Big Muff Op Amp problem

[mievil]

Hey, guys. First time post so you can tell me to bug off if this isn't the right place. But I saw someone else in a google search asking about a Big Muff and they got some good help here, which is why I'm here.

I picked up a 79 op amp Big Muff that was junk. The distortion output was only working occasionally, and would seemingly get pulled down after maybe half a second. I opened it up and found the electrolytic caps had been changed to SMT ceramics, and 2 of them were in backwards. I pulled all those caps and replaced them with electrolytics again, opened the tone bypass switch and cleaned it out.

Now, I have made progress, but it still isn't great. I have a vid I took here:

https://youtu.be/cefclh6RMWc

You can hear the drop out of the note after a brief time.

I had a scope on it and a meter. I'm referencing a 2005 schematic by Gottfried Divos for a 78 Op Amp Big Muff.

Voltages on the 4458 all seem correct.

I am a little confused after the Sustain pot, though. I have a sine wave with a 4.5VDC offset on the neg of C7, sign wave with no DC offset on the pos of C7. No AC on pin 2 of the 741.

741 voltages are as follows:

2 = 2.035VDC
3 = 1.92VDC
6 = 3.75VDC

The output waveform is really weird, but, I'm not sure what it is supposed to look like. With a signal generator hooked up I don't see a dropout on the output signal, it remains a constant amplitude.

Those voltages on the 741 are not correct, are they?

Any help would be fantastic, and like I said if this isn't the right place I understand. Thanks!

[GibsonGM]

Hi Dude,

You're in exactly the right place - welcome    Can you post the schematic you said you were working from?  That would be very helpful, since there are a few of them going around. 

And ALL IC pin voltages, please. 

I'm sure this can be fixed pretty easily.

[mievil]

Sure thing. I can't figure out how to directly attach an image here, so I found a link to the same schematic. Hopefully that's ok.




Voltages are as follows:

IC1
1 = 4.427
2 = 4.42
3 = 4.4
4 = 0
5 = 4.416
6 = 4.441
7 = 4.439
8 = 8.89

IC2
1 = NA
2 = 2.035
3 = 1.92
4 = 0
5 = NA
6 = 3.75
7 = 8.89
8 = NA

This may be many fold. The V at pin 3 of IC2 should be like 4.9V. Maybe I have a bad 741, AND a bad cap somewhere?

[PRR]

Voltages are OK.

> pin 3 of IC2 should be like 4.9V

Your meter is about 500K load. At least if I assume 500K, the 2 and 3 voltages are just what we expect for pin 6 at 3.75V. 1Meg is a little more common today, 10Meg is somewhat less liable to confuse when poking at 470K-1meg bias resistors.

However that does not explain why IC1 voltages are nominal, when P2 P3 also have large bias resistors.

I agree with Mike-- I suspect a DC bias is going wrong during strong signal. Clip leads (so you can play and meter at the same time) may help you find it. Watch the OUTputs of the three opamps. (Poking the inputs will de-bias them, we already know.)

[Keppy]

At least if I assume 500K, the 2 and 3 voltages are just what we expect for pin 6 at 3.75V.

Why should pin 6 be 3.75V? Shouldn't pin 3 bias to 4.9V, causing pins 2 & 6 to rise to the same voltage? If so, shouldn't pin 6, as a low impedance point, measure 4.9V regardless of meter loading?

I'm looking at C8, where a slightly leaky cap could pull down the bias. You could lift one end of that cap to check without doing too much surgery.

I don't see how 3.75V at pin 6 is possible without at least one bad component between C8/R11/R12/IC2. Or R13 I suppose, as that would cause the opamp output to feedback through one set of diodes at rest.

[ElectricDruid]

I agree with Keppy. I can't make 1.92V on Pin3 of IC2 make sense unless the meter is some ridiculously low load, and if it were, the reading for IC1 pin 3 should be way out. A 1Meg meter would give 500K equivalent on the bottom of that divider and 500/(500+820) = 0.379. 0.379 * 9. = 3.4V, still twice what is measured. Since IC1 is ok, we should assume the reading for IC2 is ok too. In which case something is pulling it way down - dead cap, dead op-amp, minor short, or incorrect biasing resistor?

Tom

[antonis]

Why should pin 6 be 3.75V? Shouldn't pin 3 bias to 4.9V, causing pins 2 & 6 to rise to the same voltage? If so, shouldn't pin 6, as a low impedance point, measure 4.9V regardless of meter loading?

True, only in case of one meter step-by-step taken measurements... 

[mievil]

I'm shooting in the dark here, but I did hook up to a scope with the guitar instead of a sig gen last night. I have a good signal up to R10, and anything to the right of that I can watch the signal drop out on the scope at the same time I am hearing it.

I was kinda suspecting the 741 had an issue, so I pulled it this morning and I have a solid 4.89V at the pad of pin 3. Pin 2 starts at 3.1V as soon as I start probing and then starts to drop like a cap discharge, but without the op amp in there i have a completely different circuit, I think.

I am not opposed to thinking I have a bad passive component somewhere in there, too.

FWIW, I'm using a Fluke 289, should be 10M.

[mievil]

Replaced LM741 and voltages at the inputs are back to what I expected, 4.6V ish. Haven't had a chance to play with it yet, this was just with a PS and meter. Hopefully I'll be in business tonight when the new guitar shows up.

[ElectricDruid]

Sounds positive, good luck!

T.

[mievil]

I think we're good. Sustain is back, strange light oscillation late into the heavy notes, but, meh. I'm good with where it sits. Thanks for the input.

[antonis]

strange light oscillation late into the heavy notes

Try another pin compatible general purpose OP-Amp for IC2..