Fender FM212R has destroyed its speakers

[xtian]

Client brought this solid state Fender RM212R complaining of distortion. Well, both speakers are semi-blown--I can hear the voice coils rubbing when I manually exercise them.

The first couple of times I turned on the amp, there was a brief (half a second) blast of low frequency noise (possibly 60 or 120Hz)--obviously this is the malfunction that destroyed the speakers!

I disconnected the fast-on jacks from the speakers, and tested them--no AC or DC present while amp is idling. I have a DSO with Aquire mode, but can't figure out how to use it. Finally, I decided to shoot a video, but the violent noise no longer happens at startup! However, there is a loud pop at power down, captured on video (you can hear the bad speakers complaining!):

Can this behavior be explained by bad filter caps? Any other likely place to look for bad components?

Schematic:

Frontman 212R Schematic.pdf

[The Dude]

I would say filter caps is probably a good first check. This symptom is often caused by the rails not coming up evenly. Same at power down if they don't decay evenly.

[xtian]

Thanks, Dude. I'm used to tube-amp world, where testing filter caps out of circuit is mostly useless (caps measure fine but fail under high voltage). What about these 4700uF/50v filter caps, is it worth measuring them with my LCR meter and/or multimeter?

[The Dude]

That's what I would do (test). You'll have to pull them and go beyond a simple ESR test. What I have found is that sometimes those large caps will lower in capacitance. For instance: A 1000uF cap will still show good on an ESR test, but if it's supposed to be 4700uF it will create an imbalance. Just my 2 cents.

Edit: I've also seen lots of problems with Q12 an Q13 on those amps. They will usually check good, but fail after warmup. Check voltages on those transistors and see if you note any odd voltages, especially after warmup.

[Enzo]

I have an alternate theory. Someone cranked the amp and screamed through it until the speakers died. Loud turn-on thumps pops or noises won't hurt the speakers. Those noises are no louder than a max signal through the amp.

[The Dude]

Certainly possible. I'd have to actually hear the thumps to know if it's a normal type plosive. I do agree that there is often turn on/off noises with lots of SS amps.

[glebert]

Put this in the "wild guess" column. You said the noise is only there for the first second or so. I think RT1 is supposed to give the power supply a bit of a soft start. I'd check and make sure no one put in a jumper there instead. I've had several amps of this type where RT1 started causing odd problems.

[J M Fahey]

+1 on the plain loud overblown speakers.
Which will multiply any noises passing through them.

Please post a label close up, those speakers do not look original to begin with.

Frames (at a distance) look like generic Eminence, FM212 uses some mystery Oriental speaker, which in any case sound reasonably good and mtch amp power.

Maybe owner read one forum too many and fit there "whatever SRV used" or some similar crap .

because, you know, using the same underwear as Jimi Hendrix turns you into a Guitar God.

Try the amp with another cabinet, just in case have some footswitch or a switch with a large lever or similar to cut them off from amp the instant you hear something dangerous (STRONG hum, cones moving fully forward or backward, etc.)

Check those caps, no excuses for NOT measuring anything as Enzo often says, but not highly suspect in my book, and no bad ESR would blow speakers, worst case amp would hum (excessive ripple), chirp (instability) or motorboat (same thing) but not nuke them.

Big caps might have mechanical problems , such as cracked solder pads (resolder them) or plain torn/broken legs.

Tap them and around with a BIC pen , or slightly push them around with fingertip, any pops/thumps?

Remember to post label picture and any other inscriptions on speakers (little white rubber stamps, or small stickies).

Get ready to buy a couple working cheap Guitar speakers or sell him any "pulls" you have lying around ,or Craigslist/garage sales may bring perfectly good ones for, say, $15 each.

Many replace factory ones with Celestion or the better Eminence and have NO use for the old ones, gladly accept burger or beer money for them.

[The Dude]

......because, you know, using the same underwear as Jimi Hendrix turns you into a Guitar God......

..

[glebert]

Frames (at a distance) look like generic Eminence, FM212 uses some mystery Oriental speaker, which in any case sound reasonably good and mtch amp power.
.

I'm pretty sure I've seen FM212s with those speakers, no idea what they are though.

[J M Fahey]

Thanks.
Looked around a little, found exactly same FM212 is made in widely different places, including China and Indonesia, and also they use very different speakers.
Cheapest bidder of the day I guess?
It applies to speakers too, so far I saw 3 or 4 very different ones: the very Asian frames (5 or 6 spoke with wide reinforcing ribs), Eminence type 6 spoke pinky width rib as you show, and black or silver Celestion-ish frames, most certainly some Asian copy.

Searching that also found a video *also* showing nuked voice coil speakers, where amp "works fine" for a few minutes, then burps and tries to murder speakers, so you are not the only one.

Worrying.

https://www.youtube.com/watch?v=_tMiP9NYk3Y

In this case power transistors eventually shorted .... but they started "working fine", not exactly trust inspiring.

[xtian]

Agree about pops being normal on power up/down, but BRRRROPPP I heard was certainly abnormal--the kind of noise where wife peeks into workroom to see if I'm still standing. Client mentioned hearing this as well.

I emailed client last night, spelling out the expense of repair. I also sent him a link to a used FM212R on guitarcenter.com for $165... so I think it's unlikely he'll want to repair this one. I wouldn't.

Will report.

[glebert]

I emailed client last night, spelling out the expense of repair. I also sent him a link to a used FM212R on guitarcenter.com for $165

Yeah, if the speakers are bad that makes repair pretty tough to justify. Too bad, the clean sound on them can be pretty good.

[xtian]

Client did not want to dump money into the open pit; took his boat anchor home. Metaphors successfully mixed.

[Enzo]

Hey, you go to the well too often and you get burned...