VS102R -- Low-Pitched Hum

[Cooltouch]

I have an old VS102R that I've owned for over 20 years. The VS102R is one of Marshall's ValveState line -- hybrids with a tube in the preamp section and a solid-state power section. Up until recently, it's been a decent amp: reasonably good tone and plenty of volume. Recently, however, I noticed a hum when I switched on the amp. I can quantify the pitch of this low-pitched hum. It's just a bit sharp of the low B on a guitar (2nd fret A string) -- if that helps to clarify things. So, given that the open A string is 110 cps, this slightly sharp B is somewhat north of that. It is always there, but it is loudest at certain settings.

It is quietest when I have the Clean channel selected. When I switch over to OD, it doesn't really get any louder until I turn up the bass pot. For some reason, the hum increases in volume as the bass control is dialed up. This happens whether I'm on OD 1 or OD 2. Kinda strange, I think, that this hum seems to be associated with the OD channel's bass pot. Maybe it's just dirty? But it isn't scratchy sounding, so I kinda doubt it. I'm thinking about just replacing the pot to see if that might cure things, but I'm also wondering if there might be something amiss with the OD bass circuit itself. Plus, there's the matter that, even when I have the Clean channel active, there's still a low (in volume) hum that still exists.

Any ideas what may be causing this?

 

[Jon Snell]

120HZ mains smoothing hum.
Check the main smoothing capacitors have not broken their solder joints causing this type of hum.
C11 & C12, 2200uF 50volts
Touching the capacitors may induce/reduce the hum.

 

[Cooltouch]

Thanks for that info. I'll inspect/resolder the joints. If that doesn't work, I'll look into replacing the caps.

Um, I've never been a fan of touching capacitors ever since learning of the old mechanic's joke of tossing a charged condenser to a noob, hoping he'd catch it. I fell for that only once, and have been careful about discharging capacitors ever since.

 

[PelliX]

Thanks for that info. I'll inspect/resolder the joints. If that doesn't work, I'll look into replacing the caps.

Um, I've never been a fan of touching capacitors ever since learning of the old mechanic's joke of tossing a charged condenser to a noob, hoping he'd catch it. I fell for that only once, and have been careful about discharging capacitors ever since.

Fortunately, there is a trusted high tech utensil used by engineers the world over; the chopstick