[sdiy] Help needed with ASM based VCO

[jays at aracnet.com]

Chris,

Yes your description sounds right.

On the ASM1 schematics I have R19/R20 form a 5V reference on the comparator - input. The integrator is connected up to the + input though R18 and the output is connected though the 18pf cap. The output will change on the comparator when the + input goes above the - input. R18 and C3 do make the pulse a bit longer to make sure the cap is discharged.

Couple of things with the FET discharge switch. 

Check the FET. Get the data sheet from THE MANUFACTURE THAT YOU HAVE. They come in different pin outs so maybe the one you have is different that what you expect.

Make sure you are using a FET that will work in the circuit.

Disconnect the comparator, leave pullup 3.3K in, take a jumper and tie the gate/pullup joint to -15 and then open it, do that several times. Look to see if the discharge switch is working. If not FET maybe bad. They are susceptible to static.

Typically when I have problems its more because I assume things vs construction issues.

Jay S.

Chris Manders wrote:

> Hi Jay
> 
> Thank you for your help.
> 
> I checked and I am pretty sure the LM311 is wired OK.
> 
> Please let me know if I have got this about right - 
> 
> When the CA3140 output reaches 5V (the ramp rises to
> the 5V level), the LM311 comparator switches and its
> output rises (to zero volts) and switches the FET ON
> to discharge the timing capacitor. The 18pF discharge
> capacitor (and timing resistor) are there to give a
> small amount of time to hold the LM311 'triggered' and
> the timing capacitor to discharge fully.
> 
> It is this that I think isn't working properly in my
> circuit.
> 
> At no point does the timing capacitor get discharged
> to zero volts. From my reading up, this is a common
> problem. The circuit ramps around the 5V level by a
> volt or so.
> 
> Just for a silly test, I changed the 'discharge' 18pF
> cap for 60pF, then 100pF and then even 1000pF. The
> ramp voltage swing does increase when I increase the
> capacitor value (which is good) but still, at no point
> was the main timing capacitor fully discharged.
> At no point did the ramp reach zero volts.
> 
> Any ideas anyone?
> 
> Thanks
> Chris
> 
> 
> --- jays at aracnet.com wrote:
> 
> > If my memory serves me there are some versions of
> > the schematics for the ASM1 VCO that don't show all
> > the pins connected to the LM311. Maybe you have one
> > of those and the chip isn't working right.
> > 
> > Might check that or check the schematic against the
> > LM311 <www.national.com>.
> > 
> > Good luck.
> > Jay S.
> > 
> 
> 
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