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Fender FM212R Problems
Fender FM212R Problems
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Old 26th December 2012, 10:07 PM   #1
badraven is offline badraven  United States
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Cool Fender FM212R Problems

Hello All!

My Fender amp has been a source of problems since I purchased it in 2006. It has been replaced twice by the factory.

The amp generally makes this low (most of the time) sound like crumbling paper. Generally its just something loud enough to be annoying. Sometimes it gets a little louder but never real loud. Doesn't matter if its on the clean, drive or more drive channel.
Just as a guess from what I have been reading I am thinking a capacitor is causing the problem. But I have no way to verify this that I know of.

The other issue is: The amp makes a popping sound and the power crystal light blinks like the amp is shorting out or something. It gets so bad at times that the amp will blink and pop and then stop working. It will just hum. When that happens I can turn it off and then back on and it will continue playing. Sometime it will continue to pop and other times it won't. Sometimes I can go a week or more playing it every day and it will play like it did when it was new.

I read something recently about the input jacks being an issue on Fenders and that replacement normally corrected this issue. So I ordered Switchcraft brand jacks and replaced all five jacks. Once I did that the problem went away. No pops or blinking or anything, just the crumbling paper sound. Now ( 4 weeks later) the popping sound and the humming etc. just started again.

I am of the mind to buy a new amp but the one I want is about $800 so its not something that is going to happen before summer because work is slowed down because of the cold weather.

Do any of you have experience with this or maybe know where I can find info on this subject?

Thanks for your time. The schematic is attached.


P.S. I have other Fender Schematics of someone needs a copy.
Attached Files
File Type: pdf FM 212R.pdf (733.2 KB, 261 views)
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Old 27th December 2012, 01:14 AM   #2
cyclecamper is offline cyclecamper  United States
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There are real amp techs on-list, I'm an amateur at best.

...and it's all solid-state? ...that's not cool, the red power LED is on the +42 rail. Takes a serious problem to make that blink. Does it 'thump' when you turn it on, 'cause I don't see a protection relay either...no realys at all, even the channel selection is a real switch? I would have hoped it was something easy like that, a mechanical connection. But it doesn't blow a fast 4amp fuse on the primary? Makes me suspect an open rather than a short, if one whole rail goes out so start with any plugs on the power transformer wires. I hate to mention the most expensive part in the whole thing, but...I would suspect a power transformer internal over-temp cutout except Fender usually shows them in their schematics (at least in the old days); don't even know whether their solid-state stuff has one.

Next time it makes the subtle noise, stick a 1/4 TRS plug into the poweramp jack to seperate the preamp from the poweramp. If the noise stops, then it was in the preamp. If not, it's in the power amp. That technique would divide that problem location in half, except your other symptom involves the shared power supply, dagnab it. After you do this, we can take it to the next step.

Next time the power LED blinks, put a meter on the rails and then unplug the + and - 42 cable to the power amp (I assume there is one)? For that matter, look around the rectifier bridge and filter caps for any dirty plugs? Tap them with a chopstick? This is only +-42, not 500 like a tube amp. Need to get some measurements during the problem.

I don't know these particular solid state Fender amps at all, but against the odds and normal debug technieues I would still guess two distinct different problems. And I would guess one rail is just losing power rather than shorted, as there's no smoke and the main fuse doesn't blow. It should have individual rail fuses so you would know for sure!

Looks like an interesting little 90-watt amp, so sorry it's been such a nightmare. That's why I like those "unreliable" tube guitar amps (you probably get this enough so I'll lay off).

There is one other easy possibility to rule out: some kind of bad ground.
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Old 27th December 2012, 03:35 AM   #3
badraven is offline badraven  United States
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Cyclecamper, thanks for the ideas!

As far as I know the amp is all solid state.
The amp does not thump when I turn it on. Its very quiet when I turn it on and before I turn the volume knob on the guitars I play. It does make a humming sound if I unplug the guitar chord from the amp while its turned on but nothing like the problem I have described. And the hum stops after a minute or so like something turns off.
The channel selectors are real switches and it doesn't matter if the foot switch is plugged in or not. In fact it doesn't seem to matter what I do, turn knobs, plug things into the sockets, switch my guitar cable from input 1 to input 2 etc. change channels etc. Nothing seems to make a difference on either issue.

