Historical Effects: Restoring the MuTron Phasor II

Hal Stern
5 min readDec 28, 2020


It’s frequently just one wire that separates joy from despair. This was first made evident to me in Princeton E-Quad basement lab, where a lab partner’s elaborate breadboard was tagged with a note from his quasi-evil partner saying “Moved one wire, have a great weekend!” The wire in question was of course the most obvious — the power lead — and in the days before we snapped pictures with our cell phones to document simple life configurations, this was the thinnest acceptable terminator line.

I’ve turned that lesson of looking for the most obvious defect into a small hobby of buying and refurbishing guitar effects pedals. After a few years of looking at densely packed modern pedals, I’ve drifted to the vintage 1970s and 1980s bigger boxes, packed with all manners of tubes, AC power supplies, radio-quality passive components and a tone that is usually unmistakable once restored to working order. If I can close the decades long feedback loop on good sound and help those pedals return to the hands (and feet, literally) of musicians who are both nostalgic, cash strapped and stuck at home — then I’ve wielded the soldering iron of power for good measure. I’ve hit the point where the business is self-sustaining; I’m not going to retire on it but I’m not losing money in the truest definition of a hobby.

My primary source for vintage pedals is eBay — pedals are subject to every kind of physical abuse, literally stomped on, kicked, having people spill drinks on them, or with drinks spilling people on them. Not matter the intent or impact, liquid, repeated force and other electronic vagaries don’t mix with low voltage electronics. My usual diagnostic approach is to pull the cover and knobs off, then use Google image search to find a picture of what things look like in normal wired and working states.

“Doesn’t work” has the dynamic range of a symphony, with the same few themes repeated. Typically it’s just one wire — propeller beanie tip to Phil D — usually a power wire of one of the input or output wires as these see the greatest wear and tear. Sometimes, though, the damage is more like a collision, whether it’s conflicts of power, liquids, or personalities on stage, opening up the pedal is like disassembling a sports car wreck. It’s Michelangelo in reverse — the beauty is there and you have to restore enough components around it to make it visible again.

About seven months ago, I found a vintage Mu-Tron Phasor II on eBay. This is the stompbox equivalent of a Honus Wagner baseball card on an uncut sheet. It’s the embodiment of almost everything I listened to between 1975 and now, swirly, gently vibrating tones that powered funk, prog and some of the most recognizable keyboard sounds of the last 40 years. Serial number 1005, from the first batch of these pedals, is a tank — heavy gauge enclosure, lock washers holding the pots in place, AC power line with plug that you’ve last seen behind your grandmother’s couch.

The more I rebuild old pedals, the more I appreciate my family and friends who restore vintage furniture, sports cars and houses. There is so much craftsmanship that must be respected, and with a little love (and a lot of small area mechanicals) you can breathe new life into something that was headed for the town electronics recycling.

My problem right there: These things shouldn’t melt or deform like that. Heat, sparks and possibly anger were involved.

The Mu-Tron’s power transformer literally exploded. I’m sure there was smoke that wasn’t stage effect, there was ample profanity, and something or someone was thrown. But the case has aged better than my own body modulo a few points of corrosion, everything else is in good shape.

One of the great learnings from my EE212 days is that things are not purely binary; we exist in a continuous spectrum of analog signals where tolerances matter, thermal drift happens, and layout quality matters. Noise, feedback, and signal quality all depend on things working despite the heat of the lights, that last watery rum and coke and a slightly hot power supply. Transformers are physical things — lots of wire wrapped around a magnet, moving a lot of current and using magnetic induction to step up or down voltages without generating tons of waste heat. With enough use, abuse, heat and time, the coils can get loose, they start to vibrate, and eventually pop. This one went “pop” loudly, from what I can tell.

The first schematic I can find is incorrect — it shows a voltage that can’t possibly be correct for this pedal. Along the way, I asked a few questions of the Jedi who repair vintage pedals, and found apprentice standing in their unregulated clubs, with a reference to a replacement transformer of the right voltage, amperage and physical standing. Get any one those wrong and the first power on post repair yields more fireworks and is tantamount to setting a $100 bill on fire. Kevin Kelly’s “Rule of Seven” for research — keep pushing, digging, and vetting — leads to an $8 replacement part, that found its way over the Goethals bridge where it sat on my workbench for another six months.

It was the power lead again, but with a lot of transformer between the wall main and a usable +/- 24V DC supply. Replacing the power transformer is relatively easy, although I had to open some of the PCB holes a bit, clean up some old joints and gently tuck all of the wires back into place. Pedals like this are a joy of mechanical and electrical engineering — everything has a standoff to keep the internals from shifting, it’s well grounded, and in this case, there’s even what looks like a late errata on the PCB itself, with a wire soldered around a trace cut.

Repair made, component side
Repair made, solder and transformer side.

I was eager to test out my handiwork, but I always do the simple tests before grabbing instruments — plugging in via a current limiter didn’t produce any audible pops or smoke, and then cabling in my low-end practice amp and travel bass.

If we’re going to do 1970s Yes, we need the coiled guitar cable. Lights in the right places and no smoke.

And for the next twenty minutes, I was Chris Squire, ripping through the ostinato in “Tempus Fugit,” the bass sound just rippling and bouncing ever so gently, the same way I’ve heard it 40 years ago coming out of my Crazy Eddy vintage stereo. I gave the patented Squire leg kick, out of joy and homage, careful not to knock over the test rig and send me back to the first measure.

If you made it this far, and want to buy one of the last remaining Mu-Tron Phasor II pedals, it’s in my Reverb store.



Hal Stern

By day: CIO for R&D at a drug company. Scalable computing, data privacy, performance. Non-day: husband, parent, phan, bass player, career coach and ally.