No, unless the controls are way up - whwich will hum some in most amps - it should sit there polite and quiet.
Do you mean it draws the 60w ONLY while humming, but draws less when quiet? The loud hum may not be music, but it is still output, and that takes power.
If you look inside, there will be two larger caps sticking up from the main board. Those are the main filter caps, I suspect one or both need a good resoldering. Try that.
Otherwise, in general, the two most common reasons a solid state amp hums loud is either DC on the speaker or loss of power supply filtration.
Do you mean it draws the 60w ONLY while humming, but draws less when quiet? The loud hum may not be music, but it is still output, and that takes power.
If you look inside, there will be two larger caps sticking up from the main board. Those are the main filter caps, I suspect one or both need a good resoldering. Try that.
Otherwise, in general, the two most common reasons a solid state amp hums loud is either DC on the speaker or loss of power supply filtration.
I'm sorry my question wasn't specific enough: I meant to say, it only hums loudly when nothing is plugged in. When I plug in a guitar or just a 1/4" plug adapter, the amp goes quiet. I assumed a problem with the muting circuitry, but can find nothing wrong there. I replaced the 10V zener, 10uF and .047uF in that circuit, anyway, for good measure. The schematic shows a PTC thermistor in this circuit, but I can't find it on the board. Then I looked at the input jack circuit and tested R1, the 10 ohm resistor to "chassis" ground. That's OK, I also replaced C1, the .1uF mylar that goes from the switching contact to R1. The mystery in this input, is that I don't see how R1 gets to chassis ground?
Right - here's the cause and solution:
First - the humm is a medium to loud 100Hz (120Hz in the US) straight from the PSU Full Rectifier.
It is definitively caused by the mute circuit but not every amp will do it - it depends mainly on three different things:
- the value of the temp PTC (they are not very precise)
- the HFE of Q6 and Q7
- the amount of residual humm on the -16V rail
The problem ONLY occurs when the mute circuit is engaged - ie nothing plugged in either of the two input jacks or the effects return jack.
What happens when R73 is bulled to ground - the circuit Q6/Q76 becomes a current amplifier circuit who's efficiency is governed by the HFE's and the value of the PTC. The idea is the shut down the constant current circuit formed by Q8, D20 and R82 - but depending on the amplification value the mute circuit now controls the current through the constant current circuit - in effect amplifying the residual 100Hz (120Hz) on the 16V rail.
So - that is done - now the fix:
Install a resistor 220kOhm or more (I tested 220kOhm and up to 1MOhm - they all work - depending on the amplification value of the mute circuit) from the collector of Q7 (attach it to the leg of R72) to ground Cathode of Diode D18 (is just on the diagonal other side of Q7). This absorbs the amplified current from the mute circuit and the humm is gone.
Now - have fun with the Frontman 212R !!
First - the humm is a medium to loud 100Hz (120Hz in the US) straight from the PSU Full Rectifier.
It is definitively caused by the mute circuit but not every amp will do it - it depends mainly on three different things:
- the value of the temp PTC (they are not very precise)
- the HFE of Q6 and Q7
- the amount of residual humm on the -16V rail
The problem ONLY occurs when the mute circuit is engaged - ie nothing plugged in either of the two input jacks or the effects return jack.
What happens when R73 is bulled to ground - the circuit Q6/Q76 becomes a current amplifier circuit who's efficiency is governed by the HFE's and the value of the PTC. The idea is the shut down the constant current circuit formed by Q8, D20 and R82 - but depending on the amplification value the mute circuit now controls the current through the constant current circuit - in effect amplifying the residual 100Hz (120Hz) on the 16V rail.
So - that is done - now the fix:
Install a resistor 220kOhm or more (I tested 220kOhm and up to 1MOhm - they all work - depending on the amplification value of the mute circuit) from the collector of Q7 (attach it to the leg of R72) to ground Cathode of Diode D18 (is just on the diagonal other side of Q7). This absorbs the amplified current from the mute circuit and the humm is gone.
Now - have fun with the Frontman 212R !!
