[sdiy] Vactrol - lessons learned

[Scott Gravenhorst]

Heh heh, reminds of the "old days", cheap TV sets with vacuum tubes and
all filaments in series.  Which one blew?  Take them ALL out and ohm them
if you can, or run down with the box of tubes to the local electonics
store and plug them into the tester one at a time...

Good points, I'll remember this when I finally build an H11F1 filter that
I keep thinking about - now with 100 mv input levels...

harrybissell <harrybissell at prodigy.net> wrote:
>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
>

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-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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