Solid-State Amp Techs: Hum in GK 800RB

[simojo]

I recently got an older (1987) GK 800RB with the horizontal switch. Amp sounds pretty darn good, but it has 120Hz hum. The head hums noticeably, even with no speakers or cables plugged in. That part alone is a bit odd. I assume this is the transformer making the hum.

With either the 100W or 300W speaker plugged in, or both, the speakers also emit 120Hz hum when the volume is all the way down, or all up. With the master at about 11:00, the hum is gone. This isnt a bad place to park the master, but in a quiet room when practicing I dont really need to hear the B-flat 120Hz coming off the chassis. Preamp knobs or switch positions has absolutely no effect on the hum, so it appears to be all in the power amp. Plugging in a bass also has no effect.

I know/fix/build tube amps, but I'm not real knowledgeable about solid state stuff. If I replace the six 1000uF caps, and the four 470uF caps, which all look like the main power supply, that will run me about $40. (Nichicon VX series from Mouser). Is it likely to be old caps? Or should I just do the 23-year-old caps regardless, then look for more if I still have the hum? Or is the loud hum the PT going bad? None of the caps look obviously bubbled or blown, so I'm not sure if this is really the culprit.

Any experience in this area is hugely appreciated. Thanks in advance!
--John

[bassman5959]

Check the heads on the caps, if they seem like they are bulging out, even just slightly, they need replacing. I had a similar issue with my 800rb but it was a bad pot on the volume knob.

[WingKL]

Those are some interesting symptoms with the hum nulling out at the master volume. Since according to the schematics, the master volume is at the end of the preamp, the preamp has a share in the issue. If you fix tube amps, you should probably have an oscilloscope. That should tell you where the hum is coming from and why the power supply rejection ratio is low (unbalanced operation - bad opamps or transistors?). It could also be a connection issue.

[simojo]

A few updates:
* So far, I've had two separate, experienced techs tell me if it's humming audibly without any speakers plugged in, it has to be the transformer. Specifically, a transformer that's delaminating, which causes the plates to vibrate in the spaces between plates. With a light touch on the chassis I can feel it vibrating.
* The guy who sold it to me said it was dead quiet when he shipped it, so I suspect it got dropped hard one or a few times during shipment.
* I dont have a scope. But I suspect that if the preamp & boost controls have zero effect on the hum (speakers or not), then it's all in the power section.
* I was told that the nulling may be related to lead dress around the PT. The ol' chopstick poking-and-moving should have an effect on the nulling. I'll have to try that.
* I'll try jiggling the connectors, but since it makes noise with no speakers, I'm suspecting this is not the issue.

WingKL--Please PM me if you have a schematic. I'd love to see what I think I'm talking about. :-)

[WingKL]

Here's a copy of the schematics for the 800RB.

The physical hum with no cab connected and the hum that comes from the cab when it is are two separate issues. My Thunderfunk hums a bit physically but is pretty quiet out of the cab. There are good reasons for toroidal transformers.

If you plan on fixing audio electronics, an oscilloscope is a very good tool to have along with some sort of signal generator.