch/channel switching isn't working. The footswitch itself is confirmed working.
Some one removed the 120ohm sand block resistor from the 50V tap of the power transformer and added a little 12v transformer to run the switching matrix circuit.
Now I don't know if all of these problems mentioned are related somehow.
I suspect the vactrols, but don't know how to test them properly.
Any help will be much appreciated.

[Enzo]

The switching isn't right. SOlve that, and I think there is a good chance the level problem will solve itself.

Vactrols are just an LED and a photo resistor. You can test the LED part by checking voltage across it. With power to one end and some completing circuit on the other, it is just an LED inside, so is there a typical voltage drop across the LED pins or not? The other half is a photo resistor. It has a high resistance dark, and a low resistance when lit. SO light the LED off and on and measure the resistance with a meter. It goes up and down or it does not.

[Mick Bailey]

There are two makes/types of vactrol fitted to these amps. Earlier amps run the LEDs too hard and they fail. There's a service bulletin type change + modification to lower the current.

If you're really stuck for parts/time you can get lucky and grind off the LED side until the LDR is exposed, then heatshrink another LED in place (file the LED dome flat).

I use two mutimeters to test these vactrols. I have a pair of hook connectors to connect the resistance side to one meter set to read resistance. The LED side will turn on with the other meter set to diode test. If the vactrol is good you'll get down to a few 10s of ohms just on meter voltage. Just make sure the amp is powered off and there's no voltage lurking on the switching side.

It's pretty common to find weak or non-functional vactrols in these amps. They cause all kinds of signal separation and volume problems.

[diydidi]

There are two makes/types of vactrol fitted to these amps. Earlier amps run the LEDs too hard and they fail. There's a service bulletin type change + modification to lower the current.

If you're really stuck for parts/time you can get lucky and grind off the LED side until the LDR is exposed, then heatshrink another LED in place (file the LED dome flat).

I use two mutimeters to test these vactrols. I have a pair of hook connectors to connect the resistance side to one meter set to read resistance. The LED side will turn on with the other meter set to diode test. If the vactrol is good you'll get down to a few 10s of ohms just on meter voltage. Just make sure the amp is powered off and there's no voltage lurking on the switching side.

It's pretty common to find weak or non-functional vactrols in these amps. They cause all kinds of signal separation and volume problems.

Thanx Mick. I tested some of them and some new ones from my bin ( not the same as ones in the amp)
They are vtl5c3. I get around 15k ohm. Souns a bit high.
I will check the ones in the amp and report back.

[Enzo]

How much current through the LED? The 5C3 specs show 30k on resistance at 1ma LED current, but down to 5 ohms at 10ma. 40ma gets you 1.5 ohms. Off resistance about 10 meg.