Fender Frontman 212r sound breakup and crackles diagnosis

[OwenM]

Hi all

So after fixing one 212r I find myself with a second with a more curious symptom. The amp works but breaks up in an almost bitcrushed, digital way on the fadeout of chords, and sounds generally very unwell.

It's in the power amp section, the preamp going to another amp sounds fine, and the problem remains the same sending a preamplified signal from another 212r to the power amp input.

The board looks clean, I've cleaned all jacks and pots, the power resistors that are often faulty on these amps are securely soldered (as far as I can tell) and in spec. The board looks generally good and connections check out wherever there's any question over them. The big caps are in spec from a quick test in circuit with an ESR meter.

The 4 big transistors have only been checked in circuit, but they show 0.6v voltage drop in most directions, not open in some as expected, but I've ONLY tested these in circuit and haven't checked the schematic properly to see if this is normal yet (in truth, I'm still learning to see that part of signal flow on a schematic, haven't worked on many transistor amps) so I don't know if I'm reading that voltage drop because of other transistors in the circuit. I can take them out and test but wanted to get some advice first as they are connected in a fairly robust manner that I didn't want to mess with without input from people here!

So something is out, but this type of symptom is new to me, I'm probably more comfortable dealing with 'no sound', 'loud hum', 'intermittent sound' etc.

I can start taking out things in the power circuit and testing but I really would be shotgun-approaching it at this stage, so wanted to see if the sound it makes (recording attached) points to any rough area I should start with!

All the best, and thanks to anyone who would like to help!

[The Dude]

I'll preface by saying that all symptoms are not created by the same problems. That said, I've run into several of these with leaky q12 and/or Q13. I usually find the problem by touching the transistors with my fingers. If bad, they are exceptionally hot. They will often check good on a meter, but die under circuit stress.

[Jazz P Bass]

You most assuredly need an O scope to see what is what.

[OwenM]

You most assuredly need an O scope to see what is what.

OK, yeah imagined it might need a bit more analysis but wasn't sure.. well, I could get into the idea of buying an oscilloscope, I've always wanted one but so far it wouldn't have offered much help with the things I've fixed. From what I gather its one of those things that's not on the list of first things to buy, but when you need it, you need it....

I'll have a look about and try find something, or any entry level ones that are well known and liked and have all that's needed for guitar amps?

[OwenM]

So I've been looking for an oscilloscope, I don't imagine going too deep into measuring digital waveforms and the like just yet, so should something like this be enough for working on amps? I'm NOT looking to getting into measuring the digital elements of FX circuits etc (just yet), but it would be cool to have something that covers all bases for modern as well as older amps.

https://www.ebay.co.uk/itm/HAMEG-HM-...EAAOSwlTFdl5QG

Also, what do people think of DMM's with an oscilloscope like this?

https://www.ebay.co.uk/itm/Upgraded-...IAAOSwK-1dB0kn

I want to continue learning and get to grips with all aspects of amp repair, so if something a bit more comprehensive is advised, I'm cool to spend a bit.

[Leo_Gnardo]

The HAMEG looks plenty good enough for most any use. Price is very fair. Even better if you can pick it up in person, first so you can observe it working in person, second so you don't have to subject it to the rigors of shipping.

As for pocket size scopes, they're sure a handy size. But I fear the display doesn't have quite enough resolution to show some of the things you'd like to see, like crossover distortion for setting bias, or small ultrasonic bursts that sometimes appear when testing an amp especially with a real world load aka speaker. OTOH I've never used one, so I could be wrong.

[OwenM]

Thanks! I bought it, I figured for less than £100 and 100% feedback I may as well go for it I've always loved those things since I was a kid in science class anyway, so to have an old analog one that does the job seems exciting and a no brainer.. Not near Manchester so it will be shipped, but I'll roll the dice on that I guess...

[Leo_Gnardo]

Thanks! I bought it, I figured for less than £100 and 100% feedback I may as well go for it I've always loved those things since I was a kid in science class anyway, so to have an old analog one that does the job seems exciting and a no brainer.. Not near Manchester so it will be shipped, but I'll roll the dice on that I guess...

Good on ya Onnie! For more of my views on 'scopes read what I wrote a couple days ago, posts 56 & 60 on this thread:

https://music-electronics-forum.com/...t=49793&page=2

Not that I'm any kind of expert. I just think it's good to use any tool you have, to come up with an understanding and a solution to problems. To use a 'scope certainly beats flailing around without one. And with respect to vintagekiki's comments, my hat's off to those old timey techs who managed to fix problems sans 'scope. Heck, doctors used to do without MRI, CAT & PET scanners, ultrasound & x-rays, and still managed to fix their patients. But as long as you have the insight brought by some affordable technology, why not use it.

