Mutron III Auto-Wah -- is the LDR bad?

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PRNDL

Member
Joined
May 16, 2008
Messages
20
Location
Florid
I'm fixing a Mutron III and have done all kinds of stuff, including putting in some new opamps (I used IC holders and put the old ones back).

I thought it was bad opamps because the outputs on A3 and A4 were on 10 volts DC, pretty much the rails. The HP output has -7VDC + 2VAC. The DC may be feedback from the LP and BP outputs.

Now, I'm wondering if the problem is the LDR.

The input to the LDR is DC plus a triangle wave, which is the rectified and filtered version of the sine wave input. I believe that's what it should be.

I have a scope and have been doing simple tests, but this one is kicking my EE butt.

I tried varying R23, which is a variable resistor on the board, and have left it about half-way. It didn't seem to change much.

How do you tell if the LDR is bad?

Small Bear has Hamamatsu P873-13 replacements, but I'd like to be reasonably certain before ordering one.

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

Thanks!

Ron
 
> I'm fixing a Mutron III and have done all kinds of stuff

Why? Does it work? Does it not work? Does it work sorta? You know why you are messing with it, but "we" don't.

Can you get signal through with footswitch in "out" position? In the "in" position?

> outputs on A3 and A4 were on 10 volts DC, pretty much the rails.

Rails appear to be two 9V batteries. If you found 10V coming out of a 9V battery, that's perpetual energy, a Clue, or slack measurement. The best clues are often "8.3v on pin and 8.9V on battery"... the 0.6V gives you a bone to chew.

Are you sure your negative battery isn't weak? And that -9V reaches the chips and stuff?

Or are you running on wall-wart? And is that 30 year old(?) wart still good?

> outputs on A3 and A4 were on 10 volts DC

You have more EE schoolin than me. You know the biquad. The DC bias either follows A1 out, or you have a duff ground. And on stomp-pedals, a bad/cracked/bashed connection somewheres is much more likely than a sick 1558 chip.

> I tried varying R23, which is a variable resistor on the board, and have left it about half-way. It didn't seem to change much.

It obviously varies LED current. Did you monitor that? Does anybody know what is "target" for this adjustment? IAC, a non-zero value of R23 oughta woo-woop the filter. A zero value risks LED death (though I doubt a 4558 has enough muscle).

> How do you tell if the LDR is bad?

LDR bad or good will not give the one symptom (impossible DC voltage) that you have mention. LDR is in parallel with R10 R12 so there is DC continuity.

> The input to the LDR is DC plus a triangle wave

The voltage -across- the LED will be either near-zero at no signal or ~~1.8V with signal.

The voltage across R23 should be, I think, -0.5V no-signal and several volts positive with big signal. In the "up" position, R23 voltage should follow R18 voltage. In "down" they do a trick with +4V and inverted signal. In "down", small signals should pass the filter unmolested, but large signals will light the LED, drop filter frequency, and do something un-natural to the sound.
 
You have many great questions. I actually have two of them (and fixed the first one), although they have very different PCB's and options.

The non-working unit has AC power and an internal power supply (unlike the other one). The voltages are about +- 10VDC (I didn't make exact measurements).

The non-working unit does work in bypass, and I can see the first opamp works and usually set the gain for unity.

After the post, I did some comparative measurements and found that there was no DC on the filter opamps, which is the main issue in the non-working unit.

I checked the LDR as you describe, and it seems to be working. R23 appears to be a 1.5K resistor in series with a variable pot. I set it at its lowest value, since the Neutron recommended something between 300 and 4K (approximately).

I checked many of the traces and corrected pin 6 of A3 shorted to ground, as well as a bent lead on an opamp. No change.

Any other ideas?
 

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