Frontman 212R Hum

[g1]

Also try shorting across C47 and see if it makes any difference.

[Drewline]

I've tried all the great suggestions made to me here. This is what I've found. 1)Shorting C47 did not stop the hum. 2) I shorted & removed C1 from the circuit. No difference. Shorting R1 made no difference. I've checked all the jacks, grounds & resoldered everything suspicious. My last resort was to remove R49 as suggested by Enzo. As when I had removed R73 the amp makes a low hum for a second & then is silent. At this point I'm going to go with that as an acceptable solution.
I'd like to thank everyone that contributed to this very much. I'm indebted to you all & hope to be able return the help in any way I can!

Regards,

[drewl]

I have one of these with the same problem.

Looks like you guys didn't find the problem.

I'm going to try replacing the transistors and diodes in the mute circuit to see if any of those are leaking some current.

It appears in mute mode the small ripple from the -42v supply is being fed into the differential pair that is supposed to turn off in mute mode.

[Drewline]

I'd love to know what you find. My customer was very pleased with the amp when he got it back. He told us it had the hum when he bought it which was amazing to me.

[drewl]

You just disconnected it right?

I 'll post what I find.

[patlaw]

It sounds like a ground loop, to me. It's surprising how many designs have them. It's a long shot.

[drewl]

Hmmm, but where?

Anyway, that was a waste of time. Replaced Q6 & 7, D18 & 19, C45 & 46, no change.
Disconnected mute circuit R49 for now until I get any new ideas.

[patlaw]

Hmmm, but where?

That's the magic question. The only way to find one is by disconnecting grounds. Good luck with that! They're a bear to find. Plus, if the problem is NOT a ground loop, searching for one is a colossal waste of time.

[drewl]

I don't think it is a ground loop.

As I mentioned the hum/noise produced by the mute circuit is the same as the ripple on the 42v supplies.
It must be bleeding into the circuit somehow.

That's my theory and I'm sticking to it.

[patlaw]

It must be bleeding into the circuit somehow.

Like through a ground loop.

Obviously I have no idea whether there is one. You would expect Fender NOT to have one in their design. I will tell you that I have discovered many ground loops in circuits other than guitar amps over the years. Two of them were from the factory. The problem with a ground loop is that it can manifest itself in ways that are not obvious. A ground loop in one section can show up as a hum in a different section.

It will be great to hear what the culprit is when you find it.

[Drewline]

Yes. Enzo suggested disconnecting R49. It solved the problem, the customer was pleased.

[loupy31]

Hi Guys,
Same problem here, Has anyone found the actual problem yet, its becoming very frustrating

[drewl]

I haven't.
Had two more last week with the same thing.

I'm hoping to get one in where the mute circuit works properly so I can see what's different.

One is coming in next week, we will see if that one is right.

[Jazz P Bass]

Just had an FM 65R on the bench.
Quiet as a mouse.

Maybe there is a difference between the two amps, component wise.

[loupy31]

Today while continuing to look for the cause of this hum, I discovered that when turning the amp on with nothing plugged into the inputs/P.Amp in, Q21 gets very warm/hot after 30 sec. or so, Q20 barely gets warm, The weird this is when I plug into the inputs, effectively lifting the ground tab, I get the usual few sec. of hum then the amp is quiet. the next 30 or so seconds, Q21's temperature drops to a barely warm temp, just like Q20.
I did this several times to make sure it wasn't a one of. What would cause this strange behavior