Fender 212R power amp blown

[Enzo]

It is built like all recent Fender SS amps that used the TO218s. An aluminum plate the transistors are screwed to on the edge of the pcb, the big bar-ingot under that to transfer heat to the chassis.

[lowell]

Here's some pics. Can I just unmute the damned thing and call it quits? Or bad idea? The thermistor will still function so that's good right?

[Enzo]

Unmute it and cycle power a few times. Does it make any noises that sound like they threaten anything?

[R.G.]

Actually, this is a good candidate for a "soul transplant".

It's a roomy chassis with power parts already available. Trash the PCB. All of it. Or identify and saw off just the power amp part, and simplify it back to a workable power amp.

Re-use the pots and knobs if you can. Build up the circuitry of a good solid state amp on perf or PCB, and put that on the open space where the PCB is now, and run wires to the pots and jacks.

I'm in the process of doing this to a "Rogue" amp I got for $40 from craigslist. It's a working 100W amp with 2-12s in a combo. The speakers are probably trash, but the power transfomer, chassis, power switch, heat sink, etc and the cabinet are all reuable. I'm putting the front end to a Vox Supreme into it. I recently also did PCBs for the Thomas Vox Super Beatle amps, and they'll probably go in something like this.

I make a strong distinction between electronics work, which is easy and straightforward to do, and mechanical/electrical construction, which is a lot of work and hard to get right. A working power supply, chassis, cabinet and wiring, even with a working power amp is a great start to a great amp. All you have to do is not re-make the mistakes that were in the original electronics.

But yes, if you're willing to put up with the burst of hum, un-mute it, call it good. Test that a bit to make sure it doesn't over time kill the speakers, maybe.

[J M Fahey]

Hi Lowell.
Personal opinion, I would unmute it if it solves the offset problem and leave it like that.
No, the thermistor won't work anymore, big deal. Its function was protective, yet the amp burnt anyway. I wouldn't lose sleep over it.
I would test the amp for 2 hours full blast, hope you have a basement or very comprensive neighbours; if everything fine, send it home.
If unsure , don't charge your work, just the parts, sleep in peace.
Dear Enzo: I don't get it, the amp does not look "inexpensive", it has the exact same construction and components than all other SS Fenders of the same power, the same board material, relatively expensive aluminum chassis, with its side mounting "wings", a large and complex board, it has the black inrush thermistor, good quality cables, "jewel" pilot light, generic good polyester capacitors (they might have used ceramics), a big power transformer, tiewraps, push-connector sleeves, the full Monty.
Are similar "brothers" repairable? Or they are also "replace-only?"
It's expensive to make this amp, on the same level of a similar Peavey, Crate, etc.
Of course, that's comparing it with the same market class, do not compare it to a Victoria !!.
Lowell, do me a favor, please check that C59 is correctly oriented, it's twisted 90º and it *might* be backwards. Don't consider it probable but check anyway. Thanks.
Well, I think many are already tired of this amp, and you must be even more so, try to send it through your door.
Good luck.
PS: if you are worried about thermal issues, you can add (on the underside) an 85ºC (or even 75ºC) bimetallic thermal switch, and use it to cut 120V power, á la Peavey.

[Enzo]

Remember, the only thing non-repairable about it is its warranty status.

Pretty much the entire Frontman and FM series of models is replace only, as is the Rumble series. Also Champion series, Bullet series, and the -DEC amps. Small Passports, and small SWR models.

There are a few Frontmans made in Mexico instead of Indonesia, those are limited to 1/2 hour claims ($25)

Why this and not others? DOn;t know. Assembly should be about the same as any other amp. Perhaps there are labor cost savings where these are made compared to other import models. Cheaper materials. I don't get the impression that the cabinetry and speakers are of the same quality as pricier models. Looks OK from a distance, but when I put my hands on it, it does seem cheaper. Asembling the pc board is done by machine, so adding complexity just means loading different tapes into the pick and place machines.

Comparing some more or less similar models shows me two output devices for this amps 90 watts, while others have six.

[lowell]

"Hi Lowell.
Personal opinion, I would unmute it if it solves the offset problem and leave it like that.
No, the thermistor won't work anymore, big deal. Its function was protective, yet the amp burnt anyway. I wouldn't lose sleep over it." -JM

But isn't the thermistor there to protect the amp while the amp is being played, and if the power/output transistors get too hot, AND when the amp is unmuted?

[MarkusBass]

There is 0v across R85/86.

