Musitronics Mu-Tron Phasor II LFO Tick

[PurpleOhmAudio]

Hi Guys,

I have a Mutron Phasor that had a few problems, footswitch needed replacing, mains cable was cut

Got it working and it sounds lovely, except for a ticking, even when the effect is turned off,

When I opened it up, there is a mod on the PCB, a trimmer added in place of one of the 4k7 resistors near IC-1, just checking this evening to narrow down which one exactly, the trimpot has been turned down to 3k6, this isn't in the LFO section though, but I'm unsure if it could affect the signal level, will have to turn it back up to 'stock' 4k7 and see what it sounds like, turn it back if its a problem

Whoever did the mod may have adjusted the factory trimpots also,

I have researched on a few posts here on calibration, and found the service manual https://bruynooghe.fr/public/effets_vintage/Phaser/MutronMU-01PhasorIIServiceManual.pdf that gave me some idea of whats going on with those pots,

Also some more schematics here https://freedubcircuit.blogspot.com/2017/12/mutron-phasor-ii.html

[Mark Hammer]

Replace Z6a/b - currently a 4558 - with a lower-power dual op-amp, like an LM358 or TL022/062.  That should reduce, if not eliminate, audible ticking.

Keep in mind the stock unit never completely bypasses/isolates the circuit board, such that any ticks on the power line will still find a way to your signal, even when technically "bypassed".

[PurpleOhmAudio]

Thanks Mark,

I will try that, I'm lucky as the IC's are in sockets on this version, so that's an easy one, and had seen someone asking about true bypass switching

I think if the tick was gone, its fine as it is, there's a tiny pop when using the switch, true bypass may alter the volume levels when switching though?

I have seen pictures of different layouts online, mine is a 220-240v Euro version, it has a soldered fuse, I'll get some pictures for reference incase it helps anyone

[Mark Hammer]

The bypass switching selects between connecting that 10uf cap to either the op-amp output OR the dry signal.  The 2017 schem shows a 10k resistor to ground from the common of the bypass switch, while the original factory PDF does not.  In the absence of a path to ground, "floating" electors will often pop when they are switched.  I would recommend adding a resistor like what is shown in the 2017 drawing.  Anything from 10k-100k ought to be fine.

I overlooked the fact that it is AC powered, when making my suggestion of op-amp change for the LFO.  Since they are socketed, it would be an easy swap.  But in retrospect, it may not make a huge difference in ticking, since there is adequate power.  The real solution may lie in decoupling the power supply for that chip, which is a little more involved.  But if you have or cab easily get a lower-power op-amp, try that first, just to see if it makes a difference.

[Scruffie]

Personally, I'd start by replacing the original electrolytic/tantalum capacitors, they're what, 40-50 years old? Leaky power caps could certainly cause undesirable behaviour.

The LFO has some minor decoupling via a diode on the positive rail, if a re-cap doesn't solve things, just an electro to ground from that might be all that's needed.

I suspect the 'trim' mod might be related to feedback seeing as it's originally fixed...

[PurpleOhmAudio]

Thanks,

Its easy enough to get some LM358's, the TL022 or 062 are a bit more expensive, so that's worth a try,

Thanks for pointing out the lack of a pulldown resistor, I had not seen the difference between schematics, maybe they were added on later versions along with a change of fuse holder, I saw some board pictures online that have different cap orientations too

And good point Scruffie, its over 40 years old now

Last night I was going to get pictures but got into the re-cap instead, It had 'Brel Sun' branded yellow caps [I think I've seen these in early DOD pedals] and some good old blue Philips 470uf mains caps, I got some new Vishays and Dubiliers on the way to stock up, had no axials and the power caps have to sit low on this board, but I had all the smaller ones already so that was Saturday night gone...

Board is lovely, well put together, tracks are strong and forgiving of a little pull here and there, I need some thinner braid to get around component legs

The 10uf cap on the output of this one was NP non polar, I replaced with a 22uf NP, sounds good, the switch pop hasn't changed in volume so I don't think doubling the value is a problem?

The later schematic has a polarised cap with negative to the output, and some other pictures online show a tantalum in that position

Seems a bit clearer sonically now, the tick is a little quieter, the pots were already in good shape, deoxit got them feeling mechanically great and smooth

The Trim mod does appear to be in parallel with a cap, so it probably is the one on the feedback path, would it being turned down to 3k6 decrease feedback? Unless its the one across pins 1 + 2 of IC1A, maybe that could alter volume into the phaser? I will have a play with it and see what happens,