Video of hum when I switch on the reverb of my Twin Amp

[tochiro]

I was not expecting repair lessons by Fender but I think they could have shown some interest in my case and tried to help one of their customers who bought an expensive amp in their line.

I can say now that some causes can be ruled out:
- It is not because of the tubes
- The wiring and connections are in perfect condition and were checked
- The reverb pan is in working condition and was checked

All seems to point to a bad filtering of the input current. But what can I check in this regard?

[Enzo]

Input current? You mean the power supply filters? The rest of the amp uses the same power supplies. Sems like ti would hum all the time if filters were weak.

Isolate the problem.

SO if you turn the reverb down the hum goes away. That means it is coming in before the reverb control at least. disconnect the reverb pan cables from the amp chassis. Does it still hum when the control is turned up? That will tell you if the problem is inside the amp or outside.

Dismount the reverb pan and pull it out of the amp cab, and set it as far from the chassis as possible. Did that reduce the hum? Reverb pan transducers sometimes pick up the magnetic field from the power transformer.

Did we determine the cables are in the correct jacks on the pan? A quick check is this. Turn the amp on and turn up the reverb some. Pull the cables out of the reverb pan, and touch the tip of each plug with your finger. One of them should hum through the speaker when touched. That one should be plugged into the OUTPUT jack on the reverb pan.

Have we checked the cables themselves to determine if one has an open ground or shield?

[tochiro]

Yes, I mean the power supply is not correctly filtered. The hum produced by the amp without reverb is very slight and quite tolerable but the amp is not completely silent (whereas it should be I think for an amp of this price).

I think the reverb (or tremolo) acts as an amplifier of that basic hum.

I checked what you suggest. If I disconnect the reverb pan cables the hum is still there unchanged and another noise is added on top of it. Moving the pan outside the amp does not reduce the hum. All the cables are well connected.

[knausz]

Keep in mind that the hum exists through both modes of reverb and tremolo. That would rule out the reverb pan. Nothing I have heard so far would steer me away from trying what I mentioned in post #6 and #8. Specifically, I would check the supply voltages to the four opamps that handle the reverb and tremolo channel switching functions (U1,U2,U3,U4). They are all hooked up to the same +/- 16V supply (TP55 & TP56). Measure the DC and then the AC. DC should read 16 or 17 as on the schematic and the AC should read near zero. If that checks out, I would look at the reverb and tremolo control voltages (TREM_MUTE and REV_MUTE) both AC and DC and go from there. I would do this on standby to decrease the risk of shock, because you will have the amp open. Standby will have no effect on the opamp circuits, it only kills the B+. Good Luck.

[tochiro]

Thanks a lot knausz, I'll have the amp checked by a technician and will suggest your ideas.
I'll keep you informed.
Cheers