Wiardgroup

The two LEDs in the Vactrols are in series. The one for the 555 
is the clock for the sample and hold, the other one forms a 
smoothing filter where the filter corner frequency tracks the 
frequency of the sample and hold clock (the 555). That is right 
out of the Buchla SOU. The filter is just a lowpass gate with 
bigger caps for lower frequency.

Put an LED in SERIES with the LEDs inside the Vactrol not in 
parallel. A LED in parallel will shunt the current around the LED 
in the Vactrol and it will stay dark with the photocell all the way off.

The VTL5C3 Vactrols really go way OFF. Like 100 megohms or 
more. That is why the 10 meg in parallel, it sets a limit on the 
resistance. 10 megs give a clcok rate of about 20 seconds per 
cycle.

The synch mod looks OK. To reset a 555 you have to pull the 
reset pin HARD to ground otherwise it pretty much floats and 
runs. I don't think that is the problem.

> I think I spoke a little too soon in my last post.  There's one 
thing  
> that I can't get to work and am hoping someone can help me 
> debug.  The 555 doesn't change speed if I vary the control pot 
or 
> input a CV.  
> 
> If I put an LED across the LEDs in the vactrols, it does vary with 
> the input voltage, so I think they're doing what they're supposed 
> to.  The way I understand it, the LDR in the Vactrol on the far left 
> (VC2) changes with the control voltage, allowing current to 
> bypass the 10 Mohm resistor in parallel with it.  This in turn 
> should change the rate of the 555.
> 
> I thought it might be because of my sync mod to the 555, with 
no 
> sync signal connected,  the reset pin (pin4) is set to +15 V as it 
is 
> shown in the schematic.  If I do connect a VCO to the Sync 
input 
> the 555 changes speeds with the VCO.
> 
> Any ideas?  Any help is appreciated!
> 
> Romeo