Fender Frontman 212 Intermittent Weak Distorted Output

[bobloblaws]

Greetings, everyone. I have a Fender Frontman 212 solid state amp with an intermittent issue. What happens is that from time to time it goes into a state where the output becomes weak and is distorted. I did some voltage measurements while in both normal and abnormal states. Please see attached for details. So far I've ruled out Q16, Q17, Q18, Q19, Q20, Q21, C55, C66, and D39. I didn't try changing D38 because I'm guessing that negative voltage is leaking over to the positive side as opposed to vice versa. I could use some advice on how to proceed from here, much obliged.




fender_frontman_212r_sch.pdf

[Enzo]

Quit looking for "bad parts" and isolate the problem.

The amp work, but sometimes drops output and clarity. The intermittent nature of the issue suggests a connection rather than a part. Fist suspect is FX loop return jack. other possibles are solder connections on larger parts like big caps or 5w or 10w resistors.

When this happens plug a space cord from FX send to FX return. ANy help? WHen this happens, plug the guitar into the FX return. IS the sound still affected or is it now clear We need to know if the power amp or preamp is involved. Or the connection between them (the loop jacks).

[bobloblaws]

Quit looking for "bad parts" and isolate the problem.

The amp work, but sometimes drops output and clarity. The intermittent nature of the issue suggests a connection rather than a part. Fist suspect is FX loop return jack. other possibles are solder connections on larger parts like big caps or 5w or 10w resistors.

When this happens plug a space cord from FX send to FX return. ANy help? WHen this happens, plug the guitar into the FX return. IS the sound still affected or is it now clear We need to know if the power amp or preamp is involved. Or the connection between them (the loop jacks).

Enzo, if I can actually get it to go into the abnormal state for a length of time at an appropriate time I can do those checks. However, I did previously narrow it down to power amp, or put another way, post FX return. I should have mentioned that. Another thing I didn't mention is that when it it does actually lock into the abnormal state I can "fix" it by touching my DMM probe to D21 or one of the adjacent components. After I posted this today I found this thread from pontiacpete who describes something very similar in post #27. Unfortunately he never did find a solution to his problem. Regarding solder joints, I did a complete visual inspection of all the solder joints as well as a thorough chopstick test while it was in abnormal state, neither of which uncovered anything. I suppose it might be worth just going ahead and giving every solder joint in the power amp section a fresh blast of solder and see what happens.

https://music-electronics-forum.com/...ad.php?t=35475

[bobloblaws]

Could be an anomaly, but at the moment the amp has been on for close to an hour without so much as a random crackle, something it was doing quite a bit previously. Since I'm dealing with a very intermittent problem I can't really do anything in the way of trouble shooting at the present so I reckon I will go ahead and add some solder and hope that it remains stable.

Now that I think about it, about an hour ago I hooked up a dummy load and turned the volume way up and banged out some guitar chords in an effort to see if I could get it to go into the abnormal state. That did not happen, but since then it has been perfect so I wonder if it is a fluke or whether what I did made some difference. I think I'll hold off on the soldering idea for now and see if it remains stable.

Edit: Meh, so much for that. No sooner did I click "Post Reply" than it started to crackle again. The gremlins are still hiding somewhere. I guess I didn't mention anything about the crackling part earlier. It makes seemingly random crackling noises when it feels like it but which sometimes, but not always, are an indication it's fixing to go into the distorted state again.

[bobloblaws]

OK, so presently it is locked in the abnormal state. Just to double check I did the suggested tests with the fx loop. I'm getting the same distorted waveform at the output whether plugging into the input and not using FX loop, or plugging into the input and jumpering the FX loop, or going direct to the FX loop return. Here's what it looks like:

[bobloblaws]

Also getting this waveform at times. I'm monitoring the 0V rail and in the case of the first scope pic it was at -.02V and when I took this second one it had changed to -.2V, FWIW.

[glebert]

I would get out the chopsticks and start tapping, especially bigger components like power resistors.

[bobloblaws]

I would get out the chopsticks and start tapping, especially bigger components like power resistors.

Thanks, but I refer you to my earlier statement "I did a ... thorough chopstick test".

Chopsticking has no effect in either scenario.

[glebert]

Thanks, but I refer you to my earlier statement "I did a ... thorough chopstick test".

Chopsticking has no effect in either scenario.

Sorry, missed that. Since you say probing near D21 helps I wonder if you are getting an oscillation in that area and the DMM is adding some capacitive load. I wonder if the DMM leads can even act as a "gimmick capacitor" (a term I just learned lately so I could be getting the concept wrong). I have a Peaveiy Triumph that is noisy but gets quieter when I probe some of the PI tube pins and I am going to explore if a nearby capacitor has dried out.

