[sdiy] Vactrol - lessons learned

[harrybissell]

Just finished a leslie effect simulator (vibrato that fades in/out while
speeding up/down)

Here's some free advise

If you run the Vactrols in series... if one LED dies the whole line is
GONE. Now this
SEEMS obvious...

But if you made a phaser with the vactrols to ground from the
non-inverting input, and
an LED goes open... the whole thing will DIE unless you were clever
enough to add a
resistor in parallel with the photocell.   When the cells are off for a
LONG time, they can
go up to tens of megohms.  This did not provide a bias current path from
the input, and the
whole thing drifted up to the rails.

It was HARD to troubleshoot... removing opamps one at a time does
NOTHING because...
in effect... every single stage is fvcked !!!

A good technique for finding the open LED is to bridge a GOOD LED across
each one...
when you hit the bad one, it will light up.  If two LEDS are blown I
guess you'd be fvcked
again.

I added six 499K resistors across each cell... just in case.

In my circuit, the audio signal goes through the delay at all times -
there is no bypass... I just
stop modulation.  Talk about a mission-critical failure :^P

H^) harry