ARP 2600 Bug?

Interesting. David Hillel of the New England Synthesizer Museum (Title Link) just posted a bug with the ARP 2600 on AH. Kind of odd when one crops up after all this time.

From the post:

"I was working on two 2600s with David White when he tried a strange ADSR
setting and
it didn't work. I tried replacing all the transistors, all the diodes, and
the opamp in the ADSR
with no fix. So we tried another 2600, same problem. Tried the 2600 on
display at the
Museum, same problem. They apparently all have it.

How to reproduce it:
Set Attack to maximum, Decay to minimum, Sustain to maximum, and
Release to minimum. If you hit the front panel button, you get a slow
attack, but
if you use a 3604 keyboard, the attack is instantaneous, which is wrong.
Of course,
if the sustain is at maximum you can set the Decay anywhere without
affecting the
result of a properly functioning ADSR, so the workaround is to put the
Decay up
to maximum.

Why it happens:
The trigger pulse from the 3604 is deliberately delayed, which prevents
the decay/sustain
logic from being disabled until a few milliseconds after the gate has
risen. This allows the
sustain setting (in this case, maximum) to back circuit through the Decay
control and
fully charge the capacitor before the slow attack can happen."