Mesa Dual Rec Solo Head hum

[diydidi]

Hi Guys.
I recently took in this amp for repair. I don't normally take on Mesa amps as I don't particularly like to work on them
Anyways, the amp works fine on both channels, but it has a nasty 100Hz hum. Pulling V1 doesn't help, BUT pulling V2 its gone!!
The LDR's 5&6 are good. Also the two gain pots are good and have returns to GND. I have checked the filter caps and all test good and I have no ripple after the choke. I even jumpered the capacitors with known good ones, but not even a slight change heard.
The heaters are balanced.
Replacing V2 doesn't help either.
I did notice that C12 is very sensitive to touch, increasing the level of hum. Replaced it, but no luck. Still loud hummmm.
Any ideas?

[SoulFetish]

Hi Guys.
I recently took in this amp for repair. I don't normally take on Mesa amps as I don't particularly like to work on them
Anyways, the amp works fine on both channels, but it has a nasty 100Hz hum. Pulling V1 doesn't help, BUT pulling V2 its gone!!
The LDR's 5&6 are good. Also the two gain pots are good and have returns to GND. I have checked the filter caps and all test good and I have no ripple after the choke. I even jumpered the capacitors with known good ones, but not even a slight change heard.
The heaters are balanced.
Replacing V2 doesn't help either.
I did notice that C12 is very sensitive to touch, increasing the level of hum. Replaced it, but no luck. Still loud hummmm.
Any ideas?

Are C91 and C82 in the LV power supply Brown Radial Sprague electrolytic caps? If so (and even if they aren't), check to see if any electrolyte is leaking onto the signal path. The Sprague caps I'm talking about won't necessarily show any signs of bulging, but known to leak electrolyte into stages and cause ripple voltages to couple with audio signals. That maybe one thing to check.

[SoulFetish]

Also, it looks like the value of C12 is 20pF? It's tough to read in the schem. If it's a ceramic cap, ceramics tend to be piezo-electric. So, this often manifests itself in annoying microphonics, especially when directly coupled to the grid. Plus, unless they're a specific type of ceramic (NPO/GOG, etc.) they have garbage temperature stability. Most types of caps make pretty good little microphones anyway, some better than others.