LOW COST INDUCTANCE ADAPTOR (December 1992): The oscillator stage
may not function due to differing characteristics between samples
of the BF245 JFET used in the level control circuit. This can
usually be cured by dropping the value of R21 (33k) to around
15k, which effectively increases the loop gain around the
oscillator circuit.
Also, if you are having difficulty with the calibration
procedure, try the following method:
1. Adjust RV1 (10k) for a oscillator output level (at test point
A) of about 1.5V p-p or 1V RMS.
2. Short the Lx input terminals with a wire link, connect a
multimeter to the `Volts' output terminals, then adjust RV3
(500k) for a 0V reading.
3. Replace the wire link with a 1k resistor, and adjust RV2
(100k) for a meter reading of 0V.
4. Connect a known value inductor across the Lx terminals (in
place of the 1k resistor) then adjust RV1 (10k) for the correct
multimeter reading. For a 1mH reference inductor, the reading
should be 0.1V on the normal range (SW1) and 1V on the X10 range.
5. Repeat adjustment steps 2 and 3 above, then finally step 4
where RV1 should need little (or no) adjustment.