Mutron III Auto-Wah -- help needed with LDR (light dependent resistor)

[Dr Ron]

I've got an old Mutron III auto-wah that's been giving me grief, and I thought its time to call the calvary.

The effect works on bypass, which goes through an opamp. Switching on the effect makes it not work, although you can hear a tiny bit of effect if you listen hard. The effect doesn't work if you go back to bypass, but will work if you wait a minute or so.

I have a scope and homemade signal generator and did some simple tests, but I'm wondering what to look for.

I'm wondering if the issue is with the 0805 LDR (light dependent resistor).

Opamp A5 pin 7 shows a nice square wave, but it doesn't reach the 0805  LDR (A5 pin 1). Test point 1 (on the other side of the 0805) shows +8 DC.
R23 is a variable resistor and I'm wondering if you set it so that there is a waveform on TP-1.

Is that correct?

Have I tracked this down ... or is there something else to look for?

Any help would be greatly appreciated!

Thanks!!

Schematics are here
http://files.muziq.be/schematics/musitronics_mu-tron3.jpg

[Dr Ron]

OK - I think it's working right, since the waveform at TP-1 is an 8 volt DC with a small sawtooth on it, which is the rectified and filtered envelope of the input (from my AC signal generator).

The main question I now have is how to set R23, the current scaling resistor for the LDR.
What do I look for?

Tomorrow I'll trace the signal as it goes through the filter opamps.

[mdh]

I would advise taking a look at R.G.'s excellent write-up on his Mutron III work-alike, the Neutron.  It's available on Geofex and General Guitar Gadgets, and gives guidelines for setting the current scaling resistor (he calls it Rx). Don't be afraid to use a larger value than he suggests if it sounds good, though.  However, it seems likely that if this unit ever worked, it probably came from the factory with a satisfactory resistor in that position, and the optocoupler is probably more likely to fail than the resistor.  Steve at Small Bear carries the replacement part if that ends up being the problem.

Another thing to keep in mind is that you're not likely to be able to really test this thing further with a signal generator, unless it produces nice juicy transients that resemble plucking a guitar string.  Basically, plug in a guitar and bang away, adjusting that resistor until you get a good sound.  You should probably set the sensitivity (er, gain) high enough that it doesn't distort, put the peak control maybe halfway, and select band pass.  I think those should be pretty good settings for hearing the sweep of the filter.  Also, since you seem to have a scope, take a look at what the envelope looks like when you bang on a guitar.

Good luck!

[Dr Ron]

I may have found two problems.

The output had DC on it, so I replaced the non-polarized cap C10 with a .047µ coupling cap that I had on hand until I order a replacement.

The second issue is the DC itself.
The outputs of the second opamp (LP and BP) are 10 volts DC (no AC). The output of the first one (pin 7, HP) is -7VDC +2VAC.
I believe the DC is from the feedback.

Does this mean that the second opamp needs to be replaced?

PS - Once that is replaced, will C10 work?

[mdh]

Hey there, I was hoping someone else would chime in, since I haven't really learned much about op-amp theory yet.  So I don't feel all that qualified to give advice, but I guess in the end I won't let that stop me   I don't really see where the DC bias on those op-amps could be coming from other than the op-amps themselves, so it would seem reasonable to me to take them (well, it, since it's a dual) out and put in a socket.  Also, if you have a larger non-polarized cap than 47n you mentioned for the output cap, use it.  If the original cap was leaking DC, replacing the op-amp won't really solve the problem.  You could also connect two polar electrolytics in series (negative lead to negative lead, using the positive leads as the component leads) if you have them.  A couple 22 to 33 uF caps would get you into the right ballpark.

[Dr Ron]

Thanks everyone ... it works!

It may have been several issues, including a bad opamp and a bad solder joint.

Now that I know what the waveforms are supposed to look like, the other one needs to have the LDR bias resistor adjusted correctly.
I may put in a small pot on the back of the board instead of using a fixed resistor.

That was one challenging pedal ... the support was very helpful!