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.

BR..

P.S. I have other Fender Schematics of someone needs a copy.
 

Attachments

  • FM 212R.pdf
    733.2 KB · Views: 695
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.
 
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.

Thanks!

BR,
 
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.
 
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:
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.
 
JMFahey
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.

BR.
 
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.
 
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.
 
I agree with the rest - the flickering LED can only be caused by something in the very simple PSU, and MOST likely the primary - you're not going to lose one side of the secondary without MAJOR problems (speaker blown, amp blown etc.) It's VERY possible that the fault is external, and is a faulty mains socket or lead.

As for the noises, when it's happening plug a 1/4 inch jack plug in the PA-IN socket, just a bare plug is all you need - does that istantly stop it or not?.
 
Thanks for all the advice. I am still trying to recreate the symptoms described earlier. The scratchy paper sound started earlier and I went to plug in the plug to the preamp in and it went away before I could to it. I have tried several chords and can't seem to recreate these things on demand. I think it only happens when a guitar is plugged in. But before anyone makes the assumption that its the guitar or the chords etc. These issues do not happen when I am plugged in to my other amp. I am thinking of opening the amp anyway. Which I guess I will go ahead and do that since I can still run it out of the enclosure. Then I will check all the solder joints and plug ins. When I plug a shorted plug into the power amp in the amp is dead silent.

BR
 
Enzo is absolutely correct. I saw the center tap, but you're right, it's a full-wave bridge and the center-tap mid reference is re-established later. Could be a center-tap or ground problem but more generally it can't lose half the voltage due to a transformer open or thermal-cutout. The bridge means it would be full voltage between the rails or nothing. So when it goes out, and the power light goes off, where is the power for the hum coming from? Of course the power transformer could still have an intermittent short.

If you could just reproduce it at-will, and watch with an infra-red camera...see what gets colder or gets really hot. I'm still focused on the light going out.

Whacking it is of course a perfectly valid and valuable test for solid state equipemnt. I remember when a large mainframe channel-attached storage director kept having intermittent problems at Morton Thiokol, nearly 30 years ago. They cleaned and reseated cards, then sprayed freon coolant on it, hoping to get some reaction. As service level agreements got violated IBM automatically dispatched more guys in suits. After a while, the machine room was full of guys in nicer and nicer dark suits mumbling "think we need the big guns...yeah call for an xc-5444885850-C...will it fit in the elevator..." and a few minutes later a guy appears with a redwood 4X4 about 6 feet long with a brass band at one ends and a plate with an IBM part number, and a procedure describing how several guys swing it like a battering ram and exactly where to impact the frame. Definitely poke around with insulated chopsticks.

Loose screws used to be more common than you'd imagine on the old power transistor cases. Now not so much.

Ribbon cables?

The first diagnostic step has to be finding out whether the current is not being delivered or is being burned up somewhere. If some load is drawing the power down so much the light goes off (even an LED) then something is getting hot. If it's an open circuit making voltage and curernt low, something is getting colder than usual. Still strange it's not blowing a fuse, no smoke, no hot capacitors, no burnign speaker voice coils indicating RF. To drop the rail enough to turn out the light would require a serious short or load, more than the normal output of the amp. You'd notice a 100-watt "heater". Makes an intermittent connection or rectifier in the power supply very suspect; some open-circuit condition. The fact is still hums makes me suspect a partial transformer winding short or part of the bridge. If it drops to exactly half, that might be a clue half the bridge isn't working.

Intermittents are a b*tch. I would hope your amp breaks for Christmas, then it can probably be fixed much easier; but then you hope the failure doesn't take out additional expensive parts.

If it was mine, I might solder a small fast-blow fuse into each rail right after the bridge rectifier and a 1/4 ohm resistor in series with the fuse. I have a bunch of really cheap $8 digital VOMs I bought in bulk and gave away last Christmas, so I would wire in two of them watching the two rail voltages, and two more VOMs as current meters measuring the voltage across the resistors.
 
When I plug a shorted plug into the power amp in the amp is dead silent.
BR

With it off, spray inside all the input jacks and the preamp/poweramp jacks while inserting and removing a plug or pressing on the internal switches with some wooden shish-kabob skewer or shopstick.

Plug the Fender's "preamp out" into your other amp's input, and put a shorted guitar plug into the Fender's poweramp input and another shorted plug into the Fender's guitar input.

Leave it like that until you hear the crackling noise from your other amp. That proves the problem is in your Fender's preamp.

Fiddle with knobs & buttons on the Fender to see whether it makes any difference.

