ASM test help

[Debby and Gene Stopp]

An malfunctioning ADSR can be difficult to troubleshoot sometimes...

It would be most helpful if you have an oscilloscope, although you could use
a voltmeter since envelope generators can operate at very slow rates
compared to something like a VCO.

Referring to the schematic:

First, make sure that the pair of gate conditioning comparators (LM358)  are
changing state when the gate goes from low to high and back again. One goes
high when the gate goes high, the other goes low when the gate goes high.
The one that goes high when the gate goes high triggers the set/reset
flip-flop formed by the two gates in the 4002 through a differentiator (the
.01 cap and 47k resistor). The rising edge of the comparator output sends a
positive spike through the cap, setting the flip-flop. The flip-flop causes
the analog switch to connect the envelope cap (4.7 uf) to V+ through the
ATTACK pot. When the cap charges up enough, its level gets high enough to
reset the flip-flop via the ATTACK PEAK trimpot.

When the flip-flop is reset (either by the ATTACK phase completing or the
gate input going away), the envelope cap is connected to either the DECAY
pot (if the gate is still high) or the RELEASE pot (if the gate is low).
These paths are established by the analog switches in the 4053. If the DECAY
pot is selected, the envelope cap disharges to the SUSTAIN level. If the
RELEASE pot is selected, the envelope cap discharges to ground.

Things that can cause the circuit to "lock up":

1. A bad 4053. I recommend socketing the 4053. These things can be
tempermental.

2. A bad 4002. Another good candidate for a socket. This circuit relies on
the CMOS switchpoint of one-half of V+ to reset the flip-flop with the
slow-moving envelope voltage. Some of the 4002's I've seen don't behave well
here. Try a couple of different manufacturers, like Motorola or National.

3. A solder short/open anywhere.

4. The gate input going negative by more than a few hundred millivolts. The
LM358's hate this, since they're powered between ground and V+. Use input
series diodes to prevent this if the gate input cannot be prevented from
going negative (like when you use the bipolar LFO waveforms to trigger the
ADSR).

Uh, by the way, ARE you using the LFO to trigger the ADSR? Then #4 might be
your problem, don't even get the scope out.

- Gene

-----Original Message-----
From: RevTor at aol.com <RevTor at aol.com>
To: synth-diy at mailhost.bpa.nl <synth-diy at mailhost.bpa.nl>
Date: Wednesday, December 22, 1999 10:21 PM
Subject: ASM test help


>Okay, my env. generators are all wacked out...
>That "True ADSR shape" is nowhere in sight  (yet)
>Symptoms
>-Decay and sustain pots generally have no effect
>-Attack and release pots act as if they're wired backwards (And they're
not)
>-The trim pots all seem to have no effect except for the trig thresh level
one
>    Can someone tell me some refrence voltages and where to probe for them?
>I'd like to get these sorted out...
>thanks(again)
>~SteveM
>