Fender Frontman 212R - loud popping and crackling

[olddawg]

Ok.. So I'm resurrecting this thread. Since I got this amp for $35 and had no use for it it sat in the corner even though I ordered and received the parts over a year ago, lol. BUT ... I needed a keyboard/acoustic amp for a rehearsal and show this week and figured this would work so I spent some time on it. I was correct in that the diff pair was causing the loud pop shoving DC into the output stage. I replaced them with the exact replacement transistors. The amp now works, sets quiet, no hum, all the channels work.(clean/drive/more drive) Foot switch works. All the LEDs work. Reverb works. All the tone controls etc work. The only problem is the clean channel isn't clean! It's a little hairy at all volumes. Just enough distortion to be annoying and unusable for a clean application. I thought these things were the poor man's Twin. Anyway.... I didn't notice the problem until I completely reassembled the amp of course. A little research has shown me this is a fairly common problem in various threads, but after a long discussion there never seems to be a resolution. I know this amp is a 5 year warranty exchange model. But they have been out for a while. Any of you Fender Authorized guys have any suggestions. I'm pissed at this thing. I have a schematic. (It was kindly provided in a previous post) The next step is to pull it apart yet again and scope it with a clean input signal. Actually I'm going to try a substitute speaker since I guess it's possible that the gunshot loud random DC offset from the bad diff pair may have damaged the speakers. But it doesn't sound like that really.

[52 Bill]

The only problem is the clean channel isn't clean! It's a little hairy at all volumes. Just enough distortion to be annoying and unusable for a clean application.

Divide and conquer, plug a signal into the power amp in jack and see if it still distorts.

[olddawg]

Oh I've done all of that Bill. I'm not a noob . It isn't the speakers. I did just discover the fool I got it from had the speakers wired out of phase. Effects/main amp in is clean. I'm suspecting a mute circuit problem. It's got "the crackles" but not super loud and not constant. If I unplug the main input and plug it back in it stops for a while. It isn't the jack or soldering in that area. I guess it will be scope and freeze time if someone can't point me to a known fix. The diff pairs, the slight distortion, and the rice crispies seem to be a common complaint. The diff problem was like a gun shot, lol. Like old Pioneer stereos!

[oc disorder]

I just spent far too long on a Frontman 65 R with exactly the same symptoms.. snapping and hissing and then nothing for ages, thought I was going to have to call the vet .... but it ended up being one of those taller TO92's. D12 D13 from memory one of those - possibly the 2SA1013.
The clue is they get warm but half a can of freeze and a few hours had no effect for me ....
I had a set of transistors for an Fen acoustic amp but the guy didn't end up showing up so I substituted in the end and found it that way.

[olddawg]

I just spent far too long on a Frontman 65 R with exactly the same symptoms.. snapping and hissing and then nothing for ages, thought I was going to have to call the vet .... but it ended up being one of those taller TO92's. D12 D13 from memory one of those - possibly the 2SA1013.
The clue is they get warm but half a can of freeze and a few hours had no effect for me ....
I had a set of transistors for an Fen acoustic amp but the guy didn't end up showing up so I substituted in the end and found it that way.

Thanks for that info. Funny thing. This thing has sat unplugged for a year and a half with me and who knows how long with the last guy. It looks brand new. I just let it cook all day. It's super quiet now. We will see how crackles in the morning. It might be something it only does when cold. The diff pair was a couple of 2SC2382s. Q10 and Q9. That stopped the random deafening gunshot. After cooking 6 hours the damn thing is working fine! I'm taking it to a rehearsal tomorrow at noon for someone else to use. That should make it fail if only to embarrass me...

[olddawg]

Ran an electric piano and an acoustic electric guitar through it for 4 hours straight. Worked great. I guess it will fail eventually, lol..,

[olddawg]

Back to rice crispies again. The amp is usable and sounds fine.... Except for the intermitant rice crispies. The clue should be it seems to be quiet with nothing plugged in to either channel. At the moment anyway. The second you plug in, the crackles start then settle down. Freeze doesn't seem to make a difference. Time to break out the scope.

[Glen]

Good day to you Olddawg,
I know it's been couple years since this thread's been inactive but I've been following the information posted. I have in my shop a Fender Frontman 212R with the same infamous crackling problem. I've changed various driver transistors, couple opamp IC's and resoldered connection showing heat stress and also cleaned all the potentiometers. Everything works good in the amp except for the crackling noise that is only present when a jack is inserted into the input. The noise takes about couple minutes to become very distinct and sometimes after 5 minutes or so it will become less distinct.
This problem seems very similar to yours so I was wondering if you ever figured it out?