Thank you for measuring this - finally. I assume that this is after you replaced Q10. After this I would immediatelly measure voltages on R83, R84, R87, R88, diodes D25-27. Just as a confirmation that the problem with the input stage is solved.

I replaced Q10/11 and now they are both officially OFF. No voltage across D25-27.

I think that only Q10 needed replacement. We checked that Q11 was working correctly.

Problem is, there's still DC offset on speaker jack, granted it's less than before at about -2.2.v now.

So there is still a problem with the output stage.

Q18 Vb: .94v
Q19 Vb: 3.35v
WOW

Q20 Vb: 1.6v
Q21 Vb: 2.7v
WOW again

And what exactly this "WOW" means? Are you surprised, are you happy, or what? Do you see here anything unusual? If not, why do you write "WOW"?
Lowell, I told you that in case of transistors, it is better to provide Vbe voltage instead of only Vb. Voltage measurements are "relative to". Did you provide voltages relative to the ground? If you did, either you made a mistake or there is a clear indication of a problem. If you measured the voltages relative to the ground, you most probably lost the sign of the voltage. Shouldn't the voltages on Q20 and 21 be negative (so rather -1.6V and -2.7V)? If this is the case, you have -0.65V voltage drop on Q19 (Vbe). This looks OK. And what voltage you have on Q18 (Vbe)? It should be something like 0.65V. And you have what? Definitely this 0.99V on base of Q18 looks incorrect.
I would disconnect D32-33 (since this is just a shourt-circuit protection), check again DC offset, check the voltages on Q18-20 and Q19-21, and if the are still incorrect, I would replace Q18 (first check whether you made a mistake while measuring it).
I would also look at D34-37 diodes. If any of them is shorted, or open, they could also cause this DC offset.
And don't listen to advise to build a new power amp - you just have to check 6-8 part and maybe just one of them failed - you are very close to fix the amp (and the cost of parts needed will be 1$).
Enzo is right that this amp has to few output transistors. 2SA1294 is only 130W so the amp most probably cannot deliver 100W RMS. Rather something like 70W. And in this sense I agree that the amp is poorly designed.

Mark

[J M Fahey]

Enzo, thanks for answering.
All I can say is: Amazing!!.
Considering everything, including speakers, electronics, cabinet (even the tolex !), specially transformer iron and copper (which have a fixed price worldwide by the pound, no matter the salaries involved), this amp costs more than 50U$ to produce, definitely.
You have to move them from Shanghai (or Indonesia/Hong Kong/Vietnam) for no less than 20 to 25U$S each, plus moving "X" amount of pounds of *anything* by truck through the States costs at least the same.
It just doesn't add up.
Well, maybe other economic rules apply to them. I don't understand it.

[lowell]

Thank you for measuring this - finally. I assume that this is after you replaced Q10. After this I would immediatelly measure voltages on R83, R84, R87, R88, diodes D25-27. Just as a confirmation that the problem with the input stage is solved.

D25-27 have 0v across them.
R83,84,87,88 have 0v across them.

I think that only Q10 needed replacement. We checked that Q11 was working correctly.
So there is still a problem with the output stage.
And what exactly this "WOW" means? Are you surprised, are you happy, or what? Do you see here anything unusual? If not, why do you write "WOW"?

WOW means WOW, those voltages seem way off. Sorry clearly I'm still getting the hang of the specifics of posting data on solid state repairs.

Lowell, I told you that in case of transistors, it is better to provide Vbe voltage instead of only Vb. Voltage measurements are "relative to". Did you provide voltages relative to the ground? If you did, either you made a mistake or there is a clear indication of a problem. If you measured the voltages relative to the ground, you most probably lost the sign of the voltage. Shouldn't the voltages on Q20 and 21 be negative (so rather -1.6V and -2.7V)? If this is the case, you have -0.65V voltage drop on Q19 (Vbe). This looks OK. And what voltage you have on Q18 (Vbe)? It should be something like 0.65V. And you have what? Definitely this 0.99V on base of Q18 looks incorrect.

Vbe
Q18: .6v
Q19: -.6v
A20: .57v
Q21: -.56v

I would disconnect D32-33 (since this is just a shourt-circuit protection), check again DC offset, check the voltages on Q18-20 and Q19-21, and if the are still incorrect, I would replace Q18 (first check whether you made a mistake while measuring it).
I would also look at D34-37 diodes. If any of them is shorted, or open, they could also cause this DC offset.
And don't listen to advise to build a new power amp - you just have to check 6-8 part and maybe just one of them failed - you are very close to fix the amp (and the cost of parts needed will be 1$).
Enzo is right that this amp has to few output transistors. 2SA1294 is only 130W so the amp most probably cannot deliver 100W RMS. Rather something like 70W. And in this sense I agree that the amp is poorly designed.

