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Mu-Tron Phasor II won't sweep
MUFF WIGGLER Forum Index -> Guitars, Basses, Amps & FX  
Author Mu-Tron Phasor II won't sweep
johnstilton
Hey all,

First time posting a new topic here.

I have a Mu-Tron Phasor II that won’t sweep. The filter seems to be working. When I adjust the depth pot, I can hear the filter sweep, but it is not oscillating on its own. (LFO not working?).

(And what is the big black mystery device in the middle?)

Can anyone with expertise in this circuit help?

Thx,

Stuart
PrimateSynthesis
If you have a scope or analog voltmeter, check to see if there is an LFO signal going into the depth pot. If there is no signal, try cleaning the rate pot.

If there is a signal. Then mark where the trimpot is set. Spray it out. Turn it a few times, then bring it back to where it was, and see it if works.

Check the power pins on the op-amp. Should be around +11V and -11V on each pin. If that doesn't check OK, see if the power supply is working (most likely one or more than one cap went bad).

http://www.ti.com/lit/ds/symlink/rc4558.pdf

If the PS is good, then I would do is check the connections to the pots. Disconnect the power. Then use an ohmmeter/continuity tester and go from the traces to the pins on the pots to see if the solder connections went bad. If that doesn't find a fault. Then I would test the rate pot. Disconnected, it should sweep from 0 to 25K. If that works, then look for a cap on one of the output pins of the op-amp.

If the cap tests good, then check all the resistors and traces around the op-amp. Then check the trim pot.

The "big big black mystery device in the middle" is typical Mutron. It's a "vactrol" with one LED and six LDR's.

Show me a Mutron without some weird-ass optical device, and I'll show you an octave pedal hihi
johnstilton
Thanks so much for the tips!

The LFO was stuck outputting -9V. If I reduced the depth to minimum and the rate to maximum it would unlatch and oscillate a bit but any adjustment of these controls would cause it to stick to -Vcc again.

Checking the traces I noticed that something had scratched the traces which may have bridged two of them. After cleaning up the traces and resoldering the U5 op amp, the LFO started sweeping again.

I FORGOT HOW DREAMY AND UNDERWATERY THIS THING SOUNDS!

I first attempted to repair this thing by replacing the rate pot some 15 years ago but there was no wealth of information available such as in this forum or other sites like DIYSYOMPBOXES.com where I found the prints.

I powered up my old Techrtonix scope and HP oscillator for the first time since my first job out of tech school and dang if they arn’t Still working fine.

I aim to start soldering together some DIY kits for fun but my first projects are this phas0r and and old Big Muff. I also have one of those Morley Echo Volume pedals that you are selling but the can is probably bone dry and beyond hope.

Thanks again !
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