Fender Frontman 212r sound breakup and crackles diagnosis

[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!