[sdiy] EDP Gnat envelope oddity

Pete Hartman pete.hartman at gmail.com
Fri Aug 7 02:02:22 CEST 2020


Everyone's luck has to run out sometime.

On Thu, Aug 6, 2020 at 5:39 PM Gordonjcp <gordonjcp at gjcp.net> wrote:

> Well, finally after something like 30-odd years of repairing electronic
> equipment, I've run across a dead electrolytic in a synth.  A guy at work
> asked me to look at an EDP Gnat he's had kicking about for ages, with the
> most immediate problem being that the hardboard base had fallen apart and
> the transformer had dropped out breaking the power leads.  Powering it up
> off the bench PSU revealed a very weird sawtooth waveform (and no audio out
> - more on this later) with a steep start and end and very flat middle.  It
> turns out that the 2μ2 cap in the PLL loop filter was doing more or less
> nothing to filter the loop so the VCO voltage was two big negative-going
> pulses at roughly the input frequency from the timing generator.
> Temporarily replacing this with a 1μ cap seems to have cured that, because
> of course I have absolutely no 2μ2 caps anywhere.
>
> So, there.  At long last a bad cap in a synth!
>
> The next problem is that the VCA envelope is not doing much.  Now its
> timing capacitor checks out okay even though it's the same no-name green
> 2μ2 cap, so I replaced that also with a 1μ capacitor, while I wait for bits
> to arrive.  This hasn't actually solved the problem, is that although the
> voltage on pin 13 of the LM3900 goes up nice and high, the output - and
> indeed the voltage at the junction of the two resistors where it goes to
> the VCA OTA - barely changes.
>
> I'm guessing the VCA gate going low ought to drive pin 13 high through the
> attack pot, which will set the current - and that in turn will set the
> charging current of the 2μ2 cap, which will make its way to ground through
> pin 8.  The internal circuit of the LM3900 suggests that this is just going
> to be the B-E junction of one of the input trannies.  So, why isn't there
> much change in the output?
>
> I haven't really played with LM3900s but I found some and intend to
> breadboard a few circuits to get some idea.  Anyone poked about with this
> envelope circuit much?
>
> --
> Gordonjcp
>
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