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.