[sdiy] asm2 vco problems

[Magnus Danielson]

> At 09:14 AM 11/7/2008, Dan Snazelle wrote:
>
>
>>anyone around here built the asm2? having some real trouble with the vco
>>(vco 1)
>>
>>the schematics are here:
>>
>>http://www.elby-designs.com/asm-2/vco/vco-asm2-about.htm
>>
>>
>>getting voltage on all the rails.
>>
>>oddly the outputs (square wave specifically) keeps putting out  a
>>voltage...around 4 volts
>
> This looks like a very common problem people have with modifications to
> Terry's design.  The reset pulse is probably not long enough to discharge
> the integrator cap. The solution, if this is the problem, is to increase
> C111 a bit to make the reset time longer.  Try 27p or 33p.
>
> In his original article Terry wrote "do not change anything".  There was a
> reason for that.  :-)

Indeed. The rDS(on) of the FET is critical in balance to the capacitor,
and the reset time around the LM311 is critical to ensure that the LM311
keeps the FET driven active long enought. Then, the resistor in series
with the integrating cap is needed to compensate for the reset time in
order to not cause too sever tuning problems in higher frequencies.

Come to think of it. I should get my old ASM-1 board up and running again
and fool around with my counters. When I originally tested it I didn't
have a frequency counter at home or work, I used a scope at work for
testing. Now I have uhm... 9 frequency counters... :) Two of them can't
measure that low frequiences... and the old tubie (pre-nixie!) can't get
all the way up in frequency...

Cheers,
Magnus