Fender Stage112 hum

[Wakculloch]

I've had a series of these amps in for repair with hum problems coming from the preamp. I was expecting to see this as a regular topic, but found nothing. I'm now thinking it may be a mains voltage issue, with the multitude of wires carrying 240v connected to the board, and across the board to the thermal switch.
The mute circuit works really well, but as soon as a jack goes in the hum starts. Usually worse on the clean channel.
Anyone have any experience, tips.
Moving grounds makes a slight difference and i'm pretty sure its not from the secondary side of the power supply.

Ta

[J M Fahey]

What happens if you plug a shorted plug into the input?
What if you plug guitar straight into amp (no pedals, tuner, anything) but set guitar volume and tone controls to 0?
What amp knobs affect noise?
Is it hum (deep muffled sound) or buzz?
Thanks.

In general the Stage 112 is basically the same amp as countless other Fender SS amplifiers, just with a more powerful amplifier and speaker, so it shouldn´t be much better or worse than others same family.

[Wakculloch]

All volume/ gain controls at zero, shorting jack in the input.
It's not really bad, but the owner plays in church and it has received comments.
It's 50Hz.

[J M Fahey]

Oh.
Wrong choice of amp to begin with, a 160W heavy Metal beast has no place inside a quiet Church
Same as complaining about lack parking lot space ... for his 16 wheeler truck

In any case: make an attenuation box, just a 100k Audio/Log inside an Altoids can or similar, with In-Out plugs, and insert it between Pre Out and Power Amp In.
Dial down as much as you wish.

I have done that countless times in PA systems in quiet vocational Theatres, Churches, Slide/transparency picture contests, even Guru Maharaj-Ji "meditation room" at Buenos Aires´ Prison for Women, go figure.

[Jazz P Bass]

As a rule, these amps do Not 'hum' when in proper working order.

You need to isolate the problem.

Is it the power supply?
" i'm pretty sure its not from the secondary side of the power supply"
Does that mean that you have checked the supplies?

[Wakculloch]

With nothing plugged in, ie mute engaged, silence.
With a shorting jack plugged in the OD channel hisses, but no hum.
The clean channel hums. I've checked the +/-15v supplies and there's no 50Hz riding on them.
It feels like a grounding issue, but all grounds appear good. Gonna get the magnifying glass out.

[Enzo]

Several thoughts.

Ripple on power supplies will be 100Hz, not 50Hz. 50Hz would be from grounding, or picked up fields. If you look at the schematic, the two channels share power supply, so if there was a bad power rail it would show up in both channels.

I am looking at the Stage 112SE drawing, does that match your amp? I see the input branching into U1 for clean and into U2 for the drive channel. Apparently the drive channel works well. The outputs from U1 and U2 mix into U4, so everything after U1,2 is common through the amp. To me that points to U1 as central to the issue. I could be fooling myself, but I'd look there.

Since this affects the clean channel, does the clean volume turn the hum up and down in level? DO the clean tone controls affect the tone of the hum? Does the mid-shift switch affect the tone of the hum? We want to see if the hum is coming in before or after those controls.

It is possible Q13 is leaky.

[Wakculloch]

The hum is present with all controls at zero. As you increase the volume the hiss begins to dominate.
Between the volume control and the point where the channels meet there is only Q13 and two resistors so i shall investigate.
One resistor is at one end of the board, Q13 at the other, so a nice long trace to pick up crud.

[Enzo]

How about my questions? Do the volume control affect the hum in any way? Do the tone controls affect the tone of the hum at all?

Resistors don't really have much ability to cause hum, unless maybe they are open. I might try just removing the JFET as an experiment.

[Wakculloch]

Other jobs got in the way, but back on it.
Controls make no difference. Fets checked ok but replaced just to make sure; no change. Removed the fets, hum still there; but the weird thing is, if i activate the channel change switch the hum still goes away on the OD channel, even though the fets that do the switching are not there. Ho hum- bit of a headscratch is in order.

[J M Fahey]

DID you try the attenuating potentiometer in the loop?
Does hum level respond to its different settings, hopefully down to 0?

[lowell]

Wakculloch, did you ever fix this issue? I have one in with the exact same issue. Everything you posted is exactly the same.

I've noticed however, if the reverb is off on the clean channel (using the Footswitch jack) the LOW hum goes away. U6B pin7 has +15v when the amp is in this state, per the schematic.

Oddly enough, with the Reverb pot at 0, and reverb ON, still hum. I even tried shorting the center lug on the Reverb pot to ground. No effect. Hum persists.

The hum gets WAY louder when Reverb is on if I turn up the Reverb pot. This is with no tank connected so that'd make sense I guess.

The issue must be Before U4A which mixes Overdrive/Clean channels. But why would turning the Reverb OFF cause the hum to go away?

Here's the schematic - https://schems.com/bmampscom/fender/stage_112se.pdf

[lowell]

I tried grounding U4 Pin2 and I get a super loud hum. I then tried this with a BFC, super loud hum. Is this normal? I would think if anything, it might kill the hum, not make it way worse.

I also am noticing that with the reverb knob at zero, if I touch the return RCA plug, I still hear some buzz. I confirmed that the reverb pot is indeed grounded. Maybe that's a clue?

[lowell]

Okay no bites so I assume everyone is as stumped as me. I went ahead and recapped the filters for the high and low supplies. Didn't help. Here's a recording of the hum. It truly sounds like a filter cap ground is bad...but how could that be ONLY on the clean channel? Finally...in my audio clip here... It's the clean channel, then I turn up the reverb pot which makes the hum much louder, then I switch to the Dirty channel.

Stage112.m4a

[g1]

Sounds like a bad ground to me. Does the reverb add any hum to the dirty channel? (for dirt channel you should have gain low, vol. max.)