[olddawg]

Try some freeze mist... frequently these units are riddled with loud crackles and pops from intermittent transistors.. especially the diff amps..

[The Dude]

^^^^^^ Yes, that. Also, when it's in the "broken" state, see if you can detect anything getting overly hot with your fingers.

[bobloblaws]

^^^^^^ Yes, that. Also, when it's in the "broken" state, see if you can detect anything getting overly hot with your fingers.

Unfortunately I don't think I have freeze spray at the moment. I'll order some. However, my feeling is it is not heat related. I have touch tested the transistors in the broken state (as well as I could mangae) and didn't notice anything different than the normal state. Also, the length of time the amp has been on doesn't seem to be a factor. For example, I've returned to it after the amp having been off for a couple of hours and turned it to find it "broken".

Anyway, I'm quite perplexed. I started tracing backstream to see if I could figure out at what point the waveform started going bad, in other words, hoping I could find a point where the signal was not distorted in either state. I ended up backing up into the preamp, which is crazy because my test signal is being injected straight into the FX return. For example, right now I am scoping pin 7 of U5. Generally there is no signal there as you would expect, but occasionally it will show a beautiful sine wave and I can actually hear the test signal despite being hooked up to a dummy load. So right now it is flipping between those two states and the DMM reading I am monitoring on the power amp 0V rail changes from about 14mV when there is no signal at pin 7 and 2mV when the waveform appears. I have no idea how signal is showing up at these points in the preamp since the connection from C39 is open when the signal lead is plugged into the FX return jack. In any case, checked the +16V and -16V supplies and they are both at specified voltages in either state.

Now, a little more explanation is necessary. The above is what I've observed since I tried to temporarily "fix" the issue by touching something metallic to D21 about an hour ago. Prior to that I've was messing with it most of the afternoon and the situation was more like I was describing last night. In that case the signal at pin 7 and other points in that general area was pretty reliably present and the OV rail hanging around 15mV or so, and when it went rogue the 0V rail would go to -900mV or -400 mV and the perfect sine wave would change to something wierd as in this pic (which is actually scoped on pin 1 of U5). Other than basically re-soldering every solder joint on the PCB and hoping for the best I'm at a loss.



EDIT: I was a little off track in the first paragraph of this post. I just switched my scope probe from U5 to the amp output and discovered that in the scenario where the 0V rail shows 15mV and there is no signal at pin 7 of U5, there is actually no signal at the output. So that is actually abnormal. When it flips over to the other state where the 0V rail is at 2mV (effectively zero I presume) there is signal at the output and it is somewhat audible even without a speaker driver. This is obviously the working state, and I had it backwards above. In any case, it is behaving differently than it did for most of yesterday and today. At the moment the 0V rail is not periodically going negative which is the original issue I was encountering. Having said that, I'm going to try plugging back into the input again instead of the FX return to see if that makes any difference. But I'm still mystified why I'm even getting signal on U5 with the preamp supposedly bypassed.

[g1]

Hearing a bit of 'singing' from your load is not all that unusual.
As far as U5 goes, one side of it is for reverb return, so if the reverb control is not all the way down, you might get something picked up by the springs.
Do you have anything at all connected to the 'speaker -' lug ? It must not get grounded by anything. Best to have load and any grounds connected to chassis.

[bobloblaws]

Hearing a bit of 'singing' from your load is not all that unusual.
As far as U5 goes, one side of it is for reverb return, so if the reverb control is not all the way down, you might get something picked up by the springs.
Do you have anything at all connected to the 'speaker -' lug ? It must not get grounded by anything. Best to have load and any grounds connected to chassis.

You could be right about the reverb, I thought I had it all the way off but I just discovered the pot was slightly turned up.

I'm running alligator clip test leads from speaker out to either my test speaker or dummy load, both + and -. I'm not sure what you mean that it must not get grounded. The - lug is already grounded anyway, no? So whether I connect my negative speaker terminal to the - lug on the amp or to the chassis is effectively the same thing I would think. Maybe I'm misunderstanding.

Anyway, I think Enzo might have been on to something regarding FX loop. I'm always monitoring that 0V rail and I just noticed that just plugging a cable into the return jack causes it to start swinging negative intermittently. The solder joint to R62 isn't loose but it looks sketchy. I'm gonna try re-flowing that and then possibly everything connected to that jack, or even replace the jack. Here's hoping.

[bobloblaws]

Question: In order to test whether the FX loop is part, or the entirety, of the problem, can I simply run a jumper from C39 to C40 and bypass the FX loop section entirely? It seems like a no brainer to me, but I wanted to get a second opinion. Or should I remove the jack first?

Re-soldering the return jack and connected components did not solve the problem, nor did a blast of Deoxit.