Peavey Valveking II 20W Combo Gain Switch

[Delta362]

I am working on a Peavey Valveking II 20W combo that came in with the complaint that the gain switch isn't functioning. During initial troubleshooting, I confirmed that the amp will switch from clean to lead, and the boost pushbutton works. With the channel and volume buttons pressed in and then using the footswitch, the channels and boost toggle correctly as well. But, I'm not able to get any noticeable change when using the gain pushbutton in any configuration.

I've confirmed the contacts of S2 for boost and gain both switch correctly. S13, the boost relay, contacts switch correctly. From the schematic, it appears the one set of contacts on the gain switch removes the short across D28/D29/R104 when in boost mode. That's working correctly. That leaves the other set of gain contacts that control Q8, Q22, Q4. I've checked D2, D11, D10 with my meter and they are reading reasonable diode values. Same thing with Q4, Q8 and Q22. When trying the gain on and off with boost mode on the lead channel, I get the following voltages:

Gain On / Gain Off
Q8
D -0.127 / 0.174
G -0.151 / 0.144
S -0.132 / 0.120

Q22
S -0.125 / 0.123
G -0.149 / 0.141
D 0 / 0

I'm stumped as to what I'm overlooking here. Any ideas would be greatly appreciated. Thanks!

Schematic is attached.

[J M Fahey]

Switching Gain/Boost ON-OFF , what do you measure at:

1) Q4 collector

2) R75-R23 node (node is where 2 or more component leads get together, usually joined by a track or wire)

3) D2-D4 node (their cathodes)

4) R83-R116 node, which also is S1 pin 2, try with this switch in both positions

5) measure also continuity to ground at connectors P50-P55, with nothing connected at the Channel/Boost jack J5

6) Does S2, part of Relay S13 get 15V on any of its pins 1-2-3?

All this to verify that there actually exist a control signal reaching suspect Fets Q8-Q22 or whether *they* are shorted.

FWIW,

reading reasonable diode values. Same thing with Q4, Q8 and Q22

is NOT normal, those are f*ck*ng MosFets.

[Delta362]

Thanks for the reply, Juan! Here are the results:
1) Q4 collector reads -11.8 (gain off) and +14V (gain on)

2) R75-R23 node +13 (off), +6.8 (on)

3) D2-D4 node (their cathodes) +14.05V (off), 0.09 (on)

4) R83-R116 node, which also is S1 pin 2, try with this switch in both positions +14.92 in clean, 0 in dirty

5) measure also continuity to ground at connectors P50-P55, with nothing connected at the Channel/Boost jack J5 (8 ohm, 16 ohm, 13 ohm, 17 ohm)
As a side test, I plugged in the footswitch and these pins read the following:
pins 4 and 3 read 0 all the time. Pin 2 goes from +15 clean, to +1.9V dirty. Pin 1 goes +15 (boost off), 2.285 boost on.

6) Does S2, part of Relay S13 get 15V on any of its pins 1-2-3?
left side of gain switch had -0.250 on all 3 when out, and -0.2, -0.2 and 0 when in.
right side of gain switch had 0, +15, +15 when out; and 0, 0, +15 when in
so yes, the +15 that's tied to the flyback diode D1 on S13's coil is always present on the third pin of the right side of S2.

[Delta362]

P52 resistance readings are:
Pin 1 - 330 ohm
Pin 2 - 23k
Pin 3 - 10M
Pin 4 - 0 ohm
Pin 5 - 3k and charging
Pin 6 - 24k and charging

[J M Fahey]

Thanks for the reply, Juan! Here are the results:
1) Q4 collector reads -11.8 (gain off) and +14V (gain on)

Thanks, this first reading is enough.
I suggested others along the control chain just in case the first one was not conclusive but clearly the fullchain works well, we DO have changing controlvoltages, so possibilities are:

1) C32-Q8-Q22 shorted (which I suspect since they read as bipolars or plain diodes while they should measure open in almost all psitions but one,reversed Source to Drain) or

2) open R49 (or cracked solder/tracks connecting it)

Remove Q8 and Q22 and measure voltage at their gat pins, if it now changes +/-14V , or at least a few full Volts, get a couple fresh 2N700x mosfets.

[Delta362]

I pulled Q8 and Q22. The gate voltage now switches for -12.9V (off) to +14V on. I'll order and replace Q8 and Q22 (2N7002E) and report back. Thanks for your guidance, Juan!

Next question that I have is what this circuit is doing. How does this circuit work to give an increase in gain? Thanks!

[J M Fahey]

Next question that I have is what this circuit is doing. How does this circuit work to give an increase in gain? Thanks!

You have a gain block or stage, V1 and V2 cascaded.

It has huge gain, which is attenuated to 100X through feedback, from V2 plate to V1 cathode: R8 220k into R108 2k2

When activated, Q8 + Q22 short V1 cathode to ground (we are talking signal, not DC) so both V1 gets maximum gain (since its cathode is bypassed by C51 470nF and NFB is destroyed, so that gain block gets full open loop gain.

[Delta362]

It makes sense to me now. Thank you for the detailed explanation!

[Delta362]

Sorry for the delay in returning to this thread with a follow up. Replacing Q8 and Q22 solved the issue. Thank you for your help!