Gallien Krueger 400RB Hum

[ddt]

Hi folks,
I'm working on an early version of GK 400RB and can't figure out why it's humming. The interesting thing is that when preamp is disconnected, no hum is audible, only a little bit of noise.
For those who don't know 400RB too much, the amplifier consists of a preamp board in front, power amp board at the back, and transformer on the right hand side, next to the power amp board.
Preamp is connected via 3-pin connector bearing +15V, -15V and signal, grounds are connected through the chassis.


Here are things which I've already tried:

1. Replaced power supply caps with 2 x 2200uF / 63V. Originally there was 2 x (4 x 470uF / 63V). They still seem to be good though, checked on a DMM cap meter.
2. Moved away AC wires further from the preamp board.
3. Installed shielding between the transformer and pre-amp board, which reduced some hum but the main part still remains.
4. Measured ripple on filter caps right behind the rectifier bridge, shows 120Hz 94mV RMS on top of the +-57V DC. There's no ripple behind the stabilizer on the +-15V rails.
5. Tried connecting front and back panel grounds, no difference.
6. Took out the bridge rectifier and measured all diodes with a DMM, all good.

Schematic is attached for your reference.

It's a mystery to me where the hum is coming from. Any help is appreciated.

[The Dude]

Is the hum there with master volume control all the way down? How about the input gain? Does it have any effect on hum?

[Enzo]

So isolate the problem. It is in the preamp, OK. R47 appears to be the aster, does turning that down turn the hum down? You have FX loop jacks. Connect the send jack to the input of some other amp, is the hum present? Likewise, plug a guitar into the FX return and dial it to zero. Still get hum? Do ANY of the switches affect the level or tone of the hum? The attenuator the mid cut, high boost low cut. And does R7 GAIN control affect hum level? Do ANY of the other controls affect the hum in ANY way?

Maybe getting ahead of myself, you have good 15v supplies at the supply, but is it getting to the ICs themselves? I mean right on the IC pins, are the 15v rails present? And look at each op amp, is there DC offset on the output pins of any?

[g1]

There's no ripple behind the stabilizer on the +-15V rails.

Not clear on this wording. Do you mean no ripple at points B and C in power supply? And this is with preamp connected?

[Enzo]

I read "stabilizer" as regulator, but I am just assuming...

[ddt]

The hum doesn't go down with master volume, that's what's confusing me. Also there's no hum with preamp disconnected.
The ripple is before the regulator (stabilizer), on the 57V rails, between "A" and GND and "D" and GND. No ripple on 15V rails, points "B" and "C".
I forgot to mention, yeah the hum from SEND jack is within limits but DIRECT has the hum a lot, just like in the speaker.
Regarding the controls, none of them affects the hum, neither input gain nor switches, etc.

[ddt]

I have a feeling that there's actually slightly more of the hum when the master is on 0 or around.
When master volume is turned up, the "white noise" is getting superimposed on top of the hum, which is normal I guess, these amps are noisier from my experience, depending on the BOOST, GAIN, and filter settings.
Other than that, the amp plays just normal.

[ddt]

Plugged the guitar in the RETURN, with volume down, no change, still humming.
Guitar actually plays fine through RETURN when turned up but with hum.

[Enzo]

OK, the master down does not kill the hum but hum gone if cable is detached. SO I see three pins, A,B, C. A is output and other two 15v rails. How is the ground connection between preamp board and power supply made? Looks like chassis to me. IS the input jack making solid contact with the chassis? And are the speaker jacks making good firm contact with the chassis? By the schematic those are wher the grounds connect together. A poor chassis connection with either of those jacks can make hum.

[ddt]

Thanks a lot for help, Enzo.
I've tried to clean up all chassis body connection between the front and back of the chassis, also cleaned front and rear jacks connections to the chassis, still hum, no difference.
I've previously tried connecting grounds between front and back with a wire and heard no difference either.
Should I start looking at whether all opams are getting proper voltages on the preamp board?
This one is really challenging

In the worst case scenario, I have another 400RB which is very similar to this one, so I can start poking around and compare. But it doesn't seem to me anybody touched the amp before me.

[Enzo]

The master has no effect, and disconnecting it stops the hum. SO your issue is between the master and th edge of the board. A missing ground, or a ground that doesn't belong there come to mind.

It reminds me of the old joke: Guy is looking on the ground near a light pole. SOmeone asks him what he is looking for. He says "I lost my car eksy over by the fence." SO why are you looking here instead of over by the fence? "The light is better here." I tend not to think your op amps are starving if the problem is after them.

[ddt]

I disconnected the signal wire and left only +-15 wires connected between the preamp and power amp. Guess what, it's not humming.
When both +-15V are disconnected, no hum either.

[g1]

Measure resistance from ground lug of master pot to sleeve lug of input jack. It should be zero ohms.
How did you connect the grounds from front and rear panel? At the panels? Try a wire from sleeve lug of input jack to sleeve lug of speaker jacks.

[ddt]

I think we have a winner!

I've tried connecting power supply (power amp board) ground, from one of the big 2200uF filtering caps, directly to the ground of the master volume pot. And voila, THE HUM IS GONE! No we know that it's the ground connection somewhere in the chassis but I couldn't find where. All chassis panels and lids and all surfaces I cleaned with sand paper, sprayed with contact cleaner and tirelessly put together again without much of an effect.

Previously I have also tried connecting the sleeve lugs of input and speaker jacks, no difference, resistance is 0 Ohms even without the wire.


Thanks everybody for their inputs, really appreciate the help!

[ddt]

Oh my, I was too fast with conclusions. Now the amp is squealing, depending on the GAIN, BOOST and MASTER volumes.
Also frequencies differ, depending on the EQ settings.

It's not squealing without the ground wire from my previous post.

Any ideas?