Op Amp Fuzz Issues -- Bootzilla

[Cruton]

Hello DIYstompboxes!

This forum has been tremendously helpful, and I've finally reached the point where I need to make a profile and ask a question of my own.

This story begins as so many do. I was at the local thrift, and found an old Snarling Dogs Bootzilla Bass Fuzz/Wah for real cheap. I bought it assuming it didn't work, but also thinking I'd be able to fix it. Getting it home and plugging it in, the wah function works great, but alas, no signal from the fuzz side. Just to be clear -- there are separate 3pdts for the wah and fuzz functions. Signal passes with both effects bypassed, and with the wah engaged and the fuzz bypassed, but engaging the fuzz (regardless of whether the wah is engaged) kills the signal entirely.

As soon as I saw the circuit I knew I was in a little over my head, as this is an op-amp fuzz (MC1458n dual op amp), and I've not yet built any op-amp circuits. Anyway, I found this schematic for the fuzz section here https://goo.gl/photos/1nxxv7Vzp18SqBNQ6 , and as I'm skeptical I also traced the circuit on my own and came up with the same schematic. I then started poking around with the DMM to make sure there was ground and Vcc at the appropriate places, which there was. I then checked for cracked solder joints, and found a couple, most notably on the input of the fuzz. Eureka! Except not. Repairing the joints still left me with no signal passing. I then went through and checked for continuity between all the points that should be connected and found no issues, leading me to believe there is some component issue somewhere.

So I got out the ole audio probe and went to town. I found that the signal is present at the 3pdt, and appears at the input of the fuzz when the 3pdt is engaged. The signal is also present at the junction between resistors RA1 and RA9 in the schematic linked above. However, when I move the probe to the other side of those resistors (i.e., the junction between RA1 and RA2, or the junction between RA9 and the non-inverting input), there is no signal in either case. The legs of the resistors are not broken, and the solder joints of both have been reheated to no avail.

I've read that the tracings and pads are bad for lifting on these pedals, but there is continuity from both of those resistors to their respective downstream components and op-amp inputs. So, having some basic understanding of how an op-amp works, but no experience whatsoever working with them, I wondered if perhaps the op-amp has died. I took voltage readings at each of the pins, and here's what I came up with:

1: 8.12V       8: 8.77V
2: 2.41V       7: 4.96V
3: 2.54V       6: 3.18V
4: 0.00V       5: 2.54V

Now, I've read that the inputs should be within a few mV of each other, and pins 5 and 6 seem to be toeing that line. I've also read somewhere that if the output is within 1V or so of your power rail, that the op-amp has failed; however, I can't find other sources supporting this, and everything else I've read has suggested that the relationship between the output voltage and the power rail depends on the op-amp characteristics and the voltages supplied at the inputs. So I really don't have any clue what these readings are telling me, if anything.

I have some IC sockets on hand, but will have to order some MC1458n replacements (LM1458 should be equivalent?). It's easy enough to substitute and see what happens, but I was just curious if someone with more op amp experience might know from the schem and the voltages above whether I should wait around all week for my replacement op amp or first turn my attention to a more likely culprit that I've overlooked.

Thank you all!

[Cruton]

I should also say that I have seen this response from R.G. to an earlier post

https://www.diystompboxes.com/smfforum/index.php?topic=89218.msg755480#msg755480

which would seem to indicate that this op amp is indeed bad, but I honestly do not know whether or not I've met requirements 2-3, although requirement 4 seems to be violated by the input voltages reading less than 4.5V (half the supply). But I'm not sure what having low voltages on the inputs actually means for how I can (or can't) interpret the remaining voltages?

[GibsonGM]

Me, I'd carefully remove the opamp*, make a felt-tip pen mark to remember where the "notch" or "dot" is to orient which way is "up" for the new one.   Then I'd install the new socket with its notch matching, of course.   

Then check those voltages again while waiting for my new LM1458's to arrive (a fine replacement, same thing).   Chip may be fried, and maybe by something wrong in its periphery...voltages at the socket pins can show what's going on, possibly, before you put a new chip in and fry it!   

Welcome to the forum! 

* perhaps I'd mark my chip's orientation, then cut the legs off it with "diagonal cutters' (wire snips) or a dremel too, then carefully unsolder them 1 at a time...so I don't mess up the the sensitive pads you mentioned...trying to get the whole thing out 1 side at a time can be rough...

[Cruton]

Thanks GibsonGM for the suggestion to see what those voltages look like when the old op amp is out and the socket only is in. I'll do that ASAP and report the results here while waiting on the new one. Also, thank you for the great suggestion to cut the op amp away before desoldering the pins -- I was indeed worried about getting that thing out without causing some damage.

[Cruton]

All right, having removed the op amp I now have these voltages:

1: 4.04          8: 8.82
2: 4.18          7: 3.85
3: 4.40          6: 4.24
4: 0.00          5: 4.40


These seem reasonable to me, an op-amp noob. Is there anything that I should be worried about here? 6 & 7 seem a little far apart but I'm not sure how far apart they need to be to cause problems.

[thermionix]

6 & 7 don't have the opamp in there trying to equalize them.  I'm still too noob to say for sure, but I think your voltages look correct and the 1458 was indeed the problem.

Edit:  If you have any other DIP8 dual opamps (4558, TL072, etc), the circuit would probably work fine with any of them, might even sound a little better.

[PRR]

> I've read that the inputs should be within a few mV of each other

Ideally, yes; and specifically in systems like this, you expect both to be around 4.41V (half the 8.82V rail voltage, +/- resistor tolerance).

