Copyright 2002 R.G. Keen. All rights reserved. No permission for local copies or display from web sites other than http://www.geofex.com.
|Here's the corrected circuit. The
BC184C circuit is a perfectly workable phase shift oscillator if the
battery terminals are reversed, making the emitter the most negative
terminal. That will get us a sine-wave LFO. The trimpot that was listed
as 16K and that I guessed was a 15K
or 20K trimmer turned out to be a 1M trimpot on both units I worked on.
That was a source of trouble in both units that I played with, and I
have an alternate circuit I think would be better. In any case, the trimpot,
the diode, and the 3.9K resistor provide an adjustable bias
to the BF2440.
I had quessed that the "BF2440" was really a BF244 JFET for a variety of reasons. That turned out to be dead accurate. And, as I guessed, D1 is a 5.7V Zener diode. The trimpot then adjusts for the differences between JFETs. The BF244 JFET comes in A, B and C versions. I used a photo enhancing program and blew up the close-up of the part. With some edge sharpening and thresholding, I made Dave's part to be a BF244B. That's what was in both units I worked on, too. It turns out that if you leave out the zener or replace it with an ordinary silicon diode, the thing still works, in that you can tune in the JFET to give a tremolo effect. However, the setting is then dependent on the battery voltage, so the unit will have varying operation depending on how fresh the battery is and how the trimpot is set. The zener really has to be a zener for consistent operation.
The differences between the first and the corrected schematic are all errors I could easily have made in tracing this thing out if I was in a hurry. It's only thinking about and trying to understand the circuit function that makes the changes to the second schemo make sense.
There are still a couple of curiosities. The 0.68uF cap connecting to the gate of the JFET was bad on both units I had, and both trimpots were broken. Odd, but not fatal. I'll have some recommendations later.