Over the last year however it seemed like the popping sound stayed gone longer if a plugged a chord (actually I used a converter plug hooked to nothing) into the power amp out socket. I never could firmly determine if that really helped or if I was imagining it because I wanted it to help. It seems to have the problem most often when I turn the amp up. For instance - yesterday I finished assembling a folded horn extension cabinet with 2 12" speakers. I wired the speakers parallel to get the same 4 ohm load as the stock speakers. The amp and the extension cabinet have the same combo of speakers. 1 Factory "Special Design" Fender speaker and 1 British Tonker, each is 8 ohm. When I started playing I had the amp turned up to 5 and it didn't take long at all for it to start popping and carrying on. It quit playing a couple times and would just hum loudly. I turned it off and then back on and continued to play but it kept popping. I took my meter and made sure the ohm load was correct. It was showing 3.2 ohms on the cabinet. I then wired the speaks in series and it came out to 12.6 ohms. I figured I would see what that does. It changed the tone slightly and it seemed a little bluesier but that's about it. It acted just the same. The internal amp speakers are wired in series right now. I just fired it up and played the amp without the cabinet at 2 volume on the overdrive channel and I can't seem to get it to mess up yet. I will set up my mic and try to catch a recording so I can upload it for you.

The amp has never blown the fuse.
Never smoked.
I don't know what a "fast 4amp fuse on the primary" is.
Where do I find a replacement power transformer? Fender? Or is there somewhere to get one that I don't have to pay for a name? I am sure the Fender tag adds 50 percent to the price.

Perhaps when I had the amp out to replace the sockets the simple act of unplugging and reconnecting the power supply connectors had a positive effect on the problem. Nothing was visually oxidized but like anything, looks can be deceiving. I did take Deoxit to all the pots while the amp was apart. I had a scratchy pot on the clean channel.

I replaced the speaker wires with 16 gauge speaker wire and soldered them to the amp and the speakers while the amp was going back together. I have considered taking the amp apart again and re-flowing all the solder joints on the board to make sure there aren't any cold joints.

When you say "Next time it makes the subtle noise, stick a 1/4 TRS plug into the poweramp jack to seperate the preamp from the poweramp" do you mean the preamp out or in?
I assume you mean preamp in since that kills the sound but please clarify.

My next step will be to disassemble the amp and sit it on my bench and hook it up and play it so I can try pulling the + and - 42 cable to the power amp like you suggest.

I already want to buy a tube amp. I want to buy a Fender Deluxe Tweed which is about 7 bills or a Carvin Belair which is about 6 bills delivered. Either way I can't justify the expenditure until the hours pick back up at work. I know the tube amps sound better but honestly, I love this Fender. I have from day 1. Its just not reliable. I don't play publicly near as much as I used to but when I do I can't trust this amp not to act up.

Is there a way to test the power supply and verify it is good or bad? If that's what I need I will buy one but I can't afford to spend a hundred plus and that not be the problem. I'd rather stick that aside and save for the tube amp.

How do I check for a bad ground? I can open it up and clean all the connections with emery cloth and/or deoxit. I'm open, just give in an idea on the direction to go.


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Old 27th December 2012, 12:45 PM   #4
JMFahey is offline JMFahey  Argentina
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cyclecamper's answer basically says it all.
Agree with the "plug something into the power amp in", only adding it's better to build and use a shorted plug there , to better "kill" any interference.
This will confirm where the crumbling paper sound comes from.
As of the popping/thumping/blinking led symptoms, he's also correct, it must happen at the PSU.
There's a chain of connections there, any bad one can cause it, you must check all of it.
1) your 120V power line may have a problem !!!
Don't discard it just because it's "outside" the amp !!!
Bad wiring inside the wall, bad or worn 120V socket , do you plug into an extension strip?
Is any "hungry" appliance connected to the same line? Even in another room.
Say: freezer/washing machine/electric or microwave oven/air conditioning.
2) bad power cable/worn/weak IEC connector (female on cable or male on amp).
Carefully wiggle it yo confirm.
3) bad/poor contact power switch/some of its spade connectors/fuse holder.
The fuse clip contacts may be worn/weak or the wire *inside* the fuse may make poor contact. Just in case, replace it. ALWAYS WITH THE AMP UNPLUGGED.
4) check spade lugs P6/7/8 . These CAREFULLY wiggle with a chopstick, with the amp on.
5) the NTC thermistor (usually a black ceramic disk) may be cracked (the body or one of its legs) or the soldering on the PCB.
6) secondary connectors P3/4/5 . Also CAREFULLY tap/slightly wiggle them (using a chopstick) with the amp on.
7) it's a quite usual problem that big heavy main filter capacitors C71/72 have cracked solder; resolder adding a little normal (leaded) solder and a good hot iron; the lead free solder applied at the factory is somewhat brittle and often cracks.
Well, as you see, there are a lot of "mechanical" suspects, it's *very* unlikely you have a transformer proble,.
Not impossible of course, but it lays way back in the list.
Also forget the "it may be *a* capacitor" riutine.
Unless somebody can point (correctly) to a specific one, that "help" is useless.
The FM212 is a nice little amp, with surprisingly good sound, for peanuts, which also causes that nobody wants to spend much time repairing it .
Not even Fender, who usually just replaces it , go figure .
And remember, that crumbling sound *can* be coming from outside (including your guitar/cables/pedals/etc.)
Good luck.
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Old 27th December 2012, 05:15 PM   #5
cyclecamper is offline cyclecamper  United States
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If you do determine that it is the power transformer making one rail cut out, then take off the bell end covers from the transformer (assuming it has covers and isn't open or torroidial) and look for a thermal cutout (any heat-shrink-covered blob on the wires). You can replace the thermal cutout or just remove it rather than replace the transformer, as it mostly protects the transformer anyway. Some are bimetal thermostats that deform after a certain number of trips, then they open early.

