Frontman 212R Hum

[Drewline]

I was wondering if anyone has encountered a Frontman 212R with a hum when turned on which disappears when the center contact of either input or poweramp in jack is lifted. I've checked for bad solder, bad grounds, subsituted caps in the power supply with no luck. If I lift R73, disconnecting the mute circuit the amp gives a short buzz when turned on & is then silent.
Thanks for your help"

FM212R_schematic.pdf

[g1]

So this is a hum that occurs only when nothing is plugged into the amp? How bad is the hum? Why not just turn the amp off if not in use?

[Jazz P Bass]

With the unit off & nothing in the jacks, measure R73 where it meets C46.
It should be grounded.
If it is not then you have a jack problem.

[52 Bill]

It should be grounded.

Well, almost grounded. There is a 10 ohm resistor in the circuit.

If I understand what he is saying, when the mute circuit is turned on, the power amp has a hum. When the mute circuit is turned off the hum goes away.

The mute circuit works by disrupting the power amp input differential pair circuit. It will be difficult to figure out what is causing the hum in the circuit.

[Drewline]

Our customer brought in the amp because of the hum. I've tested all the jacks & made sure that they all go to ground which is actually the 10ohm R1. I tried replacing Q6 because I could not tell if it was working properly by the pin voltages. I know I'm missing something. I'll get back to it Monday.

Thanks,

[TimmyP1955]

My guess is that the hum is from current across R1. Short it and see if the problem gets better or worse.

[Enzo]

By any chance are the input jacks no longer the originals? The input jacks have little tiny points on the face of them, these points poke into the metal panel when the thing is installed in the chassis. In the schematic, look just under each jack and connected to C1. If you install plain jacks or some other type jack, they won;t have this grounding point, and hum can result.

The amp hums in mute is the issue, eh?

I know one way to fix it, remove R49. I like that better than lifting R73.

[J M Fahey]

Problem is the amp hums when muted and it actually mutes by shooting a 9mm bullet through its brain (literally, killing the differential input stage) so actually no classic hum problem at the preamp or any jack can reach it.

[Enzo]

I wasn't thinking so much classic input jack hum. I was trying to think of ways that grounded line could cause the mute circuit to inject hum. Only thing came to mind was maybe that lack of a chassis reference at AC made some difference. Basically kicking C1 out of the circuit causes issues or it does not. Just a quick look answers the question either way.

The ground reference in the power amp is GNDA, while the preamp circuits are referenced to plain old GND. So the mute circuit sits on GNDA, while the mute feeler, the line through the jacks is grounded to GND. It may be nothing, but I don't think it is outside the imagination to consider the grounds may be involved. If GND and GNDA are not properly mated - wherever that occurs - something might result.

[Drewline]

All the jacks are original, cleaned & checked. The amp hums when the middle contact of the inputs or power amp in are all closed. Break any of them like a plug is inserted & the hum is gone. I'll try lifting R49 & see what happens.
Thanks for your help.

[Enzo]

I was being facetious. Removing R49 is the same thing as lifting one of those contacts - it breaks the mute line. The amp will think there is a plug in the jacks. it will work, but is sorta cheating.

[g1]

I'll ask again how bad the hum is. This is not an expensive amp. Fender would not be too concerned about a minor hum that can only happen under non-playing conditions.
If it's not real objectionable, you would almost have to A/B it against another amp of exact same model.
Sorry if this question seems ridiculous, but you have not quantified the hum yet. Maybe an AC voltage reading across the speaker when humming would help.

[Enzo]

I do agree there is a difference between something you notice and something that is a problem. This amp hums in mute. No one plays in mute, and no one records in mute, so a little hum in mute is what I would consider a cosmetic problem.

Having said that, I expect a muted amp to be at least as quiet as the amp in active mode. So it is at least an academic interest.

[g1]

I would kind of expect that too, but I'm considering the point of the mute circuit:
1) avoid turn on thump
2)kill the signal if over-temp condition
Getting rid of noise with no input connected is more of a bonus than a design criteria, IMO.
I would think shorting across C47 should tell if there is a problem with the mute circuit.

[Drewline]

The hum is loud enough to be annoying. That is why it was brought in. I guess I'll need to find a solution. I'll check & see if any AC is present on the speaker & let you know.

Thanks,