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.







Adding a sub out to an amplifier

Adding a sub output is not the kind of work I usually do. However, I decided to give it a shot.

The item to work with is a Skytronic karaoke amplifier.

So I simply tapped the signal after the volume pot and attached it to the active subwoofer circuit I used,  I found inspiration from this circuit. Since the amplifier already had +/- 12VDC supply I did not have to use a dedicated power supply for it.

I also changed the output terminals for the speakers as requested by customer. Here’s a view from the back, with some new stuff.





Guyatone Rolly Phase Sonix

Got hold of this old phaser in a no working contition.

Tried to power it up but  found that it’s short circuited. The problem was not located in the circuitboard, found that one of those blue old tantalum caps on the power inlet for the DC-jack was  shorted.

I had them replaced and yes! There’s phasing. However it seems that it’s in need some more tending to, it’s way to noisy.  Lets see what a total recaping can do for that.

To be continued….




Maestro PS-1A Phaser

Here’s a continuation on the Six stage phaser post.

So I finally got the fets matched and ready to go into the circuit. It all seems to work as it should. Here’s a short soundclip recorded as I take it through the presets, slow, medium, fast and then back to medium and slow. Poor quality on the recording though, using a tone generator and cellphone for recording.

You can hear the nice feature of the ramp working.

Now the work continous, it’s time to modify. The input buffer needs some tweaking and the I’ll have to decide how to the speed presets will be set, who knows were it will end.


Yamaha RA-100 rotatating speaker repair

Got  a Yamaha RA-100 in for repair. A quite nice looking solidstate amp with the feature of two rotary speakers. It’s in a nice condition but it blows fuses in the poweramp amp.

Here’s a caption of the rotary speakers and the poweramp.

I ripped the poweramp out and after some measurements I found that the power trannys on side was blown, causing the fuses to blow. I replaced the power and drive trannys for both side and had the circuitboard recaped since some of the electrolytic capacitors was in a bad state.

Look at the size of an old electrolytic capacitor from the -70:s compared to a modern one with much higher voltage rating.

The circuitboard totally recaped with new power and drive transistors.

So it was finally time to hook everything up and fire it up.  Here’s a video from the tryout.

I must say it was nice bringing this one back to life.





Six stage phaser

Currently working on this right now. It’s based on the Maestro PS-1A Phaser. Made a layout and etched the board

Started populating, just waiting for the fets I’ve ordered to show up.


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.


Rozz R-5 Flanger repair

So I got an old Rozz R-5 Flanger on the workbench. It is in a no working condition. The outputjack was missing, the circuitboard is missing at least one diode and a transistor and some of the offboard wiring is loose. Seems like someone has tried to fix it but given up.

Rozz FL-5 Flanger

Did some search on the net but could not find a schematic for it but it seems identical to the Ampeg A-5 Flanger .

Ampeg A-5 Flanger

Searching on a schematic for the Ampeg Flanger did get me a schematic that seems correct.

The missing transistor was a 2SK30 for muting the Flanger effect, missing diode was connected to the gate of that transistor.So I got the offboard wiring reconnected and a replaced the missing components and fired it up. It just pass unmodulated signal. Measured the clock IC MN3101 with oscilloscope and found it did not send any clock frequency. Replaced it just to find that the BBD chip, a MN3006 was also broken. Replaced the BBD circuit and yes! There’s a flanging effect.



Patch 2000 project completed

So it was time to get the power supply mounted in the enclosure for the Steiner Microcon Syntheziser. Power inlet, transformer, rectifier and so on. A pretty tight fit but it all went well.

My main concern was that due to the tight fit, I would have transformer hum picked up in the audio path. I must admit it felt god firing it up and realizing there was no interfearence from the powersupply.

Chassie with transformer and PSU electronics. As seen I mounted a fused powerinlet with built in main switch and a poweroutlet for the Ampeg Patch 2000 pedal.

Steiner Microcon

Steiner Microcon

A picture of the unit mounted

Steiner Microcon

Here’s the complete setup all hooked up for testing

Hagström,Ampeg Patch2000

And a video