Mu-Tron Phasor II - guitar signal not getting to the phaser section

[Little Jerry]

Hi guys! I just got an old Mu-Tron Phasor II. Powers up, bypasses fine, but the effect is weird... If I click it on and turn up the amp I can hear the phaser phasing noise, and a quiet guitar signal, but the guitar signal isn't affected and both are very quiet. I checked the wires and retouched all the solder joints...no change. I'm assuming the problem is just that the guitar signal is just not getting into the phaser section - which seems to be working okay?
I might just resolder/socket all the ICs if I get the time... but before I do that I thought i'd see if any of you have had similar issues with a Phasor II or a Bi-phase.

Thanks!

[Jazz P Bass]

Nice little writeup with a schematic & text.
Link: The Technology of Auto-Wahs / Envelope-Controlled Filters

[Mark Hammer]

Thanks for the nod, but not everything that came from planet Mu-Tron was an envelope-controlled filter.

There IS a poorly scanned copy of the schematic here: http://music-electronics-forum.com/a...4014-schem.jpg and more information about the pedal (including gut shots) here: Mu-Tron Phasor II

If you are "hearing phasing noises" at the output (just minus the guitar its supposed to be phasing), then that suggests nothing is amiss with the LFO, so we'll check that off the list.

The Phasor II does not use true bypass switching, but takes the "clean" output from the input buffer stage. Since that same dual op-amp is used for the input buffer and the stage that combines input and feedback signal to drive the phase-shift stages, we'll assume that chip is fine. Dual op-amp IC5 is used for the output mixer stage, which MIGHT be suspicious, but half is also used for the LFO biasing to feed the light source. And if you can hear sweeping of hiss, then the LFO is fine which means IC5 is also fine.

But since it IS fine (or appears to be), then any residual hiss you might have in the phase shift stages should be increased if you max out the regen/feedback. If diming the feedback control does not result in any audible increase in hiss, then that suggests there is a stage along the the phase-shift path that is fried, either IC2, IC3, or IC4. These are not legended, but they are the 3 chips surrounding the black thingamabob that houses the six LDRs. The sensible thing to do is to desolder all 3 chips....GENTLY, with as minimal heat applied for as short a period as possible. I'd recommend using either brand new solder wick or wiping some liquid flux on solder wick so that it flows off the solder pad onto the wick efficiently. Then, replace the chips with 8-pin sockets (orient them right so you don't forget which way to stick the chips in) and stick 3 dual op=amps into those sockets that you KNOW work. There is absolutely NOTHING special about the Raytheon RC4558s in this particular application, so don't worry about detracting from the sound if you stick a 1458, 358 and TL072 in those sockets. Once you know you have a functioning phasor, THEN you can pop out one of the known good chips and stick in one of the originals to verify that it does or doesn't work.

I'm not guaranteeing this IS the problem or that it will solve it, but it's an educated first step towards reviving a classic that deserves to be rehabilitated.

[Little Jerry]

But since it IS fine (or appears to be), then any residual hiss you might have in the phase shift stages should be increased if you max out the regen/feedback. If diming the feedback control does not result in any audible increase in hiss, then that suggests there is a stage along the the phase-shift path that is fried, either IC2, IC3, or IC4. These are not legended, but they are the 3 chips surrounding the black thingamabob that houses the six LDRs. The sensible thing to do is to desolder all 3 chips...

Great reply - I might have ignored those ICs otherwise! The feedback pot does not seem to affect the noise or phasing in any way, so you could be on to something... Time to hit up mouser for some sockets & 4558s and see if that's the problem (or maybe just a bad feedback pot). I really want to get this thing up and running!

[Mark Hammer]

No reason, I suppose why it couldn'ty be an issue of BOTH the feedback pot and chip/s. But if there were an interruption in the signal at any point along the path from the 1st to 6th phase-shift stage, even a 100% functional feedback pot would not provide any discernible effect.

Note, as well that a discontinuity between successive phase-shift stages seems only minimally likely, based on what I know. If any of the caps ain't working, then there is another path between stages via the resistor going to the inverting pin. Now, if there was a crack in the board at the output of any of the first 5 stages, that could do it also. So check what's coming from pin 1 and pin 7 for those three chips with a magnifying glass, and try reflowing the solder there.

[Little Jerry]

Yeah, the only reason the pot is a (very slight) suspect is that it looks like somebody stomped the shaft clean off of it! I don't know why I forgot to just check it on my meter - I'll check it out once I get a chance to work on it again. It was just a great thrift store find, so I don't know it's history.
The first thing I tried with it was check the wires and reflow all (well, I think I got 'em all) of the solder joints and add some solder to some particularly bad looking ones. The board/traces/joints generally look pretty good, but might need closer inspection. Unfortunately reflowing didn't change anything but I might (carefully) check around those suspect ICs again while I wait for the new ones to come in the mail...

No reason, I suppose why it couldn'ty be an issue of BOTH the feedback pot and chip/s. But if there were an interruption in the signal at any point along the path from the 1st to 6th phase-shift stage, even a 100% functional feedback pot would not provide any discernible effect.

Note, as well that a discontinuity between successive phase-shift stages seems only minimally likely, based on what I know. If any of the caps ain't working, then there is another path between stages via the resistor going to the inverting pin. Now, if there was a crack in the board at the output of any of the first 5 stages, that could do it also. So check what's coming from pin 1 and pin 7 for those three chips with a magnifying glass, and try reflowing the solder there.

[Mark Hammer]

There's a gazillion phasers that lack any feedback whatsoever, which still sound nice. So if there was a problem with the feedback pot only, that would not interfere with audible phasing. Again, there may well be an issue with the Feedback pot/path, but there is something else amiss in the phase-shift pathway, in addition to whatever might hypothetically exist with that pot.

[Little Jerry]

Phasor II lives!!
It was a bad IC (probably). I replaced the phase shift ICs first, no luck, then the output/mixer, no luck, and finally the input/buffer IC and it now works! (To be honest though, I could have the locations of one or two of the ICs on the board mixed up)
I haven't gone and re-inserted the old ICs to find the bad one yet, so it could've been a cold solder joint or something too. But I'm a happy man now though I tell ya! Sounds great
Thanks for all the info & help!

Excellent!! Glad we were able to suss this one out. That's the nice thing about forums like this: you can turn to other folks for a fresh perspective on how to fix things.

[kurtdaniel]

that's a rare box.. I have heard from a guitar how to magazine that Mu-tron is long gone.

[Gtr_tech]

I had one of those for a short time. They are good sounding phase boxes. This one had bad solder on most of the IC connections. It was hard to see at first....you really had to flex the board to see the chip pins moving. After taking car of that it worked great. But....It was worth more on ebay than it was to me so I offed it. I think I got around 350$ for it IIRC.

Musitronics didn't make it out of the 70s alive IIRC....