Uni-Tron 5 repair

Pretty rare piece I got in for repair. An Univox Uni-tron 5, a clone on the famous Mu-Tron III.

Here’s what’s said about it by Mike Beigel (Musitronics co-founder & designer)

The “Uni-Tron 5” was a copy of the Mu-Tron III, sold by Unicord. It was a time when Japan was copying US designs. Unicord was the precursor to Korg. We at Mu-Tron approached the president of Unicord, who acknowledged their product infringed our patents, and paid us a royalty. However, they only made about 500 units total. I have one in my collection too. The circuitry is a direct copy of the Mu-Tron IIIcircuit diagram, though the units don’t always sound quite the same.

That’s the story, firsthand.

The pedal was in a no working condition, it had been modified and also attempts was made to repair it. Here’s a picture of the homemade power supply that came with it, two 9VDC adapters connected for a +/- 9VDC supply.

First thing I decided to remove all the modifications restoring it back to its original state, removing the power supply connector, the potentiometer inside the chassis and restoring the off board wiring. I guess the pot on the inside was an attempt to attenuate the volume boost.

I tried firing it up after restoring it back to its original but it did not do what it’s meant to. The filters kind of went on and off with no slope and only with the gain pot turned up way to high. I found that it was the precision rectifier opamp A5 (looking at the Mu-tron schematic) that was faulty and the envelope capacitor C8.

Here it is in its working state

Now, earlier in my post I quoted Mike Beigel from Musitronics, he said “The circuitry is a direct copy of the Mu-Tron IIIcircuit diagram”  Let’s take a look at my findings.

The Mu-tron III used a dedicated opto electronic module seen in this picture with the A805 letters on it

Here’s the module in the Uni-Tron 5, it got its own module no markings on it.

Here’s a picture of the precision rectifier/led driver A5/A6, seen at the right of the transistor. Now that transistor is not to be found in the  Mu-Tron III schematic.

Actually it’s a NPN emitter follower connected to the output of opamp A6 driving the opto electronic module. Here’s the schematic for it.

Other than that I’ve found no difference.

/Krister

 

 

 

 

 

Mu-tron III

Got on old Mu-tron III in for repair.  It appears not to be tinkered with before. As I got it up I realized there’s some difference as compared to the schematics circulating the web.  I found two caps that differs from the schematics. The 4,7uF capacitor in the drive section for the led is actually a 3,3uF in this unit.

The 15uF nonpolar cap at the output is 10uF.

Looking at the schematic you can see them where I made notes about values in red text.

Here’s a pic of the old lady

Turned out the rocker switches for range and drive did not work as they should. Some cleaning sorted that out.

/Krister