Fender Frontman 212 Intermittent Weak Distorted Output

[OwenM]

I am not the one qualified to advise here, but FWIW I did read a tonne of threads about this amp (which many here posted on), and got some help here directly when I had the same amp on my bench recently.

It really was a nightmare job and most of the normal or suggested culprits tested fine. The tonne of negative feedback it uses meant it was really hard to see where an injected sine was becoming distorted in some way. And the shape of the fault on a sin wave changed day to day as it slowly (fully) failed, which appeared in various, slightly different ways, all over the power amp circuit.

But my symptoms were similar to yours, and I got mine fixed in the end and oddly it was something that hadn't come up before, one of the fuse-able resistors on the power rails, r87 in my case. It started by clicking and giving out a crappy weak signal on and off, with the sin wave a bit messed up. And by the end (though I only saw it, didn't hear it) one side of the sin wave was very cleanly clipped. By the time it had failed entirely, and I was at the stage of just checking everything because I had no logical process to work on, I just came across it as failed at a few MOhm. replaced it and job done.

I've been suspecting various secondary problems (or maybe more accurately, primary problems that caused it to go in the first place) but it gets a lot of use and all good so far so maybe it was just a crappy batch of resistors...
may help I guess!

Heres some threads I also bookmarked when working on this, some with folks from this here forum giving good advice - You're probably a fair bit further on than I was when I started fixing it, but plenty of times these threads gave me something to look at

https://www.thefretboard.co.uk/discu...r-buzzing-help
https://www.diyaudio.com/forums/inst...er-fm212r.html
https://www.music-electronics-forum....ad.php?t=41201

Good luck! I found the whole thing a total bear to try and understand, but satisfying to get there in the end

[bobloblaws]

I am not the one qualified to advise here, but FWIW I did read a tonne of threads about this amp (which many here posted on), and got some help here directly when I had the same amp on my bench recently.

It was a really nightmare and most of the normal or suggested culprits tested fine. The tonne of negative feedback it uses meant it was really hard to see where an injected sine was becoming distorted in some way. And the shape of the fault on a sin wave changed day to day as it slowly (fully) failed, which appeared in various, slightly different ways, all over the power amp circuit.

But my symptoms were similar to yours, and I got mine fixed in the end and oddly it was something that hadn't come up before, one of the fuse-able resistors on the power rails, r87 in my case. It started by clicking and giving out a crappy weak signal on and off, with the sin wave a bit messed up. And by the end (though I only saw it, didn't hear it) one side of the sin wave was very cleanly clipped. By the time it had failed entirely and I was at the stage of just checking everything because I had no logical process to work on I just came across it as failed at a few MOhm, replaced it and job done.

I've been suspecting various secondary problems (or maybe more accurately, primary problems that caused it to go in the first place) but it gets a lot of use and all good so far so maybe it was just a crappy batch of resistors...
may help I guess!

Heres some threads I also bookmarked when working on this, some with folks from this here forum giving good advice - You're probably a fair bit further on than I was when I started fixing it, but plenty of times these threads gave me something to look at

https://www.thefretboard.co.uk/discu...r-buzzing-help
https://www.diyaudio.com/forums/inst...er-fm212r.html
https://www.music-electronics-forum....ad.php?t=41201

Good luck! I found the whole thing a total bear to try and understand, but satisfying to get there in the end

Thanks Owen, I'll take another stab at it ASAP with all that in mind.

[g1]

Did you get any freeze spray yet? Active Tech or Sayal have it for around $20 a can after tax.
When you consider how much time it saves you, it's a lot cheaper than shotgunning parts.

[bobloblaws]

Did you get any freeze spray yet? Active Tech or Sayal have it for around $20 a can after tax.
When you consider how much time it saves you, it's a lot cheaper than shotgunning parts.

It has never appeared to be a heat related issue but all the same I tried the upside-down canned air on Q9 and Q10 as you suggested. No difference.

[g1]

I think it was discussed earlier in the thread, but it is not just for thermal issues. The shock/contraction effect can be very effective at weeding out hard to find crackles etc.
It's hard to describe just how valuable it can be. The compressed air can work, but is not quite the same. Even for suspected resistors like the ones Owen mentioned, a quick shot to the right resistor could make it exhibit the fault, and now you are essentially done.

It's your call, but you said you were at the end of the line, short of 'shotgunning'. My point is that you are not there yet till you've tried freeze spray. For anything beyond hobby level, it's a real game-changer.
(BTW, apparently some use it to kill warts too. That's why you see it on some med. supply sites for twice the price. )

[olddawg]

There’s really nothing “special” about these amps except they seem to have a lot of intermittent problems. But as Enzo will tell you.. you don’t hear about the thousands that work fine. The last one I have I Picked up for $35 off of CL because it had pops. Mine had diff amp issues. They are really bright amps... as in trebly. Almost unusably so imho. You mentioned building a tube amp in it. Shouldn’t be a problem. Strip the chassis and build a 2x EL84 or 6V6 amp would be my suggestion. Strip the chassis and find a junker/donor amp if you want to keep the price down. The speakers work but are nothing to write home about..

