[sdiy] ReReReRe: ASM VCO tips wanted..

[Oren Leavitt]

Ooops, I meant connect (- input of U3,C2,Q3) to V- (not V+) through a 
100K resistor...sorry

For the JFET, you'll really want the 2N4391or 2N4856 or other good 
chopper FETs like the J111, etc, for both their fast switching time and 
low 'on' resistance which will discharge the capacitor C2 completely 
within the short < 1uS reset pulse from the 311.
The 2N3819 may or may not work. If it does, C2 may not be completely 
discharged within the reset time as it is.
You'll see this as a smaller faster sawtooth that's floating at the 5V 
level that doesn't reset all the way to 0V.
This can be remedied by increasing the value of C3, which sets the 
duration of the reset pulse.
This will give C2 more time to discharge but will also increase the high 
frequency tracking error substantially.
The increased HF tracking error can be offset somewhat by increasing the 
value of R17.

More info on the original VCO (that this core is from) by Terry Mikulic 
that appeared in Electronotes (recommended reading!) here:

http://home.att.net/~synthsource/ens76/vco1.htm

Make sure the gate, drain and source of the JFET are connected properly 
- the symptoms you're describing suggest that this might be the case...

It's a great oscillator - it's been my favorite for the past 20+ years.
Hope that helps!
Oren

RevTor at aol.com wrote:

>Well, thanks for the tips so far, but still no workee.
>
>I still have a few things to try.. 
>The voltages at the cv summing opamp are all normal, and adjust as expected, same as the other osc. I can get it to 0v with the trimmer no problemo..  I checked the JFET orientation a few times, im 99% sure its correct. 
>
>Im going to try this next:
>  
>
>>>If that checks ok, then check the oscillator core by >>disconnecting the collector of Q2 from Q3,C2,U3.
>>>Connect this (- input of U3,C2,Q3) to V+ through a 100K >>resistor - it should oscillate.
>>>      
>>>
>(thanks Oren)
>
>one question for the more JFET/tranny savvy of y'all out there..  Do you think that my 2N3819 is a suitable replacement for the recommended 2n4391 or 2n4856?
>Hmmm  and howabout the fact that the V at saw out is a slooowly declining ramp? About 1 cycle in 20 seconds approximately from +5 to -5.  Maybe this is just some V leaking through the..  uuuh..  FET or just the natural cap discharge???
>anyway,
>thanks!!  Im going to get this bitch going!!
>
>~Steve
>
>  
>

-- 
Oren Leavitt
oleavitt at ix.netcom.com