Get your Fender special jacks from here
http://www.musicpartsguru.com/thumbnail.asp?cid=131
Is cheap in the US (bit more from where I am)
http://www.musicpartsguru.com/thumbnail.asp?cid=131
Is cheap in the US (bit more from where I am)
You are right.
Fender took a lot of questionable decisions when designing FM212R (it sounds good though) , one of them being that cheesy "nuke the input differential stage" mute.
Yes, it will mute the amp all right , but by killing its "brain" : the input stage which does a lot of things , including rejecting rail lines ripple (hum) ... rings a bell?
They should have used a muting FET shorting the input, which makes the amp real silent.
SWR (which was later acquired by Fender) is fond of that kind of muting, so I guess there's where the idea came from.
I know this is an old post so I have not read all the remedy's. Let me tell tell you what I accidentally did when I took mine apart. I forgot to install the mica insulator between the power amps and the heat sink, thereby shorting the transistor to ground. So I went back and put them in and no more blown fuses. Lesson learned
Hey guys I'm new here, but I too have FM212R probs. The amp makes a cracking noise, I put a scope on it and have tracked it down to crossover distortion appearing at pin 7 of U6B a TLO72. I have replaced it and did not cure it. I am suspecting D17-D16. Can anybody tell me what those do in the circuit? I'm stuck. If they are bad then probably not turning on and off at the right time causing the crossover distortion. Am I on the right track? Pin 6 and pin 5 of the TLO72 are showing a good clean 1khz test sine wave.
Not really
Any crossover distortion you might hear would appear straight at the speaker out terminals.
U6b is basically getting negative feedback from the output, through R77+R79 (voltage) and R80 (current) , this is a mixed feedback circuit, U6b will try to compensate for errors so typically tou will see an exaggerated version of distortion at its output.
D17/16 are there across the amplifier differential input pair bases, for protection in case of extreme overdrive.
Normally they do nothing and certainly don't affect crossover or bias.
Crossover distortion does not sound like a ticking sound, so you must have another kind of problem.
Keep scoping the output, you should see some kind of glitch when it happens.
Good luck
Ok, so U6b is correcting and has a clean output and Q20 and Q21 shows a good sine wave when it has no load on it(speaker). However when I hook up the speaker it starts showing noise. High freq spikes all over the sine wave. I need a dummy load so I don't have to listen to that 1k audio. I guess I should get one to further troubleshoot this amp. I did run the pre out to another amp and it was clean.
Hi Guys
There is nothing inherently wrong with solid-state amps - many sound great and perform well.
All the symptoms sound like poor solder connections. The reality of modern mass production is that most PCBs are wave-soldered. Wave soldering is fast and applies the same amount of heat and the same amount of solder to every connection regardless of the size of the leads involved. So, at the end of the wave soldering line a few people have to go over the boards and do hand touch-ups on the larger connections. To save money, manufacturers aim to apply the least solder possible.
A friend of mine who operates the largest service shop in the country routinely solders the entire PCB of whatever comes in. This fixes 90% of symptoms even before they test the unit. I would wager that this would fix your FM212, too. This amp is not too complicated, so resoldering the PCB should not take too long.
Surface-mounted components present a further challenge to reliability in portable equipment. The solder connections are even smaller than with through-hole parts and the life expectancy of such connections is even lower than for wave soldering. As I've stated on my site and in my books, solder connections are eroded by electron flow. This means you want the largest solder connection that is reasonable and can be assured to be of good quality for every connection. You still want to see proper flow, proper wetting and proper cooling without crystalisation.
Also: avoid water-clean-up solder as it is truly nasty stuff. The resin will eat the PCB over time and wreak havoc on high-impedance boards like tube circuits. The Kester "no-clean" 245 type is much better. The resin dries clear, at which point you can remove it mechanically by chipping it off if you wish. Flux removers should be avoided unless you can really flush the board well and properly dry it. The flux remover breaks the resin into acidic components that eat the board and cause leakage between traces - so either use none at all or use a massive amount.
Have fun
There is nothing inherently wrong with solid-state amps - many sound great and perform well.
