MU-Tron III repair

[pontiacpete]

This mu-tron is not making any effect. I replaced electrolytics and cleaned pots and switches. I’ve read that the optocoupler in these goes bad. So after some research a came across the recommendation to replace it with 2 NSL32s. I attached the original schematic and the one that somebody posted with the NSL32s. I’m not understanding how these are wired up to the circuit. It seems clear that the leads to photo resistor goes off to ICs but the other 2 wires on each opto for the LEDs is not clear to me. Does anyone know how this is done? Thanks Pete

[x-pro]

Since the optocoupler LEDs are on the same circuit and controlled by the same element, it doesn't matter which LED is opposite which photoresistor. The main thing is to observe the polarity of the LEDs.

[Mick Bailey]

They're in series off IC A6. Connect one anode to the other's cathode, then the free anode goes to A6 and the free cathode to the trimmer wiper.

[52 Bill]

Have you tested the original optoisolator? Diode test the led side and see what it reads. Resistance test the photocell side and see what they read.

[pontiacpete]

It measures open on the resistor side. The led side might also be gone . I replaced it with the NSL32s and the effect sounds like it’s on the verge of starting but still nothing. I installed a 50K trim pot in place of R23(?) which was a 330R resistor. Not sure what I did wrong.

[pontiacpete]

It looks like this mu-tron III is an early model, I think. The ICs are the round top type and there is an old TO92,? These can be seen in the picture I posted previously.
There are no markings on these. I’m not sure where to go with this from here. Does anyone have an idea? Like I said, it appears to be close to working with the new NSL32s installed. What i mean by this is that it has that nasally sound like a Wah but no Wah effect, if Wah is the correct way to describe this effect.

[Mick Bailey]

It looks like the filter is working but isn't sweeping. There should be a varying DC on the output of A6 that responds to picking. There's a significant difference in the way they the LEDs are wired in the two drawings that I hadn't spotted earlier; in the original the LED side if the vactrol is in the opamp feedback loop and R23 is the current path to ground. In the modified drawing the LEDs are outside of the feedback loop. This makes a difference in the filter response because in this case the modulation signal has to overcome the LED forward voltage before any change occurs in the filter. In the way it was originally wired the opamp effectively eliminates the forward voltage drop.

You say you replaced R23 with a 50k pot. If this is now in series with the NSL32s this is the current limiting resistor for the vactrols so is too large to pass any meaningful current unless it's close to the end of the range. Bear in mind that there's no current limiting at all if it gets to zero. A 1k pot in series with a 220r resistor may give a better adjustment.

[pontiacpete]

It looks like the filter is working but isn't sweeping. There should be a varying DC on the output of A6 that responds to picking. There's a significant difference in the way they the LEDs are wired in the two drawings that I hadn't spotted earlier; in the original the LED side if the vactrol is in the opamp feedback loop and R23 is the current path to ground. In the modified drawing the LEDs are outside of the feedback loop. This makes a difference in the filter response because in this case the modulation signal has to overcome the LED forward voltage before any change occurs in the filter. In the way it was originally wired the opamp effectively eliminates the forward voltage drop.

You say you replaced R23 with a 50k pot. If this is now in series with the NSL32s this is the current limiting resistor for the vactrols so is too large to pass any meaningful current unless it's close to the end of the range. Bear in mind that there's no current limiting at all if it gets to zero. A 1k pot in series with a 220r resistor may give a better adjustment.

Thanks for this. I made the change of the trim pot to 1k and 220R resistor. Now there’s loud siren noise the comes and goes along with the sound from the instrument. Adjusting the pot doesn’t solve this situation. It still isn’t sweeping. There’s no continuous signal from the instrument.

[Mick Bailey]

Check the voltage on A6 output both without a signal and with a picked note. You can also clip a red LED across one of the Vactrol LEDs to give a visual indication of what's happening while troubleshooting.

Do any other controls affect the noise?

[pontiacpete]

This is a little different on this particular mutron, there is no A6. There’s a transistor with white(or silver) dot on the top of it, it’s located to the left of my LED. And OK I went and reversed the positive and the negative because of this confusion . But the polarity is sorted out now and the siren noise is now only happening when I shut it down…weird? After all this I snapped off one lead of the NSL32 and now I’m subbing in a vactrol VTL5C1. My LED tacked across is lighting when I strum or pluck a note. All the controls only vary the tone or the gain only, no sweeping. The DC voltages goes up on the dotted transistor, but is 0Vdc with no signal. The ‘drive’ switch when in the down position my LED stays lit
thanks!

[Mick Bailey]

Does the LED vary in brightness to correspond with the string pluck envelope, and is it very bright when lit? The siren noise could be the result of resonance in the filter.

[pontiacpete]

The LED gets bright when plucked and is brighter as the gain pot is increased and as to the strength of the pluck.

[Mick Bailey]

I've breadboarded an NSL32-R3 and a VTL5C1 using a 9v battery and trimmer on the LED side. Both still give a fairly low resistance even with a 20k trim pot on maximum resistance. I'm now thinking that your 50k trimpot was the correct choice for this circuit. Given that the LDR side of the vactrols in your pedal are paralleled with the 220k fixed resistors, the filter could be over-modulated.

Also did you include the additional resistors on the LDR side shown in the modified drawing? I don't see these in your pictures.

Edit; Perhaps consider mounting the vactrols and 4x resistors on a small piece of stripboard and using solid core wire to link it into the main pcb.

[pontiacpete]

I set up a breadboard with the NSL32 and Vactrol and I put 100K resistor and switched in a much lower value resistor in series with the 100K across the photoresistor of each opto. Is this the correct way I should be testing this?
The results don’t seem to be getting anywhere just different degrees of the same thing. I switched in several resistors from 470r to 1K and nothing. Also I put the 50K trim pot back in. I wonder if a sound clip would help. But when I turn the trim pot it goes from what I was experience before, just the LED lighting up with strength of each pluck, to being completely on and the sound of loud rumbling and a pop when plucking.
Here’s a YouTube of all this:

https://youtube.com/watch?v=KZbcKBNE...gVmQ7CXxx6Anj5

[Mick Bailey]

The LED looks to be operating as I'd expect in terms of how it reacts to the guitar signal. In the down mode though it should operate the opposite way - LED stays on with no signal and goes dark with picking. One thing to bear in mind is that the Vactrol LED is likely to behave slightly differently than when you have an indicator LED wired in parallel due to differences in forward voltage between the two, but this would only make a minor difference.

As the optical part of the circuit is just a side-chain it doesn't pass any audio. All it needs to do is vary the brightness of the LED. The schematic has the input opamp always connected whether in bypass or not, which at least removes that as well as the side-chain from the main source of the problem. I presume the pedal works fine in bypass mode.

To my way of thinking the problem lies in the filter circuit. Did you check the DC voltages on the opamps? Also check the outputs on your equivalent of A2, A3, A4 - they should be close to 0v with no signal. Have you checked the continuity of the mode and range switches? Re-check how you have the LDR side of the Vactrols wired.