Mesa Dual Rec Optocoupler

[ThisLifeILead]

Howdy folks,

Got a friends Dual rec on the bench. Noticeable hum on both channels But i feel like im on a goose chase with this thing.it seems to be moving around when looking for it on the scope.Hum disappears when first tube is pulled. Did mesa use V1b for something? i cant find anything on the schematic.
Checked all grounds and replaced all the tubes. Heater winding centertapped.
Scoping the switching legs of LDR1/2/3/4/5/6, it seems 1/2/4 seem to be leaking a couple mv of signal when closed, strumming a guitar. the rest show nothing. Would it be better to test the LDRs on their LED side? (voltage drop etc etc)
Do optocouplers even leak or would that be a switching supply issue?
Lastly V3a's Plate is quite noisey compared to other plates. the resistor measures ok, BUT i just read on this forum that mesa laid resistors over other traces and caused problems. This amp is from 1996. Goona go look into that now.

Schematic link:
https://goo.gl/images/EZuu6O

[nosaj]

Howdy folks,

Got a friends Dual rec on the bench. Noticeable hum on both channels But i feel like im on a goose chase with this thing.it seems to be moving around when looking for it on the scope.Hum disappears when first tube is pulled. Did mesa use V1b for something? i cant find anything on the schematic.
Checked all grounds and replaced all the tubes. Heater winding centertapped.
Scoping the switching legs of LDR1/2/3/4/5/6, it seems 1/2/4 seem to be leaking a couple mv of signal when closed, strumming a guitar. the rest show nothing. Would it be better to test the LDRs on their LED side? (voltage drop etc etc)
Do optocouplers even leak or would that be a switching supply issue?
Lastly V3a's Plate is quite noisey compared to other plates. the resistor measures ok, BUT i just read on this forum that mesa laid resistors over other traces and caused problems. This amp is from 1996. Goona go look into that now.

Schematic link:
https://goo.gl/images/EZuu6O

Looks like v1b is unused. Just because a resistor test ok it can still be noisy, if it's noisy change it out. Just work with the facts on your amp not what is out there on the internet it'll have you running in circles.

nosaj

[ThisLifeILead]

That makes alot of sense haha. Ive been going around in circles on this one.
Just for clarification the grid on v3a looks good. The signal directly off the tube socket v3a plate is ugly. Thats enough evidence to swap it? Mesa boogers are a nightmare to disassemble.

[nosaj]

That makes alot of sense haha. Ive been going around in circles on this one.
Just for clarification the grid on v3a looks good. The signal directly off the tube socket v3a plate is ugly. Thats enough evidence to swap it? Mesa boogers are a nightmare to disassemble.

Have you swapped tubes?

nosaj

[ThisLifeILead]

swapped the tubes and about a month later the hum started.

[nosaj]

swapped the tubes and about a month later the hum started.

If you play musical chairs with the 12 ax7s does the signal at v3 change?

nosaj

[ThisLifeILead]

Yea the noise stays on v3a. so with that said i will resolder the tube socket connectors and replace the Plate resistors on V3 and perhaps V1 while i have it apart.

[ThisLifeILead]

Well i didnt get the board out i just replaced V3a's plate resistor from the top. it was super tedious and the pad started lifting slightly. SO, if im goona do more i think i am going to remove the whole board for easier access. Looking at it, disconnecting the whole front panel should allow enough wiggle room to flip the board over. The exception is the Channel LED wires are too short to flip the board over. Would you guys clip them and resolder after? to me that seems alot simpler and less invasive then unsoldering ALL the critical wires to the board to get the whole board out.
The resistors swap made no difference in hiss n hum. I am going to re-scope the grid/plate signals throughout the amp and report back. no point focusing on the LDRs.