Repairing an EDP Gnat Special
About a year ago a colleague of mine gave me an EDP Gnat Special to repair. It sat round the workshop at work for a bit until I finally got a chance to take it home and get it on the bench.
Now for those who have never seen one it's basically a single-VCO version of an EDP Wasp - same touch keyboard, same 4069 inverter-based analogue bits, but the Gnat Special is in a rather nice wooden case made of what appears to be unfinished mahogany. The base was made of hardboard which had at some point got wet and broken away from the wood, allowing the mains transformer to drop down and breaking off the wires. Sadly I don't have a lot of pics of the repair but the power supply is basically just a small 9V transformer like you'd get in a wall-wart, with one diode and a 1000μF capacitor - that's it! It generates about 9V and could more accurately be thought of as a "battery eliminator" since the normal plastic-cased Gnats were powered by a 9V battery.
I fired it up off the bench PSU, slowly increasing the supply from 5V up in case the little TO92 7805 regulator had failed dead short. This would have caused a few problems for some of the logic ICs on the board which might not tolerate more than 5V. Tim Stinchcombe has drawn out the circuit for itwhich made my life a hell of a lot easier, since there's a lot of very clever stuff going on inside.
The most obvious problem was that there wasn't any sound coming out! Although the oscillators could be selected (with some wonky-looking waveforms, which is what I realyl wanted to write about) the VCA never did much. This turned out to be a blown LM3900 quad Norton opamp. That one chip is used for a bunch of stuff - the VCO, the VCF exponentiator, and both envelopes - but the quarter used for the VCA envelope seemed stone dead. A replacement chip cured the problem. It's a fascinating chip and I'd love to write more about it, but for now it's enough to say that if you check the datasheet you'll see that both inputs are basically the base-emitter junction of a tranny, and may have been zapped by the timing capacitor. Or something. Who knows?
The bit that generates the oscillators is incredibly clever. There's an analogue master clock oscillator, a bunch of multiplexers to select which touchpad is being tested, a thing that detects key touches, and then a decoder that changes the key number into a divider ratio to divide the master clock down for each note. This gives a little narrow pulse which is fed to a 4046 PLL, which then generates a nice clean square wave at the same frequency and also derives a control voltage, which is fed to the VCO circuit.
The VCO circuit FIXME is similar to the Juno 106 VCO, where an integrator charges up at a rate set by the incoming control voltage and is discharged by an external clock rather than a comparator that detects the top of the ramp. In this case the clock is taken from the squarewave output from the 4046. But as you can see in FIXME the ramp wasn't all that ramp-shaped! The square output was a funny double pulse FIXME and the VCO control voltage was all over the place FIXME.
I've hardy ever come across "bad caps" in synths, although in a certain make and model of radio gear I've done thousands of little 0.1μF SMD ceramics that go leaky and turn into 1k resistors. The 2μ2 capacitor was completely open-circuit! Never seen that before, ever. Now I'm not a fan of the shotgun approach but I figured if one really was bad, the rest couldn't be far behind and indeed changing the VCF envelope timing cap made the VCF envelope a useful length.
You can see the voltage on the 4046 VCO input kicks up and down a little as it pulls the squarewave into lock with the pulses from the note divider. The sawtooth wave FIXME looks a lot happier now too. Notice the slight "s-shaped" look? That's soft clipping from the 4069 inverters used in the filter. I'd expect that to give a nice "valve-y" clipping if you could push it harder.
Anyway that's the sort of thing I do for fun. It's hard to play the touch keyboard and hold a phone at the same time, so you'll have to excuse the potatocam video. RIXME