Crate VC50 Head unit with only channel “A” working

[Repairfreak]

Hello,
I have another Crate VC50 amp on the repair bench. This time its the VC50H
Head amp, rather than the combo model with 3 cabinet speaker. The “A” (clean) channel is working, but have virtually a completely dead “B” (dirty) channel. I checked the continuity of the push switch for changing channels and this switch seems like it doing its job. All tubes test fine. I checked the footswitch jack for proper switch closing, and it is fine. Trying to wrap my head around how the switching is accomplished. It appears to be by the toggling of a +10v to 0v on one line and -10 to 0v on another line. Should I be checking the -10V +10V power supply section off main PT separate winding, or could the Vactrol “like a optoisolator”, but light dependent resistor instead of photo transistor be defective? It appears the no 4 Vactrol might be used in the channel A/B switching, as well as the -10/+10v supply “if I understand correctly?” Any suggestions of where I should be looking for a defect in the circuit?
If not, could someone please break down the channel A/B switching method down into plain english for my understanding.
Thank you
Schematic found here: https://drive.google.com/file/d/1V1M4gZApJuNIbGpUFbJfuBiAL1BCHRp7/view?usp=drivesdk

[Jon Snell]

If you have signal on TP8 but not on TP9 look on the wiper of P8 for signal.
Check the Test Points for a clue with your oscilloscope.

[Enzo]

It switches from the good clean channel to the bad dirt channel. That tells me the switching itself seems to be working, at least the switches and most of it. Switching runs on -10v, and since the clean channel switches off, we know you have -10v. I don't see +10 involved in switching.

SO turn on the dirt channel and track down where the signal stops. We are not looking at parts, we are looking at a circuit. it may be a bad part, it may be a solder joint, it may be a cracked copper trace, etc. Once we find WHERE the problem is, the what will fall into place.

First some basics: are both heaters glowing in both V2 and V3? DO we have B+ on pins 1 and 6 of those tubes? Do pins 3 and 8 all have non-zero DC voltages?

You have an excellent chart of clearly marked test points. Use them. APply some sort of signal - I use a CD player - and see if it progresses through the channel. TP-3,4,7,8.

Switching isn't hard. VActrols are just like relays, just like switches. You pull current through the LED part and it makes the resistor part go low resistance - ON. Let teh LED be dark and it has high resistance - OFF. So for clean, we turn on OC1,2,4. OC1,2 are series in the clean signal path, so they pass signal along. OC4 is on the dirt channel. and it shunts (grounds out) the signal path on its way to V3.

Dirt channel turns all those off, so no more shunt at V3, plus OC3,5 turn on to pass along the durt signal.

There is no way for OC4 to stick on when you turn off OC1,2, so I doubt it is bad. And TP11 will tell you if the dirt vactrols are being energized or not.

Vactrols can fail just like every other part in an amp, but I have no reason to focus on them at least not at first. That is what the signal; path test points are for.

[Enzo]

MAybe this PDF be easier to use?

[Enzo]

And just noticed, down by TP10,11 those control lines also serve as S and S-not which control which reverb control is active.