Marshall Valvestate 80V Clean Channel Dead

[misterc57]

Clean channel has no sound. I pull the board up and see 4 pins on the channel switch soldered together. Probably not suppose to be that way. Compare to next switch to the right.

Was this a hack fix (for what issue)? Any guess? What do you think will happen when I remove the extra solder?

I have attached what I believe to be the proper schematic.

Thank you. MC

Marshall valvestate 8080_8100.pdf

[J M Fahey]

Not an issue.
They wired a DPDT switch as an SPST one because thatīs all they need.
Check "Panel SW2" wiring , to the right of the FSW jack.
They simply paralleled the unused half for better reliability.

This amp switches channels by varying voltage at M5201 pin 1, check it changes following switch operation of FSW pushing.
Mind you, these are in parallel, so keeping either of them pushed will make the other "useless".

Check that voltages change at TR1-TR3 collectors and Red Led Boost Led 2 turns on-off
Then follow voltages to actual Pin 1 in IC3-5-7

[misterc57]

Glad I asked before doing anything. Here is an update.

The LED lights work and toggle for both the clean and gain channels when pressing the channel switch.

When toggling the CLEAN/CRUNCH switch, there is a loud pop.

The clean channel partially works. Only in CLEAN setting, not in CRUNCH setting.

In clean setting I only here audio if the volume pot is set to about 2. Any other setting no sound. Also when there is sound, it does not sound right. Lots of HISS and signal sounds faint.

All pots have been cleaned. Solder connections look good on bottom of board.

[g1]

You didn't say if the lead channel is working normally or not. I assume so but better make sure.

[Enzo]

When you work on an amp that was (apparently) working before it broke, my tip is to resist the urge to change things. AFter all whatever appears to be "wrong" didn't stop it from working before.

[misterc57]

Lead channel is working fine. In both OD1 and OD2 mode.

I have not attempted any repairs yet. Just sprayed the pots and switches.

I am suspicious of the Clean channel volume pot but also wonder why only the CLEAN mode has some sound and the CRUNCH mode has nothing.

Thank you! MC

[misterc57]

More on the clean channel

VR1 test good and smooth on the meter, on the entire rotation of the pot..

Channel one Clean/Crunch mode switch appears to work when measuring resistance from R4 to C30. In one position it has very low resistance, in the other position there is about 8M resistance.

Are LED 4 and 5 on the board suppose to light up when switching between Clean/Crunch mode? I do not see either of them lit, in either position.

Thank you, MC

[g1]

I would just replace the IC that VR1 is across, for starters. If putting that pot in some particular position makes it work, I suspect that IC.

[misterc57]

I would just replace the IC that VR1 is across, for starters. If putting that pot in some particular position makes it work, I suspect that IC.

IC2 TLO71 I will see what I have for op amps. Thank you

[misterc57]

More test results. I tried tracing an audio signal before replacing anything. Signal is never getting to IC2.

I have audio signal on both sides of R3, one side of C2, and one side of C3. Not sure how the circuit should work but I would think there would be audio on both sides of C2 and C3.

No audio signal at pins 2 or 3 of IC2.

I noticed that in CRUNCH mode LED 4 is lit when the VR1 pot is above a setting of 1. LED 4 is not lit when dialing the pot to zero or switching out of CRUNCH mode. I never see LED 5 lit regardless of CRUNCH or CLEAN mode.

Thank you. MC

[g1]

You usually won't see signal at op amp inputs due to them being at 'virtual ground'. That you do see it at one side of C2 and C3 means the signal is most likely getting there.
If you were to swap IC2 and IC4, you would probably find the fault moves to the dirt channel.

[misterc57]

Progress but...

I replaced IC2 with a new TLO72

This brought the clean channel back. Both CLEAN and CRUNCH mode working.

However the volume pot is not working. The volume level is constant, the volume pot dialing from zero to 10 changes the sound slightly from bright to muffled, no effect on volume.

Also after fiddling around for a minute it seemed like the sound was starting to become distorted and the volume level was fading. I shut it down.

Before this I verified the volume pot was working. When signal tracing I could change the level of the signal using the volume pot when I listened at C3.

Gain channel is working fine.

[misterc57]

Argh... just realized that IC2 is supposed to be a TLO71. I thought it was a TLO72 that went there.

[misterc57]

Put in a TLO71 replacement and it is working now! Yay!!! Thank you all for the help!

MC

[misterc57]

What is the best replacement for the TLO71? I will add some to my next parts order.

Available are NE5532P. Is there a better replacement available?

Thank you