Univox SR-55 Analogue Drum machine repairs. Korg Mini-pops!

My very good friend, and confirmed member of the Taffia, has passed this in my direction.

"Just sort of humms, can you take a look?"

... of course I can.

Good grief, what a rats nest! Not a single integrated circuit! Discrete transistor logic. Simple power supply. A real treat to fathom out!

 Connecting to the workshop amplifier and switching on reveals not so much of a hum, as a scream. A little reminiscent of 1980's Casiotone PLL chord generators, only very loud. Listening carefully I can hear some very faint drum sounds though the noise. Given the age of the equipment (possibly early 70's) I was hoping for a simple power supply repair, but no, those 40 year old electrolytic's test superbly.


On the top of the unit is also the sequencer. It's job is to provide pulses, once per beat. It's a discrete shift register. The clock speed (and thus, tempo) is controlled via a pot on the front panel.

Each  pulse is sent to the switch board, which decides which pulses should go to which "instrument" when. It's a huge wiring nightmare. The rhythms are hard wired in this switch bank.

So our pulse has left the sequencer, been routed through the switch bank, and on to the diode matrix. This is used to turn each timed pulse into the instrument we hear. Each diode passes the pulse to a whole bunch of near resonant filters on the rear board. Each filter is tuned to a different frequency, and two are noise generators. One instrument can be several filters being triggered at once, each triggered by the pulse sent via it's own diode. All the filters are mixed together, buffered and output via a volume control. There's also a tone control, which is simply an adjustable high-cut filter. There's a start button, which stops and resets the sequencer, or starts it running.

It's a simply stunning piece of equipment. With any sort of equipment like this, it's important to take your time and work through it. It's won't be complicated. It may look like it, but there's no odd ICs, no firmware and no pre-programmed bits and bobs only the manufacturer knows how they work!

So, what's wrong with this one? One of the filters seems to be resonating all the time, and howling. After some time poking around with a scope, it transpires that there's an inductor open circuit in one of the filters, and is causing it to oscillate. I have no idea what value the inductor is , so it's un-wound and re-wound with new wire of the same gauge... all 1,400 turns! Replacing it and a leaky 2SC828 transistor near by, followed by a quick set up produces some improvement. Some instruments in some rhythms sound distinctly odd though... One diode in the matrix is short circuit... replacement restores operation ...




Now... where's the Jean Michel Jarre LP???!