Mark

I already stated that I disconnected D32/33 and the offset was still there.
I also stated that I replaced D34-37 and still offset.

Q18-21 Vbe look good.

So back to my other voltage readings to ground: (corrected)

Q18 Vb: -.97v
Q19 Vb: -3.39v

Q20 Vb: -1.6v
Q21 Vb: -2.76v

I'm not sure what to tell of these readings. I would expect Q18/19 bases to be the same but reverse polarities. IE: 1v/-1v. (2v apart) I guess it's hard to get an accurate reading on the bases of transistors because of the meter impedance?

[MarkusBass]

So back to my other voltage readings to ground: (corrected)
Q18 Vb: -.97v
Q19 Vb: -3.39v
Q20 Vb: -1.6v
Q21 Vb: -2.76v
I'm not sure what to tell of these readings.

If you'd provide these voltages in this order: Q18, Q20, Q19 and Q21, they would be more "readable". Then, if you'd shift them by 2.2V, you would see that they are symmetric. The output stage in this amp are just emitter followers (they follow voltages from the input). Can you check what voltage you have on the input of the output stage (collector of Q12 and Q13)? Isn't it -2.2V? If it is, can you check the voltage on R90 (to see whether Q13 is conducting)?
The idea to disconnect this input fromthe differential input wasn't that bad. Is it posible (is there a trace that could be cut?).

I guess it's hard to get an accurate reading on the bases of transistors because of the meter impedance?

Typical multimeter has very high input impedance and it is not a problem unless you have a very old meter.

Mark

[Enzo]

Fahey - There are additional costs supporting things like this. For example, I first found out the FM212R was on the do not repair list when I tried to get a replacement speaker for one. I knew about small amps, but it never occurred to me something this large would also be on the list. The parts guy says, "Uh, you know these are not repaired in the field, don;t you?" Well, no I don;t know that.

I could have gotten the speaker but it would have to have been special ordered in from overseas. AT least at the time.

SO to support something under warranty by repair means they also have to catalog and stock all the parts for said amps. All that and the warehousing adds to the cost per unit. Now that the product is in the market, there are some listed parts, the speaker, the controls, the reverb pan, and the knobs. Woops, and input jack. That is the entire parts list.

There are accounting costs as well. I once called my support guys over some small amp a customer had just bought, the store had no stock to exchange with, and I looked and it was just a loose wire or something I fixed in minutes. I knew I wasn;t supposed to fix it, but for a few minutes of my time, I made the customer happy. I asked if I could get special dispensation for doing the repair, and he told me no. He explained that just processing the warranty claim cost them $18 on their end. In that case, as a thank you for looking out for the customer, he gave me a quarter hour special authorization labor and told me to just add that labor to my next claim for something else.

And consider if the warranty station was having this much trouble, as a gold level tech, I can claim as much as two hours on these things, that's $100.

Even their SR6325 powered box mixer is on the list.


As incongruous as it may seem, I am pretty sure their bean counters have figured it out, and they come out ahead this way.

[J M Fahey]

Enzo, thanks a lot.
You offer a vision on things that nobody ever comes even near to, unless you are , as you say, a "Gold Level Tech", which clearly was won over years and years of effort and dedication.
Not to forget other very high level Techs which also contribute here generously.
Thanks again.

[lowell]

"Can you check what voltage you have on the input of the output stage (collector of Q12 and Q13)? Isn't it -2.2V? If it is, can you check the voltage on R90 (to see whether Q13 is conducting)?"

Yes.
R90 has 0v across it.
Q13 Vbe is -75mv

Guess Q13 is indeed off.

"Lowell, would you please cut the track that, on the schematic, goes from the lower leg of R95 to the left, separating it from the collectors of Q12 and Q13? (and C51/52).
Everything to the right is symmetrical and has no gain, and *should* have 0V offset if
the string of R91, 92, 94, 95, 96, 97,99, 100.
is symmetrical, as RG pointed."

I don't understand this. Don't the collectors of Q12/13 move the power amp up and down thus making the amp work?? Maybe you're suggesting this just for troubleshooting the offset, to separate the first half of the PA from the second half?

[Enzo]

Bingo.

At this point you are trying to find the source of your DC problem, not amplify music. If the right half of hte amp settles down and centers on zero when left alone, then we know that side works.