[The Dude]

I've had several of these. Have you tried freeze spray? It's usually heat related. Also, feel the transistor bodies (careful not to touch anything that might shock you). Most likely something will be getting hot. There are a couple of TO92 Giant case style drivers that often are the cause. Maybe Q14 and Q15 if memory serves? One or the other is usually very hot. It will test good with a DVM, but crap out under load.

[olddawg]

Good day to you Olddawg,
I know it's been couple years since this thread's been inactive but I've been following the information posted. I have in my shop a Fender Frontman 212R with the same infamous crackling problem. I've changed various driver transistors, couple opamp IC's and resoldered connection showing heat stress and also cleaned all the potentiometers. Everything works good in the amp except for the crackling noise that is only present when a jack is inserted into the input. The noise takes about couple minutes to become very distinct and sometimes after 5 minutes or so it will become less distinct.
This problem seems very similar to yours so I was wondering if you ever figured it out?

The main problem I see with these amps is the diff amp pair. I resolved my problems by replacing every one of those transistors with the same designation. The 2sc2382s

[Gingertube]

Glen,
that is telling me it is definitely in the power amp section.
Note the current source (Q8) in the tail of the diff amp/splitter (Q9 and Q10).
The current source is switched off by the circuit around Q7 when nothing is plugged in (or it overheats, sensed by the RT2 thermistor). Follow the signal labelled "MUTE" to see how that happens.

The power amp section is a "***" to follow. but FWIW this is how I think it works.

The phase splitter drives a push pull driver stage Q12 and Q13 and the output from the collectors is the audio drive signal into the power amp.
Q14 and Q15 are Vbe multipliers to provide the bias points for the output transistors. The Vbe multipliers are transparent to the audio signal, they just provide the DC offset (approx 1.4 Volts) to bias the output transistor darlington pairs.
Q16 and Q17 are protection clamps which operate when the output transistors conduct too much current from a shorted speaker cable or similar.

WAG - check C59 which provides some AC current feedback to the power amp input.
Also check C55 and C56, I think these are bootstrapping the loads of the Q12,Q13 push pull driver (by making R92 and R99 look bigger than their actual value).
These are all electrolytic caps. not sure why one side has 10uF while the other has 47uF. Would have thought they should be the same although the different value bootstrap caps may be introducing some 2nd harmonic at lower frequencies.

There are also a heap of diode clamps around the place. My guess on these is that they are to assist overdrive recovery.

I have one of these which started with the fault you describe (noise) but eventaully went terminal with a totally shot power amp. Gave up trying to fix it. I have stripped the poweramp completely and am in the process of putting a KT66 push pull tube power amp in the bottom of the cabinet. The entire preamp section still works fine.

Cheers,
Ian

[Randall]

I have a FM212 in with the intermittant gun shot problem at cold power up, then it won't do it for quite a while. olddawg, I see you fixed yours by replacing the diff pair Q9 and Q10, but earlier you designated them as 2SC2362K and twice later you are calling them 2SC2382K. I found a source for 2SC2362K as found on the schematic, but before I pop for them I wanted to be sure I was getting the right ones, abd that I do not have to worry about matching them?

https://www.bdent.com/nsearch.html?q...BoCu50QAvD_BwE

[Enzo]

This is what freeze spray is for...

[Randall]

I have had no luck with freeze spray in this case.

[Glen]

Problem Solved!
I will continue to keep my fingers crossed.
Even though my Frontman 212R suffered with this Rice Crispy's crackle, the pop eventually became worse with loud gun shot sounds that could wake up any sleeping congregation. I decided to not let this dilemma defeat me and I adopted a thorough approach.
Gingertube supplied me with enough information that had me thinking and troubleshooting with a fine tooth comb.
It becomes very frustrating for us technicians to have to remove transistors several times in search for a faulty culprit. I remember someone mentioned Q9 & Q10 would get hot and testing them with th DMM they would measure correct but may be breaking down under heat stess. I could not place my hands on the original part 2SC2362K so I used a better equivelent which is slightly larger in case size handling better heat dissipation, part number 2SD667A. So Q9 & Q10 are now 2SD667A.
However this was not the only change that was made Q8 & Q13 use 2SC2383Y and while measuring both parts I found the measurement of Q13 odd. Usually base to collector and base to emitter readings are relatively close example 0.624 and 0.628 respectively. However the reading on Q13 was 0.624 and 0.687 which I found strange even though it other wise seemed good. I went with my gut instinct to change the part with an original 2SC2383Y. Even the new part 2SC2383Y had a better reading of 0.623 and 0.627
After assembling the circuit board back into the chassis and Jacking in a plug into the input with no audio I did not hear anymore crackling or gun shot popping. The Fender Frontman was very quiet and problem free.
Testing with audio it is clean and loud and I am monitoring the unit for any sudden outbursts but the repairs that I did drastically solved the problem.
I hope this info can assist else...good luck.