Then I'd use the other amp as an AC probe at various test points; just convert a guitar cable, with a wire to an alligator clip on the ground and a pointy solid wire on the hot. If you want to be careful you should put a big (10m?) resistor and nonpolar cap in series with the hot, but I've probed around with just the stock 10K guitar input network before without damaging either amp. Don't know whether I was just lucky or desperate, but it was a useful technique when my old 'scope had drifted completely off-screen. Maybe Enzo can tell whether that's dangerous at the test points labelled on the schematic. Anyway, the point is that you should be able to use the second amp to listen and find the first test point where the cracking noise appears. If it's everywhere but TP7 then suspect the zeners and caps that give you the =/- 16V.
 
The "Bump Test" didn't produce anything other than the sound of the spring in the reverb tank rattling. Plugging in a shorted cable in the Preamp in didn't produce anything I can report other than silence. The amp has a very low hum like all electronics seem to have but its is very low and I have to listen for it. Plugging in the shorted cable didn't change it. The only sound I got was when the metal of the plug came into contact with the socket. Of course my fingers were touching the metal of the plug and we all know that creates hums and scratches when we do that. But all went silent when the plug slid into home base.

I removed the amp from the cabinet last night and went over the top of the PC board looking for anything obvious. Nothing is burnt, scorched or broken. There were a couple small caps lightly touching a resister body but no metal to metal connections. I will be removing the the PC board shortly and checking all the solder joints. I plan to re-flow all the joints unless someone things that is a bad idea.

I would like to note though, last night after I removed the amp and was looking over the board I laid my hand on top of the transformer and it was pretty warm. Not hot but it seemed hotter than I expected. I don't know how hot these things get while the amp is being played but I played it on low volume - about 1 to 1.5 on the drive channel for about half an hour.

I rechecked all the connections from the power supply to the PC board and all connections are correct. I labeled each before I removed them last time. So if they were correct from the factory, they're still correct.

Cyclecamper. Tell me where to solder in these connections on the schematic so I can get these points installed and ready to check things if it starts shorting again.

BR.
 
FM212R in pieces w/pics

Its all apart and I have been over it visually and I don't see anything out of order, no strange smells, burn marks etc...

Attached are some pics. The Power Supply has varnish that ran out of the coil(s) which I assume happened when it was originally dipped.

The two big Caps are not discolored, misshaped etc.

There is the residue all over the solder joints that I am under the understanding is supposed to be cleaned off after soldering because that substance is supposedly corrosive. But I am not positive about that.

Not sure if I should reflow all the joints or not. I don't want to cause myself any more problems.

I think I will re-assemble the amp and leave it out of the cabinet, hook up the speakers and power and try wriggling wires etc.

BR
 

Attachments

  • Fender FM212R PCB1.jpg
    Fender FM212R PCB1.jpg
    840.7 KB · Views: 337
  • Fender FM212R PCB2.jpg
    Fender FM212R PCB2.jpg
    923.5 KB · Views: 288
  • Fender FM212R PCB3.jpg
    Fender FM212R PCB3.jpg
    608.8 KB · Views: 300
  • Fender FM212R PCB4.jpg
    Fender FM212R PCB4.jpg
    686.5 KB · Views: 805
  • Fender FM212R PS1.jpg
    Fender FM212R PS1.jpg
    512.8 KB · Views: 285
  • Fender FM212R PS2.jpg
    Fender FM212R PS2.jpg
    473.8 KB · Views: 160
I reflowed the solder on the main connections. i.e. Power supply terminals, the large Caps, all 5 of the jacks I recently installed and a few of the smaller ones. No I didn't over heat anything that I know off. I then mounted the PC board back in the chassis. I made a quick check of all the connections and discovered that the crimp connection on P7 moved within the connector. It wasn't flopping about, it was just loose enough to move it if I tugged and pushed back. I got really happy assuming that I had found the underlying problem. I recrimped all the push-on connectors and then I plugged the amp in without the speaker hooked up. I flipped the switch and it took about 2 seconds and it blew the fuse on the PCB.

I pulled the PCB again and rechecked all my solder joints and found what looked like a hair sized piece of solder between two of the small joints I had resoldered. I desoldered it and then soldered again. I checked all the joints again and found nothing so I reinstalled the PCB and installed a new fuse and hooked up the speakers this time and when I turned it on the speakers hummed for about 2 seconds and the fuss blew again.

So... Either I am missing something or I caused another problem OR I inadvertently caused the humming issue to finish "going out". So... I'm not sure where to look now but either way I don't have any more fuses. Tomorrow will be another day I guess...
 
Still blowing fuses

The joints I thought I had shorted are a ground leg for one of the pots and the ground leg of the adjacent "More Drive" switch.

If there is something shorted from my endeavors I cannot locate it.
I got more 4 amp fuses and removed the PCB from the chassis and hit the on switch and it just hums and then pops the fuse after a few seconds. I'm pretty sure what's happening now is what was going to happen eventually anyway. It used to blink out and hum intermittently, now its doing it non-stop.

Anyone have enough understanding of how to troubleshoot this? I have a Good quality DVOM.

BR