Fender FM 212 buzz

[moralyson]

I recently purchased a used Fender FM212 off craigslist and it has this annoying buzz (I purchased it knowing the buzz existed).when i click it on it buzzes for a sec and then stops, but if i do not have an instrument cable plugged in the buzz is continuous. Any help would be greatly appreciated.

[Enzo]

Turn ALL the knobs to zero. Does it still hum? Ball up your fist and whack the top of hte amp. Does that make the hum stop or start, even for a moment? The amp should not react to the whacking, if it does, something is loose. My primary suspect in Fender amps is the solder cracking on the main filter caps on the circuit board.

[TD_Madden]

maybe one of the input jacks has an open-ground? I've seen quite a few of those that don't ground themselves properly when nothing iis plugged-in.

[moralyson]

thanks for the suggestions. I'll give them a go.

[Twist]

Same issue

Frontman 212R Schematic.pdf

thanks for the suggestions. I'll give them a go.

I've got the same amp on my bench and have been pulling my hair out trying to track down the problem. The amp works fine when you plug into the input or power amp input. I think it has something to do with the muting circuit since plugging into either of those inputs you are lifting R49 10 ohm resistor from ground thus disengaging the mute circuit. I've been all over the mute circuit checking transistors and diodes in and out of circuit. When nothing is plugged in it throws a small DC voltage on the speaker out.

[Enzo]

First, have any of the jacks been replaced? It even is shown on the schematic, but the input jacks have a little spike on the front surface that makes contact with the metal chassis. Replacement jacks won't have that. See R1 10 ohms right by them (on the print)? Is that open? And of course if you operate the amp with the board out, that ground is missing. Your input jack grounds the guitar to circuit common. R1 connects to chassis

Back to R1, see the "to pot brackets"? Fender runs a ground from pot to pot with all those brackets wired to the ones on either wide. The brackets are more or less in series. A broken connection or a replacement pot lacking those brackets will interrupt the ground strip.

[DrGonz78]

Yeah I definitely second the idea to look at all the traces on the pots from pot to pot groundings. I had the older Frontman 212 for a repair a while back and mostly it was fixing traces that some Newbie had damaged changing out a pot or two. Those circuit boards are prone to breakage along that ground strip path (trace to trace) if someone is unkind when replacing pots.

Also, might be a good idea to run a ground to chassis on the input jacks ground solder joint. This always has helped many fender reduce general hum for me time and time again. Just make sure you pick a sleeved part of the jack that is still grounded even after the 1/4 inch TRS plug is inserted. Do a continuity test of that soldered leg to make sure which is always grounded. Lots of small cheap amps have this already installed on the input jack. I know some will scoff at that recommendation but I stand by this claim that it never has hurt anything. Actually, this has only helped the amps that I have put ground to chassis on the input jack.

[byronboy]

Hi.I have a frontman 212r in for repair.It had no output which was due to R145 300ohm 5watt resistor broken leg,so no -16v supply to the op amps.Replacing that got the amp working again but I notice the same issue that many of you have experienced.A noticeable hum from the speakers when a jack is inserted into inputs 1 or 2.Even with all controls turned to zero it is still there. The amp is untouched,so no previous repair has been attempted.The customer could not be sure if the hum was present from new.He has only used it a few times as a standby for his Marshall.I made sure all pot brackets were grounded and checked for dry joints and broken print but all seems well.I notice the amp uses a rather crude method of producing the negative and positive 16v lines.I would have preferred a couple of ic regulators instead.The guy is quite happy to have it working as it is.He says in a noisy club it,s not too noticeable.If a jack is plugged into the power amp in socket the hum is no were near as loud. I tried the two 100mfd caps on the 16v LT rails with no effect.The mute circuit is obviously working when the jack is removed it is dead as the grave.Just curious to know if someone has solved this problem.
Cheers, Peter. [Byronboy]

[Jazz P Bass]

Your problem is the exact opposite of the original post.
Going in the 'power amp in' lessens the noise but does not eliminate it.
Not like the 'mute' circuit does.
Start by checking all of the power rails.
Vdc & Vac.
Look for symmetry between the + & - rails.
And the measure the Vac ripple.
On the opamps, you want to see zero.

[DrGonz78]

I know a whole bunch of people are gonna not like this answer...

Connect a ground wire from the two input jacks ground legs to the chassis ground. Does this improve the hum that is present when something is plugged in? I am not saying to modify the amp in this manner unless you want to do so. I have found that all these SS Fenders have less hum when you ground the input jacks to the chassis in a more solid fashion. May not work on all SS Fender designs but I am dead serious that it has helped on a Princeton, 112plus, and a Frontman 212. Let me know what you think?

[byronboy]

The guy has taken his amp back.He says the hum is not that bad and can live with it.At least it,s working again now,but if it comes back in the future,I will try your suggestions.
Thanks,guys.