[The Dude]

Look at it like this, if you tech for a living like I do. Let's say I worked at a tire shop. Sure, I could loosen all the lug nuts with a standard manual tire wrench. But, I can change well over twice as many tires with an air wrench and bill out a lot more $. A scope is a tool,...... much in the way an air wrench is to a tire guy. It may not be absolutely necessary, but it will save a hell of a lot of time,...... rather than "flailing around without one", as Leo says.

[Jazz P Bass]

I would buy the signal generator too:
https://www.ebay.co.uk/itm/Griffin-S...MAAOSwKhpdl5I5

[OwenM]

I would buy the signal generator too:
https://www.ebay.co.uk/itm/Griffin-S...MAAOSwKhpdl5I5

I thought about it but bench space is a premium and I was going to run a cable from my mixing desk on the next table, generating a wave from Logic or similar.. that seems to be the cheapest and easiest way but does using computer generated test waves have some kind of drawback?

[Leo_Gnardo]

does using computer generated test waves have some kind of drawback?

It depends on how low distortion sine waves you need. Some of the old fashioned - physically large - generators had a very low distortion factor. Then solid state function generators became popular. They derive their sine wave by filtering other wave forms, and often have a significant distortion. I have a Philips fg, and can easily hear a "fuzz" riding on the sine output. Not a good thing when you're trying to track down distortion in a circuit. OTOH I've been using a Loftech solid state generator for decades now, and its sine wave is pretty darn good. Maybe not the very best but I sure can't hear any obvious distortion. I'd have to say it's served me very well.

There are some pocket size signal generators that would certainly not take up too much room on your workbench. How pure their sine waves are, I can't say. You will have to read specs, also heed the experience of anyone here to cares to talk about them. Same for computer generated waves. With modern high speed computers there's no excuse for anything but high purity sine waves. But that would depend a lot on the program generating them, also the interface circuits between the computer and our analog world. I hope some experienced users will sound off and inform us. I expect we will be hearing about some excellent audio analysis programs that not only generate tones, but produce sweep and warble tone functions, produce response graphs & more.

[Enzo]

To me it is important to have both level and frequency be completely adjustable. IN other words, for like chasing a cabinet buzz, I can turn the knob up and down to sweep the frequency to find a resonant spot. Also, a continuously variable output acts like a volume control.

[OwenM]

Thanks all. I received the scope and have got to know it and calibrated it, as far as the basic calibrations in the manual outline. It's pretty simple all in all isn't it? I could get the basics and the way to read the voltages against the display and the V/Div on a once over the manual. I made a sin from my phone and got the hang of interpreting the frequency on the scope screen too. Next I'm gonna see if the sin made in logic can be reliably made to conform to a desired rms voltage and see how pure it is. I've got good D/A converters so I reckon that the purity will be fine for amp testing. Then I'm gonna go in and follow the signal from the power amp in and see where (if) it goes fuzzy. It has a component tester on it too, so I'll probably check out all I can in the power amp circuit and make sure it reads correctly. If I can't find something dodgy, or can't find a point where the sin massively degrades, I'll post back here and beg for further advice

One question, for testing a power amp, what's the a good starting Vrms for the sin wave given to the power amp under test?

[The Dude]

All of the test points and expected voltages are labeled on the schematic.

[OwenM]

All of the test points and expected voltages are labeled on the schematic.

Thanks! I was meaning more though, what voltage should I output with a test oscillator? By the diagram the pre amp out is giving 91mV, and one it goes through the first op amp in the power amp it’s 31mV I think.. should I just meter it and set the voltage on my test oscillator until I get 31mV at the first test point in the power amp?

[Jazz P Bass]

The expected input voltage is stated on the schematic: 5mv ac.

[Helmholtz]

....and 91mV at the PA-IN.

[OwenM]

....and 91mV at the PA-IN.

Thanks, understood, lets see what I get

[OwenM]

OK! I'm enjoying this scope thing, seems much more pragmatic than just having a multimeter and no real idea of how this circuit is supposed to work! Learning a tonne..

I've found some oddities all over the place, where the signal changes or looses shape or similar. The sin on the output has turned all sorts of odd at various different points, but usually with one half of the sin gone or wonky.

Signal at speaker output terminal


However it seems like the very first op amp is suspect, U6, sometimes it passes the sin ok ish, other times it looks like this, including the DC offset


So I'm gonna order one of those I think and then see what else I can find after replacing that...