[bobloblaws]

I think it was discussed earlier in the thread, but it is not just for thermal issues. The shock/contraction effect can be very effective at weeding out hard to find crackles etc.
It's hard to describe just how valuable it can be. The compressed air can work, but is not quite the same. Even for suspected resistors like the ones Owen mentioned, a quick shot to the right resistor could make it exhibit the fault, and now you are essentially done.

It's your call, but you said you were at the end of the line, short of 'shotgunning'. My point is that you are not there yet till you've tried freeze spray. For anything beyond hobby level, it's a real game-changer.
(BTW, apparently some use it to kill warts too. That's why you see it on some med. supply sites for twice the price. )

I appreciate that and I'm thankful for the help, maybe down the road it will come in handy. But presently it's exhibiting the same behavior that Owen described, gone from bad to worse. It no longer "works" for any duration longer than a second or two, so when blasting a component I think it would be impossible to know if anything happening signal or voltage wise was a result of the coolant or just a typical fault given the conditions.

In any case, I'll hold off on completely abandoning that idea for now. I'll take a video when I get a chance of what's happening sound-wise in conjuction with the DMM reading on the 0V rail.

[bobloblaws]

There’s really nothing “special” about these amps except they seem to have a lot of intermittent problems. But as Enzo will tell you.. you don’t hear about the thousands that work fine. The last one I have I have I got for $35 off of CL because it had pops. Mine had diff amp issues. They are really bright amps... as in trebly. Almost unusably so imho. You mentioned building a tube amp in it. Shouldn’t be a problem. Strip the chassis and build a 2x EL84 or 6V6 amp would be my suggestion. Strip the chassis and find a junker/donor amp if you want to keep the price down. The speakers work but are nothing to write home about..

I only played it at bedroom level but I thought it sounded OK for a SS amp. 2 x 12 should provide adequate bass response and can always roll off treble.

As for a tube amp project, it still might come to that :-)

[olddawg]

Have you changed the diff pair? They are about $1 from MCM last time I looked.

[bobloblaws]

Have you changed the diff pair? They are about $1 from MCM last time I looked.

Yes I did. If recall correctly it wasn't the exact transistor but specs were close. Symptoms were identical.

[olddawg]

They use the same transistors all over the amp so you can swap them. The important thing with a diff pair is they have to have a matching hfe (beta). In the old days we used to have to get beta matched pairs. Specs are tighter now. But I would never use a sub for a diff pair without measuring the beta.

[g1]

The good news is that a 'no sound' condition is likely easier to track down than a 'random crackles'.
Back in post #3 you were able to get it to change by probing around D21, maybe see if that still has any effect.

[bobloblaws]

I've been systematically swapping in replacement components (typically in pairs, one on the positive side as well as the counterpart on the negative side). If I start out by touching a screwdriver tip to D21 it typically runs correctly for a couple of minutes, then the voltage on the center rail goes to some negative value (e.g. -0.05 VDC) and the output is distorted. So then I would reconnect the original components and assume they are not the culprit.

Then I got to D34 and D36. Swapped in a couple of replacement zeners and I'll be damned if it didn't run fine for a half hour. That wasn't enough to convince me that I'd found the problem though, because it's been very unpredictable in general. So I decided to reconnect the originals and wait for it to fail so I could see if swapping in the new ones again would toggle it back to non-fail. However, since doing that the amp seems to be running without issue. I've left it on for over an hour at one point and I've played guitar through it at various times over the last 24 hours and it appears to be stable. The voltage on the center rail sometimes changes from 0 to 0.001 or 0.002 and if I touch D21 it will go to 0.004 or 0.005, which is what I've observed for the entire time I've been trying to solve this. At one point yesterday that voltage was reading a few millivolts on the negative side, but there was no failure in terms of distorted output.

So it's hard to say whether it's good to go or whether it's just playing games with me. If it is in fact OK it would have more satisfying to be able to point to a specific component that was bad and was replaced. As it is, it seems quite strange that it just magically seemed to sort itself out after disconnecting and then reconnecting D34 and D36. Can't look a gift horse in the mouth I guess.

[Sparky2]

I think it was discussed earlier in the thread, but it is not just for thermal issues. The shock/contraction effect can be very effective at weeding out hard to find crackles etc.
It's hard to describe just how valuable it can be. The compressed air can work, but is not quite the same. Even for suspected resistors like the ones Owen mentioned, a quick shot to the right resistor could make it exhibit the fault, and now you are essentially done.

It's your call, but you said you were at the end of the line, short of 'shotgunning'. My point is that you are not there yet till you've tried freeze spray. For anything beyond hobby level, it's a real game-changer.
(BTW, apparently some use it to kill warts too. That's why you see it on some med. supply sites for twice the price. )

I have used "Spray Cool", canned spray Freon, since the 1970's to find thermally failing electronic components. I fixed my first old 1935 radio using it when I was 21! I have not gotten up the nerve to try any myself to freeze off Warts or other skin lesions. So far I have left that to my Dermatologist, since my insurance pays for it.

[bobloblaws]

I have not gotten up the nerve to try any myself to freeze off Warts or other skin lesions. So far I have left that to my Dermatologist, since my insurance pays for it.

Personally, I swore off playing with frogs and haven't had a wart since!