Hi list,
So, here is my modification to the A-140 so that it can be made
to `auto-repeat', i.e. re-trigger when the sustain level is reached.

It turned out to be a little more complicated than I was at first
hoping for because in its first incarnation it didn't perform that
well, and so I added a few more bits to improve it. The basic idea is
to turn the op amp which currently inverts the envelope (so you lose
that function!) into a comparator which switches when the sustain
level is reached, and thus feeding this signal back into
the `Retrig.' input makes the envelope continually cycle. To
retrigger properly, the switching action of the comparator needs to
be crisp, but this tends not to be case when the envelope
is `levelling out' at the end of a long decay, especially if the
sustain level is low. Hence an extra resistor and capacitor are used
to `speed up' the comparator switching action.

It works quite well, but it will not repeat if the sustain level is
too low. With the component values below, I measured these levels to
be approximately:

Time Sustain (Sustain
Range knob voltage)
L 1.5 200mV
M 2 500mV
H 2.5 1V

(all at short to medium delay: they increase gradually with much
longer delays, but note the envelope goes up to around 7V). It also
(obviously) stops re-triggering when the sustain level is very close
to the top of envelope. Another thing is that since the `Re-trigger'
function (as designed in by Doepfer) re-starts the envelope from the
_beginning_ of the attack phase, when it re-cycles you can get a
sharp negative-going `glitch', but dialling in some `Release' slows
down the capacitor discharging to the point whereby to all intents
and purposes it recycles from the sustain level itself, which looks
much nicer.

In spite of all this, there is still lots of room for manoeuvre and
experimentation!

The last part of mod (step 8) drops the re-trigger output down to
within +/- 8V, so that there will be no problems switching it with an
(unmodified) A-150 if you want voltage control of re-trigger off/on.
If you are not bothered by this, just leave out this last step and
the re-trigger output will be approx +/-11V or so.

New components needed:
10pF ceramic capacitor
1k, 22k, 47k resistors (22k & 47k not needed if last step not to be
done)
1N4148 diode

("Top, bottom, left, right": for the component side, when viewed with
the face-plate downwards; for the solder side, face-plate upwards.)

1) Immediately below the TL084 chip find two sets of a 47k resistor
(yellow, violet, orange, gold) and a diode, one set above "D6", the
other above "D7". Remove both resistors – the top one is seen to have
been "R13", the bottom one "R12".

2) Cut the track immediately next to where the left end of R13 was
soldered, so that it is isolated from pin 13 of the IC (second pin in
from the right) and what was the left end of R12.

3) Locate pin 12 of the IC (third in from the right end, and directly
below the left end of R13. Cut the track connected to this (which is
actually 'ground') - I did it just below pin 14, the right most pin
of the IC.

4) Solder the 1k resistor between the middle connection on the
Sustain pot (just below the middle of the middle jack socket) and pin
12 of the IC (third in from right, now disconnected from ground by
above cut). (Take care not to get IC too hot...!)

5) Solder the 10pF capacitor where R13 was - bend the left end over
the bottom end of the 1k resistor and solder, so that resistor &
capacitor are both electrically connected to pin 12 of the IC. Double
check that solder has not bridged the cut in the track made in step 2!

6) Solder a plain wire link where R12 was.

7) Locate the 47k resistor above the Decay pot, just above
capacitor "C1" (looks like this is actually "R2"). Solder the diode
across this resistor, black stripe to the left.

8) Locate the 1k resistor (brown, black, red, gold) between
capacitors "C2" and "C3", and remove it (revealing it was "R14").
Solder the 22k resistor in its place. Solder the 47k resistor between
the top end of the 22k one (connected to the 'inverse output'
socket), and the top pin (negative terminal) of C4, which is actually
ground.

Re-label the 'inverse output' socket so you know module _is_
modified! (I called it 'Retrig Output'.)

To test, I suggest patching the envelope output into an A-110 pitch
CV input so you can hear what is happening, gate it with a slow-
running LFO, and patch the bottom socket (the new retrig output
signal) into the re-trig socket.

I shall put some photos (thanks again to Mike) of it in the photo
section straight away!

Cheers,
Tim