One question,

It's not a worry, and the scope seems to work fine when testing stuff, and I can get it to go away. But on some time/div steps it shows two traces for the sin, one perfectly out of phase. On some settings it just flicker, in others it freezes like in the attached photo. The invert function on channel one is dodgy on this unit (though that said it does it on channel 2 two, and with two different sin sources). If this is just an old ratty scope then that's cool, it's not interfering with the job at all. I was more curious if it's more down to something I'm missing about a scopes function..



Cheers!

EDIT - sorry, they rotated as attached!?

[Helmholtz]

It's not a worry, and the scope seems to work fine when testing stuff, and I can get it to go away. But one some time/div steps it shows two traces for the sin, one perfectly out of phase. On some settings it just flicker, in others it freezes like in the attached photo. The invert function on channel one is dodgy on this unit (though that said it does it on channel 2 two, and with two different sin sources). If this is just an old ratty scope then that's cool, it's not interfering with the job at all. I was more curious if it's more down to something I'm missing about a scopes function..

The double trace in the picture is a scope trigger artefact. A signal cannot have different values at the same time. Try to find a stable trigger setting (trigger source and level) to get a single trace.
You need to "tell" the scope via the trigger setting where to start writing the trace. For instance a trigger level of 0V is ambiguous as it could mean positive or negative slope.

[OwenM]

The double trace in the picture is a scope trigger artefact. A signal cannot have different values at the same time. Try to find a stable trigger setting (trigger source and level) to get a single trace.
You need to "tell" the scope via the trigger setting where to start writing the trace. For instance a trigger level of 0V is ambiguous as it could mean positive or negative slope.

Thanks, I figured there was a little I was missing, I wasn't triggering at all, just running it in 'normal', on AC trigger mode I can get a stationary drawing, and on the 'waveform icon' I can get it to output a slowly scrolling one, both better than what I had thanks!

[OwenM]

Hi guys,

so after looking around and getting my head HALF around what's going on, I've started to zone in on some stuff that I could do with some help on..

For now I'm curious about this; the TL072 op amp seemed to work fine (earlier...), and now has issues, quite possibly something I have done and not noticed perhaps.

I'm feeding in a 91mVrms signal to the power amp, The voltage on the + of C40 is confirmed 91mVac, no DC. By the time it's hit TP18 at PIN 7 on U6, its way off it's target 33mVac, hitting about 120mVac. The waveform is also a stretched lumpy sin now.

I put in a new TL072 I had and same deal. So, although I'm sure there's other stuff later in the amp to fix, I think first I'm looking at something around the array of R67 R68, C42 C43, D16 and D17, is that right? I'm not too good on OpAmps although I understand the principle roughly, but it seems to me like something throwing off the signal from one side to the other of one OpAmp has to be something nearby in this circuit? Maybe? Possibly?

Cheers for any help anyone can offer!

[g1]

Don't forget R66 & C41.
If all those parts check out, I would resolder them all.

[OwenM]

Don't forget R66 & C41.
If all those parts check out, I would resolder them all.

Thanks, I've had a good root around and resoldered everything. The only thing out of spec was c42 which measured about 35pF. I'm can't get my head around how that circuit actually works, but figure that c42 is about the same order of magnitude out of spec that my TP18 reading is, so I've ordered another of those! I'll scope it out again when it arrives..

(I know that’s probably not the most thorough thinking but it’s all I’ve got to go on )

[Jazz P Bass]

Be aware that U6 is in the feedback path of the output. (pin #6).

Also the - speaker connection should not be considered as at ground potential.
There are those 2 .22 ohm resistors in there between it and ground.

[OwenM]

Be aware that U6 is in the feedback path of the output. (pin #6).

Yeah...I see that, I guess once I replace C42 then I can move on and tick the OP amp circuit off the list of suspects...

Also the - speaker connection should not be considered as at ground potential.
There are those 2 .22 ohm resistors in there between it and ground.

Cool, I see what you mean, but not too sure how that will affect my measurements or how I should conduct them? Could you elaborate a little?

[g1]

Don't connect any grounds to speaker negative. Ground all equipment to chassis.
If you are using a load other than the speaker, it must not be grounded in any way either.

[OwenM]

Don't connect any grounds to speaker negative. Ground all equipment to chassis.
If you are using a load other than the speaker, it must not be grounded in any way either.

Ok cool, thanks. Yeah, no problems with that this end, I've been using chassis ground and my load is just connected as the speaker would be. But good learning thinking through why that was a point worth mentioning!