It does sound like the worst problem is heat-related. So anything mounted on the heatsink near the output transistors (to thermally couple) like to adjust bias with a thermal curve or something like that is suspect. The way it "latches" to stay in the problem state and "resets" on power-off would probably be a clue to a better tech than me.

I see there's a 'mute' circuit that silences the power amp when you remove the guitar cord, and involves Q6 Q7 Q8, which affects Q9 Q10. No relay to get dirty. Can that remove the turn-on thump???I'd have to think about that, I guess it depends on whether the outputs make the thump as the rails come up asymmetrically or if the thump comes from something earlier in the circuit. So what is RT2 PTC100 with a little "T" in the circle with the resistor symbol? Looks like a thermal resistor sensor in the mute circuit. That's another suspect just because it's temp-related and might be intended to latch but might just be noisy if it has a problem? When it's making the subtle noises, do they disappear if you pull out the guitar cord? If the noise remains, that means it's generated later in the power amp, in the mute circuit or later (or power suppy).

Last edited by cyclecamper; 27th December 2012 at 05:22 PM.
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Old 27th December 2012, 10:49 PM   #6
new2tubes2 is offline new2tubes2  United States
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have you taken a 1/4" plug in/out of all of the connections? sometimes dirty fx return jacks can cause a problem. more likely if they're never used.
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Old 27th December 2012, 11:25 PM   #7
nigelwright7557 is offline nigelwright7557  United Kingdom
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Originally Posted by badraven View Post
Hello All!

I am of the mind to buy a new amp but the one I want is about $800 so its not something that is going to happen before summer because work is slowed down because of the cold weather.
If the amp is getting old then you might fix one problem only to find more problems later.

The usual problems with older amps is bad connections and dried up electrolytics.
PCBCAD51 download or CDROM 2019 version out now http://www.murtonpikesystems.co.uk
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Old 28th December 2012, 12:42 AM   #8
badraven is offline badraven  United States
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The power amp in jack is a stereo plug. Are you saying to make a dummy plug and tie all three wires together and plug it in and see what happens?

I will disassemble the amp this weekend and try your suggestions and those of Cyclecamper.
As for your questions about eh chord, power circuit etc.

This amp has exhibited this issues in three states: Arizona, Arkansas and now Mississippi.
I ran a separate circuit into my music room from the power box just for my computer and musical instruments.
I have several guitar chords, none of them made any difference.
The power chord has been changed several times over the years and nothing has ever changed.
I have never inspected the fuse at the power plug. Honestly I never even considered that it may be a problem since the amp always come on when I hit the power switch.

Cyclecamper I will thoroughly inspect the transformer when I open the amp.
As for the heat issue, I have had times in the past in hot weather in Arizona when the amp would fade out and I would have to turn it off for a bit to let it cool down. Not sure if that happened to this amp or the one that was originally replaced.

nigelwright7557 You never know, the amp is nearly 8 years old now. Everything looked nice when I opened it up to replace the sockets last month. All the connections were tight etc.

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Old 28th December 2012, 12:49 PM   #9
JMFahey is offline JMFahey  Argentina
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You are right, they use a stereo jack because they use some trickery with the preamp input jack grounding at the same time , but the actual signal path for the power amp uses a conventional mono plug, which in this case you should short, to make certain nothing external gets in.
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Old 29th December 2012, 10:59 PM   #10
Enzo is offline Enzo  United States
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The power transformer will not be able to cause just one polarity to cut out, the rectification is full wve, meaning both polaritied of AC are rectified both directions. A bad winding could drop both at once though.

Crackling paper noises are most likely a noisy semiconductor, find out if the noise is in the preamp or power amap and we can go on from there. After semiconductors, resistors are next most likely, though in my experience, that is mostly in tube amps.

Hum and pops and drop outs? First thing I'd do is resolder the two main filter caps. I'd also whack it. Turn the reverb down, ball up your fist, and whack the top of the amp. If the amp reacts, then some connection is loose. If hitting it makes a pop or crack noise or really any noise, then something is loose. If you do it while music plays through it, if the music cuts out when whacked, or if whacking makes it start working right, either way, something is loose.
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