Marshall Valvestate VS65R Channel Two Out

[misterc57]

Starting to debug this amp, clean channel works fine, gain channel has no output.

No footswitch involved, using the front panel channel select switch. When you toggle from clean or gain, there is a loud pop.

Trying to understand the clean and gain path through the switch. Which ICs are in the gain path.

Thank you

Marshall-VS65R-65W-Schematic.pdf

[misterc57]

and this seems odd. IC6 appears wet, like it has sweat on the top of it, which I was able to wipe off. Everything else in the amp is dry

[misterc57]

12AX7 test fine

[g1]

The 5201 IC's do the switching (IC 2,4,5). They are switching type op amps. Control voltage at pin1 determines which set of the IC's inputs are used.
Check that DC is changing on their control pins (pin1), and output is changing at output pin (pin5).

[misterc57]

The 5201 IC's do the switching (IC 2,4,5). They are switching type op amps. Control voltage at pin1 determines which set of the IC's inputs are used.
Check that DC is changing on their control pins (pin1), and output is changing at output pin (pin5).

Clean Channel

IC2 pin 1 14.3 VDC
IC4 pin 1 14.5 VDC
IC5 pin 1 14.5 VDC

IC2 pin 5 0.1 mVDC
IC4 pin 5 0.32 mVDC
IC5 pin 5 19.2 mVDC


Gain Channel

IC2 pin 1 13.8 VDC
IC4 pin 1 13.4 VDC
IC5 pin 1 13.4 VDC

IC2 pin 5 0.21 mVDC
IC4 pin 5 -0.45 mVDC
IC5 pin 5 13.6 VDC

[misterc57]

IC5 only has CLEAN output
IC4 only has GAIN output

IC2 has both GAIN and CLEAN output

Should all of these ICS always have an input signal present from both the clean and Gain channels?

[g1]

So from IC2, for either channel, it goes to the tube. If clean is working, tube is ok. At pin8 of tube it splits off. Ch.1 goes to clean tonestack (VR2,3,4). Ch.2 goes to VR6 and 8, then to IC 6B. Do you have signal at IC6 pin 6 ? From there it goes to VR7 and VR9. If you have signal all the way up to VR9 wiper, IC5 is likely bad.
Also, you reported DC at IC5 pin5 when ch.2 selected, which further points at IC5 being bad.

edit: if you do end up needing a 5201 IC, there are lots of counterfeits out there that you want to avoid. These should be legit:
https://www.amplifiedparts.com/produ...itch-8-pin-dip

[misterc57]

So from IC2, for either channel, it goes to the tube. If clean is working, tube is ok. At pin8 of tube it splits off. Ch.1 goes to clean tonestack (VR2,3,4). Ch.2 goes to VR6 and 8, then to IC 6B. Do you have signal at IC6 pin 6 ? From there it goes to VR7 and VR9. If you have signal all the way up to VR9 wiper, IC5 is likely bad.
Also, you reported DC at IC5 pin5 when ch.2 selected, which further points at IC5 being bad.

edit: if you do end up needing a 5201 IC, there are lots of counterfeits out there that you want to avoid. These should be legit:
https://www.amplifiedparts.com/produ...itch-8-pin-dip

That explanation of the circuit really helps me. Will report back. Thank you

[misterc57]

In Gain Channel mode


I have signal at IC6 pin 6, C55, IC5 pin 6. No signal at IC5 pins 7 and 5.

So it looks like a bad IC5

[g1]

Bad ch.2 volume pot (VR9) would also result in no signal at IC5 pin7. You could try desoldering that pin7 and then check if signal appears at the solder pad.

[misterc57]

Bad ch.2 volume pot (VR9) would also result in no signal at IC5 pin7. You could try desoldering that pin7 and then check if signal appears at the solder pad.

Will check that out. Thank you

[misterc57]

Bad ch.2 volume pot (VR9) would also result in no signal at IC5 pin7. You could try desoldering that pin7 and then check if signal appears at the solder pad.

VR9 is open. Ordering a new one

[misterc57]

Replaced VR9, problem fixed.

Thank you everyone!