[bobloblaws]

I have a digital scope (Rigol) that I put to use from time to time, most often for measuring output power before clipping. In this thread there is mention of using the scope for observing crossover distortion when setting bias as well. I have to admit though I don't understand the general importance placed on owning and using a scope as indicated near the beginning of this thread. I'm sure there are uses that I'm not yet aware of so I was hoping you guys could give me some insight. I find I can do a hell of a lot with just a DMM and a makeshift audio probe. I generate test tones with software on a laptop PC. So what am I missing? I feel left out of the party!

[Enzo]

Imagine playing guitar always at the end of the neck. E and A are easy, as are G and D. F? B-flat? You work around that, lea ving out strings on some chords. Then one day someone says "Learn to barre chords." Now you always got a.long playing E in first position, but now you can play E all over the neck.

Imagine your amp gets gravelly sounding. Your meter tells you B+ is OK, and a good meter will even measure ripple. Your listener will tell you what it sounds like. But a scope will show you the waveform - you can see the nature of the distortion. And that leads you closer to understanding the cause of the problem. Is that distortion clipping in the phase inverter? Or is it crossover distortion. A meter can't tell you that. On a scope it is readily apparent.


I understand meters, I use mine for most work. I can get close enough with it I don't always need the scope, but my scope is always warmed up and running when I work. SO many times I use the scope first even though I could have done without.

It is a tool. I used to have an A-440 tuning fork in my guitar case. Tune up my A string, then tune the rest to it. Then one day I got a little tuner. Now I can just tune all six strings without trouble. I could do without, but it is so much more versatile than my fork. You don't have a good idea what teh scope is all about, but regular use will cause you to learn maany things is can be used for.

[OwenM]

OK... I don't know enough to really interpret what I'm getting back here, though it's definitely illuminating, learning tonnes as ever.

I wondered if anyone could point me in the direction of a few places to test? I can't really see where the signal is meant to go, and what levels of DC are acceptable in various places.

Heres a photo of the output -

Quite a lot of that ends up back at the beginning at pin 7 of the op amp, so it's hard to find a definitive place where the signal goes awry.

It's a new TL072 chip, and the first OpAmp's circuit has been thoroughly verified and tested. At pin 7 of the op amp I get 140mVAC, and -200mVDC

The following Q9 and Q10 have the expected 41vDC on the collectors.

Q9 Emitter has -700mVDC and 30mVAC. I can't see what's happened to the signal it seems as I can't adjust the Y on my scope enough to bring it into view when at a zoom level enough to see the 30mVAC signal. This might be user error and I just don't know how to do this!

Q10 Emitter has -600mVDC and 30mVAC

Q8 has about -30VDC on it's base and emitter.

I'm kind of stuck there, I don't know these circuits and what they are meant to be doing, and YouTubing 'transistor amp circuit's' and the like just brings up tonnes of very simple 1-3 transistor amps. I need to go and do that, and get a book on tranny amps too but I'm still only half way through the one on tubes!!

I see the signal bouncing around further down the amp but not really sure what to look for.

Anyone have a few pointers of where I should be looking, or what I should start with?

Best, O

[Jazz P Bass]

The bottom half of the output is being cut off.
If you look at your voltages you will see a lot of Minus dc measurements.
That is hint.

That said, solid state amps can be a real bear to repair.
Unlike a tube amp in many ways.

I see you mentioning Vac And Vdc test points.
Are you measuring Vdc with a signal present?
I find it is best to measure the Vdc with no signal.

[OwenM]

The bottom half of the output is being cut off.
If you look at your voltages you will see a lot of Minus dc measurements.
That is hint.

That said, solid state amps can be a real bear to repair.
Unlike a tube amp in many ways.

I see you mentioning Vac And Vdc test points.
Are you measuring Vdc with a signal present?
I find it is best to measure the Vdc with no signal.

Hmmmm, so, a lot of negative DC voltages getting somewhere they shouldn't? Some kind leaky capacitor on the negative side perhaps?

Yeah I'm learning that!! It's just seems a mess of stuff that all effects everything else simultaneously

Yesss, I was, mainly because I wanted to see if my AC signal went up so I could see when I went past a point of amplification... but as I'm not sure what transistors are meant to be amps and which are for current reg or whatever else I should probably stop doing that! I'll look around at some DC voltages, sans signal, on the negative side.

Thanks for your help

[olddawg]

I find with these amps that a can of freeze helps a lot. I’ve seen several with lots of pops and snaps from intermittent/temperature sensitive transistor.