ASM-1 and JFETs

[Magnus Danielson]

Hi!

First: Check out your pinning properly! If the pinning of your JFET is
wrong you will have to correct that first... I had to! The pinning
really matches the PN4391 and not the 2N4391 (it's the second pin
which has to be bent over to the other side of the pin 1 and 3).
The 2N4391 is not really critical in type... I use a 2N4393 with great
result. One key issue is the Rds(on) value which should be kept low.
I beleive it's about 40 ohms for the 2N4391...

Second: If the pinning is correct you migth try this:
If you take an oscillioscope an look at the output of the CA3140 you
will most probably find that it is about 15 V and has a fairly high
frequency downward spikes... if you have those then you will need to
replace the 18 pF cap (which branches that LM311 op-amp) with
something higher (I hooked on an additional 30 pF to ground... not
very scientific but it works :). Changing the cap will slow the LM311
down so that the spike which drives the JFET will be longer and this
will allow it to empty the cap long enougth. A much too short spike
will not bring it down to 0 V and that's good bye tracking... the HF
oscillation comes from when the LM311 just barely stops pulling.. so
that the track current pulls it over the edge again... maybe would a
schmitt-trigger type of comparasion be better here.. but then, you
can't win them all :)

There is a PDF file somewhere giving reference to the PN/2N4391 JFETs
if you would need it... in worse case I can mail it... (I think I
linked it of one of my pages but I am not sure... should have at
least).

Tips: Trim the start frequency trimpot to about 0V (check after the CV
summer op-amp) cause then you can expect the oscillator at about 910
Hz. This is close to an A5 (880 Hz) so it can be a good kick-off
frequency... also, it is the free-running (non-modulated) frequency of
the VCO...

Good Luck!
Magnus