120 Hz Background Hum Question On Marshall 2210 Amp Head 800 Series

[BrianC]

I've got a 1988 Marshall 800 series 2210 amp head in the shop for a background 120Hz hum, not affected by volume controls. All filter caps check out OK so far. Tubes checked also.
What I have found is, when I remove (see schematric link below) the wire to Pin #2 (grid) of the pahse inverter V5 (junction of C25 & R41), I now lose that hum. Unfortunately, keep in mind this is a PC board amp, so I am limited on what I can do as far as quick component substitutions and bypasses. I would have to constantly remove and remount the pc board with the possibility of breaking wires and messing up the traces..etc.
So the 120Hz background hum is creeping in at the #2 pin grid I assume. So some part of that PI circuit is making that hum AFAIK.
To me, it has to be that circuit due to it's the only tube that affects it...pulling any or all pre-amp tubes doesn't kill it...and pulling any combo of EL-34's doesn't kill it. Different PI tubes don't help either. BTW--I don't have a scope or much else beyond a good digital VOM to work with.
For convenience, here's the schematic link again...https://schematicheaven.net/marshallamp ... w_2210.pdf

[Enzo]

Don't assume automatically it is a bad part.

Just to please me, check all four power tube sockets - is there good B+ on pin 4 of each? And as long as you are there, pin 3. And whatever the bias voltage is on pins 5 of each, it is totally free of ripple?

Pins 1 and 6 of the PI, are they both up to similar B+? SImilar to each other, not similar to the power tube screens.

Does just pulling the PI tube sound identical to removing your wire?

[BrianC]

OK Enzo-----EL-34's....Plate=473v Screen=470v Grid= -45v
PI (V5)…….Plates=303/300v Grid=43/49v Cathode=53v
I have no way to check ripple...no scope.
Pulling PI knocks out the hum.
Just to be sure---This is my first Marshall repair in many years...Aside from some of the normal amp hiss, Can I assume the model 800's should have no lo-level 120Hz background hum when no signal is input?
I notice haven't found any Marshall schematics of this model with the voltages indicated..usually most makers have some voltages included on their schematics.

[g1]

Can you try inserting a plug or cord into FX return, and short the tip to the sleeve?

[SoulFetish]

I just had an 800 in the shop for an unrelated rush repair job. After fixing the amp's issue it came in for and installing 4 new EL34s, I noticed a moderately bothersome hum just like the one you describe. The customer said it's always been like that as far as he could remember, and needed it for a gig, so I couldn't spend the time investigating this any further.
The source of the hum in my case seemed to be generated in the phase inverter stage, just like you are describing. I pulled each tube one at a time and the only one which had any effect on the hum level was the PI. Once that was pulled, the amp was dead quiet. Checked the filtering at each power supply cap, and the caps seem to be functioning fine. Hum level was unaffected by plug in the input or volume control.
This indicates to me these particular amps are suffering from noise induced by a ground loop; most probably isolated in the stages withing the global feedback loop(PI to speaker/DI out).
If I had the amp back in the shop, I would start there. If I'm wrong, at least that can be eliminated as the issue.
Good luck, and let us know how you're making out!

[Enzo]

Brian, a trick for measuring ripple. Better meters can measure AC sitting on DC. Cheap meters get confused. Test your meter. get out a 9v battery, set your meter for AC volts and measure the battery voltage. WHen you first touch probes to it, it will read something, but will settle to zero pretty quickly. If so, that is how you would measure ripple. Meter on AC volts, and measure power supply that way. The small voltage will be your ripple. On the other hand of your 9v battery reads something like 12vAC, then your meter is not up to the task.

BUT... even then, you can set your meter on AC volts, and add a cap in series with the probe. I use a handy 0.047uf I have a lot of, a 0.1uf even better. Just has to be sufficient voltage for your circuits. That cap blocks the DC in your circuit but passes the AC (ripple) so you can read it.

[BrianC]

Thanks for the tips guys. It's getting late here, and the old brain and eyes are shutting down...LOL. I'll get on your suggestions fresh tomorrow.
SoulFetish---Before I read your reply, I was studying the schematic and zeroing in on that global feedback/DI circuit also. Just have to decide how to attack it. Maybe lift R47 (100k) resistor and see what happens..then work my way back to the PI?

[BrianC]

OK...Fresh start today. I tried your tip on AC ripple Enzo...didn't see any readings above .2 VAC...I used the .1 cap trick on the probe.
I went ahead to explore the idea SoulFetish had about the ground loop possibility in the PI circuit to the DI out jack. I lifted R47 at the connection to the OP tranny secondary, and no change in the hum. Not sure what else to do right now in the circuit to try to kill the 120Hz hum.
I could assume it's possibly a design defect...but Marshall has been using this PI circuit for quite a while. Maybe a flaw in the OP tranny or some other wiring error? Open to ideas as it seems to definitely be in the PI circuit based on what I found and SoulFetish's similar experience.

[SoulFetish]

I went ahead to explore the idea SoulFetish had about the ground loop possibility in the PI circuit to the DI out jack. I lifted R47 at the connection to the OP tranny secondary, and no change in the hum. Not sure what else to do right now in the circuit to try to kill the 120Hz hum.
I could assume it's possibly a design defect...but Marshall has been using this PI circuit for quite a while. Maybe a flaw in the OP tranny or some other wiring error? Open to ideas as it seems to definitely be in the PI circuit based on what I found and SoulFetish's similar experience.

So, lifting the 100k resistor eliminates the negative feedback. But it doesn't necessarily solve a problem cause by a ground loop. In order to start troubleshooting that you have to start to trace all the physical locations of signal ground connections and power supply ground returns.
One place to look might be in the PI itself. Where does the ground side of R48 connect to?

[BrianC]

Looks like it's fixed now! I was trying to check out where the ground went for R48 for SoulFetish..So I also went over all the ground strap connector nuts, since they were easier to get to with the board up. I also checked and cleaned the circuit traces where overflow rosin was, which can conduct at times. C48's nut was slightly loose, so I say that was the culprit. Quiet as a mouse now....Thanks for the assistance guys...got me going in the right direction.

[SoulFetish]

Looks like it's fixed now! I was trying to check out where the ground went for R48 for SoulFetish..So I also went over all the ground strap connector nuts, since they were easier to get to with the board up. I also checked and cleaned the circuit traces where overflow rosin was, which can conduct at times. C48's nut was slightly loose, so I say that was the culprit. Quiet as a mouse now....Thanks for the assistance guys...got me going in the right direction.

Nicely done!

[BrianC]

Thanks! Your suggestion to check R48 kind of set up the eventual solution. Was a real head scratcher....Done!!!