In practice, your meter loads-down the "normal" voltages, more or less depending how much resistance is in the line. You report 4.04V on one input. Without looking, I'd expect that to be biased by a high value resistor like 1Meg, and assume it really is 4.41V when not being poked, OK.

> if the output is within 1V or so of your power rail, that the op-amp has failed

Audio swings both ways. We normally bias so, no-signal, the output sits "half-way" between the two rails. In +/-15V work, near zero DC. In +9V work, near 4.5V. That leaves ample room to swing both ways. In most 9V, 4.0V or 5.0V is all about the same.

The output can't go any closer than X volts from the rail. If it is already there no-signal, JAMMED against the rail, you have no room to swing further, distortion will be gross.

You like to park your car in the center of the garage for best space all around. If it is a little off-center visually, maybe you aren't looking straight, it may be OK. Or if it is a bicycle and you check "by feel", the feeling may have shifted the light bike. Anyway a few inches off-center is no big deal. However if you find the car jammed right against the wall, NO clearance, more likely the car is "broken", the steering failed. (In a car you might notice, but you can't watch opamps the same way.)

Yes, the only way to take out 2-bit DIP chips is to cut the legs off. There are special tricks when you face a VALUABLE chip, but '4558 and '072 and the like are too cheap to try to "salvage".

[Tony Forestiere]

You like to park your car in the center of the garage for best space all around. If it is a little off-center visually, maybe you aren't looking straight, it may be OK. Or if it is a bicycle and you check "by feel", the feeling may have shifted the light bike. Anyway a few inches off-center is no big deal. However if you find the car jammed right against the wall, NO clearance, more likely the car is "broken", the steering failed. (In a car you might notice, but you can't watch opamps the same way.)

Why do Paul's analogies always make so much sense to my "Mind's Eye"?  I can visualize the concept of large, poorly biased voltages slamming into the garage walls.

[GibsonGM]

You like to park your car in the center of the garage for best space all around. If it is a little off-center visually, maybe you aren't looking straight, it may be OK. Or if it is a bicycle and you check "by feel", the feeling may have shifted the light bike. Anyway a few inches off-center is no big deal. However if you find the car jammed right against the wall, NO clearance, more likely the car is "broken", the steering failed. (In a car you might notice, but you can't watch opamps the same way.)

Why do Paul's analogies always make so much sense to my "Mind's Eye"?  I can visualize the concept of large, poorly biased voltages slamming into the garage walls.

Why does slamming the car into the garage walls just sound so GOOD when you do it right, though??   

I think you are on your way to solving the problem, Cruton!  Let us know what happens when you get the new one in.  Hope you are getting at least 10 opamps, by the way (30 would be better, ha ha).   You'll want to experiment with them on a breadboard! 

When I was starting out, every time I'd get something inexpensive like that I would get many extra, which proved very helpful later...parts collection.   

[Cruton]

Thank you all for the help!

I didn't have any other DIP8 op amps around, but I took the advice to go ahead and stock up on a bunch and went ahead and ordered several different kinds (LM1458, TL072, JRC4558 NE5532). I will try a few out in the socket and see what sounds good in the Bootzilla, but I'm more excited to have all those on hand for tinkering around. I also looked around in the electronics drawer at work and found some LM741 that I've cabbaged for future tinkering. Any suggestions on fun first builds for these op amps? I guess that's kinda off topic...

Thermioinix, your response about the inputs not currently having the op amp in there makes sense, thanks for clarifying that. And to PRR, that analogy was spot-on. Now I'll always be sure to mind where I park op amps in my garage

All right, once again I greatly appreciate you all coaching me through debugging this pedal. I'll be sure to report back once the op amps arrive.

[PRR]

> large, poorly biased voltages slamming into the garage walls.

Guy I know has a better example: his road is long and skinny. Ignoring oncoming traffic, he aims his truck pretty much up the center of the road. By being "biased" in center, he can wobble a bit (dodging kids, cars, porcupines, ice, and potholes) without dinging his truck on the "rails" (which are trees, rocks, rocky-snow, and drop-offs).

[Tony Forestiere]

Guy I know has a better example: his road is long and skinny. Ignoring oncoming traffic, he aims his truck pretty much up the center of the road. By being "biased" in center, he can wobble a bit (dodging kids, cars, porcupines, ice, and potholes) without dinging his truck on the "rails" (which are trees, rocks, rocky-snow, and drop-offs).

Unless the "steering failed".  Got it. Again.
Poor kids and porcupines. 

[ElectricDruid]

I don't know about op-amps, but I'm never getting in a truck with any of Paul's friends.

T.

[thermionix]

LM741...Any suggestions on fun first builds for these op amps?

If you got 6 or more, Phase 90.  I highly recommend the GGG PCB, it is based on the orginal but slightly improved to avoid that awkward resistor stretching across the board.  If you build with small, low-profile, or bent-over parts, you can fit in a 1590B on top of the jacks just like the originals.

Mine, for reference:



For something simpler that you can easily whip up on perf or vero, there's the MXR Dist+ or DOD250.

Many more to choose from, the 741 is/was probably the most popular single opamp.

[BetterOffShred]

Opamp big muff of any variety.  I think they sustain for days.  Clip the 100k to ground at the output though.. louder 

[Cruton]

All right, finally time to report back. I installed the new LM1458 and the fuzz has been revived! The Bootzilla is now in fighting form. Thanks for the help everyone, and for bearing with me through my first journey into op amps.. I'm excited to build something using all the spare ICs I have now. I think an op amp muff or dod 250 might be the way to go.