Marshall 2210 hum

[wizard333]

Did you check those jacks yet? They are right before the PI, passive insert jacks with switching return jack.

To give an idea of signal levels into the PI on these amps: If you plug your guitar directly into the return jack, it is louder than the signal from the preamp unless you have the master turned up quite a bit. That 10k resistor to ground before the effects send pulls the level WAY down.

[lowell]

I have V4 pulled so I'd think those jacks aren't "in circuit."

[lowell]

Nickb, So you're saying if the hum is from the very last supply node on the B+ line, that the hum could still appear at the PI supply node? Through the 10k dropping resistor?

I also disconnected the channel switching supply to see if that was in play. I disconnected the two diodes from the heaters. Didn't help.

I'm gonna toss in the towel on this. Spent hours and hours ....and hours haha. Got other items awaiting my attention. The hum is there, but it's not loud enough to be much of a bother. If it were my amp I wouldn't care.

[lowell]

Last ditch effort idea. I'm not that great with a scope other than checking signal and clipping etc.. How can I use the scope to locate this hum, what settings should it be on, and is this a good idea. This is one of the most elusive noise problems I've ever encountered. Hate to not be able to fix something. Hate it.

[nickb]

Nickb, So you're saying if the hum is from the very last supply node on the B+ line, that the hum could still appear at the PI supply node? Through the 10k dropping resistor?

I also disconnected the channel switching supply to see if that was in play. I disconnected the two diodes from the heaters. Didn't help.

I'm gonna toss in the towel on this. Spent hours and hours ....and hours haha. Got other items awaiting my attention.

I'm not saying that at all. PS hum on an earlier stage(s) is passed with minimal rejection through to the next and amplified, e.g V4a to PI via C25. I didn't notice that all the preamp stages shared the same node at that rather kills the idea off since the hum would be massive. Turns out it isn't...

The hum is there, but it's not loud enough to be much of a bother. If it were my amp I wouldn't care.

...it would have been nice to know that from the outset.

I calculated the attenuation of all the PS filters at around 160dB. That implies about 50nV of hum at the node is question. So given that and the fact that the audible is low level, I think the measurement of 27mV is suspect.

[Pedro Vecino]

As possibilities: Have you tried to remove +B to reverb transformer? and what to try with another choke?

[lowell]

I did remove the B+ from the reverb transformer, didn't help. Haven't tried another choke. I'll try a smallish resistor in place of the choke and see if it helps... and/or make sure the choke isn't a short.

[lowell]

The choke isn't a short. Getting ~100ohms DC resistance. One thing I noticed was that on the clean channel setting, there is slightly less hum.

[g1]

Don't go by the choke resistance, try the sub.
It can measure DC resistance ok and still have a problem.

[lowell]

Ok will do. I figure it's just a coil right? Or does a choke have a magnetic core like a pickup?

[g1]

No magnets, but you can have shorted turns yet measure ok resistance. Same as transformers. Also you could have leakage to the casing.

[lowell]

Well, subbed a choke, still hum. Ah well. Thanks for the help.

[Jazz P Bass]

Last but certainly not last, have you subbed in another OT?

[J M Fahey]

Or does a choke have a magnetic core

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[lowell]

Ok I admit I could a just read this before asking.

https://en.m.wikipedia.org/wiki/Choke_(electronics)

I knew what a choke DOES just couldn't remember off hand if it had a ferrite core.