Electro Harmonix Micro Synthesizer

This Micro Synthesizer is sporting a 1/4" jack that will take a Moog (or similar) expression pedal and sweep the filter. This is a modification I did per a customer's request.

I like the Micro Synthesizer. This is another David Cockerell design. David designed a legendary effect known as the Synthi Hi-Fli for EMS (David also designed the VCS3 and a lot of other cool stuff for EMS). A real Hi-Fli is a hard thing to come by. But you can assemble a decent approximation of it with a Micro Synthesizer paired with a Small Stone. It was the phaser part of the Hi-Fli that David used to create the Small Stone.

The Micro Synth has a very cool design. I enjoy working on them. The circuits are somewhat challenging for the beginner, but a deep understanding of them isn't necessary for repair.

Like all EH products in this enclosure, you will often see broken power switches. I put the link to a good replacement in the Memory Man info. Faulty stomp switches and bad jacks should be next on the basic checks.

Broken sliders are to be expected. Note that it is quite easy to just step right on the poor sliders. A guitarist might even try sliding them via the tip of their boot, only to lose balance and stomp right on them. You'll want to disassemble and really take a good look at the sliders. The other side effect of stomping on sliders is cracking the circuit board. I've seen Micro Synths with horribly cracked circuit boards. For cracks boards, you must carefully jump with little wires all the places where the traces are broken. It's actually not too difficult once you find the crack. The hardest ones are where you don't realize it's cracked initially.

Several of the circuits rely on OTAs. OTAs are easily fried, so try swapping them out.

One trick the pedal can play on you is with the RATE control. I can never remember which way to set it for a quick change, and which way to set it for a slow change. If you power the pedal up and don't realize the RATE is set for maximally slow... you may think the pedal has some weird start up problem. It will take a seemingly LONG, but strangely consistent time to turn on. If you're experiencing this, change the RATE to the opposite end. This will solve it.

A note on disassembly. After removing the back cover and slider caps, you actually have to solder and remove the input jack wires. Otherwise, you can't really get it apart easily. Mark the wires and board with sharpie if you have any doubt you'll remember where they go. Next, unscrew the 3 screws you see on the circuit board. Now you can flip the first board. The slider board is held down by 3 standoffs. Remove those next and you can get both boards out. Note the board to board wires. These are a trouble spot. Handle the boards to much, and these wires are sure to break off on you. Minimize the strain here while you work, check them often, and fix any suspicious ones just to be sure.

Do not remove the screws from the face of the pedal. That is a typical mistake, and I often find these either loose or the customer has already tried to take it apart and messed this up. You never need to remove the screws from the face.

Voltage regulator failure is common. Any pass transistor will do. The last one I did was a reissue unit, and the pass transistor specs were crazy high. The thing was rated to pass some really high number of amps and take 50V or something. This is overkill. I think my replacement was also amp rated, but a normal number of amps, and the voltage was plenty high but probably around 40V. If it's in a TO-220 package, it will probably do.

This pedal is prone to user error and false reports of problems. The TRIG control is misunderstood, as is the input gain trim pot you can access through the bottom panel. I made a youtube video at one point to illustrate the common user error stuff. Maybe I can find that link...