All the symptoms sound like poor solder connections. The reality of modern mass production is that most PCBs are wave-soldered. Wave soldering is fast and applies the same amount of heat and the same amount of solder to every connection regardless of the size of the leads involved. So, at the end of the wave soldering line a few people have to go over the boards and do hand touch-ups on the larger connections. To save money, manufacturers aim to apply the least solder possible.
A friend of mine who operates the largest service shop in the country routinely solders the entire PCB of whatever comes in. This fixes 90% of symptoms even before they test the unit. I would wager that this would fix your FM212, too. This amp is not too complicated, so resoldering the PCB should not take too long.
Surface-mounted components present a further challenge to reliability in portable equipment. The solder connections are even smaller than with through-hole parts and the life expectancy of such connections is even lower than for wave soldering. As I've stated on my site and in my books, solder connections are eroded by electron flow. This means you want the largest solder connection that is reasonable and can be assured to be of good quality for every connection. You still want to see proper flow, proper wetting and proper cooling without crystalisation.
Also: avoid water-clean-up solder as it is truly nasty stuff. The resin will eat the PCB over time and wreak havoc on high-impedance boards like tube circuits. The Kester "no-clean" 245 type is much better. The resin dries clear, at which point you can remove it mechanically by chipping it off if you wish. Flux removers should be avoided unless you can really flush the board well and properly dry it. The flux remover breaks the resin into acidic components that eat the board and cause leakage between traces - so either use none at all or use a massive amount.
Have fun
Are we still talking about the ticking sound?
Please clip your scope to the output , show us a clean signal and then the noisy one.
Be certain that anything strong enough to be heard will be important enough to show some visible abnormality on the waveform.
Guaranteed.
This will sound sweet:
none of these will:
this is an ugly overdrive tube Marshall
this is an unstable Acoustic 360 where the bottom of the waveform plain clips while the top briefly oscillates:
I show this not because of the waveform but because of the terrible spikes, which would be absolutely unbearable in an amplifier:
here's the same, mounted on a clean sinewave and making it raspy, annoying:
this is an output stage missing the bottom half drivers:
in a nutshell: when you hear something weird, you must be seeing something weird.
That may (should) help troubleshooting.
Have your camera ready
EDIT: there is one way to hear ugly/weird stuff and see nothing: when you have a *mechanical* problem at the speaker: junk/loose wire/bubble/torn/unglued voice coil - frayed tinsel wires - cracked/split cone - loose dustcap - loose speaker - etc.
To check that, try amp with another, known good speaker.
Please clip your scope to the output , show us a clean signal and then the noisy one.
Be certain that anything strong enough to be heard will be important enough to show some visible abnormality on the waveform.
Guaranteed.
This will sound sweet:
none of these will:
this is an ugly overdrive tube Marshall
An externally hosted image should be here but it was not working when we last tested it.
this is an unstable Acoustic 360 where the bottom of the waveform plain clips while the top briefly oscillates:
An externally hosted image should be here but it was not working when we last tested it.
I show this not because of the waveform but because of the terrible spikes, which would be absolutely unbearable in an amplifier:
here's the same, mounted on a clean sinewave and making it raspy, annoying:
this is an output stage missing the bottom half drivers:
in a nutshell: when you hear something weird, you must be seeing something weird.
That may (should) help troubleshooting.
Have your camera ready
EDIT: there is one way to hear ugly/weird stuff and see nothing: when you have a *mechanical* problem at the speaker: junk/loose wire/bubble/torn/unglued voice coil - frayed tinsel wires - cracked/split cone - loose dustcap - loose speaker - etc.
To check that, try amp with another, known good speaker.
Last edited:
No never was a ticking sound, it a crackling sound, like wadding up paper. The wave form above that you describe as making it raspy and annoying is what I have. The sine wave is there, but it has noise all over it. I will try and get a pic to you. It's hard because sometimes it's intermittent, so I will have camera ready.
Not at all, you can hear distortion LONG before you can see it on a scope.
However, severe distortion, or high levels of noise are easily seen on a scope.