Paul Burridge wrote:
Hi gang,

I've never had a lot of luck with GDMs for some reason. Even with a
decent meter, it seems such a drag tuning across such a vast range
looking for a tiny, easily-missed dip which you have to screw out of
the meter by forcing the sensing coil so far into the circuit
concerned you practically break the circuit board. Am I alone in
finding this potentially invaluable device practically useless in
practice? Is there a more viable alternative?

p.
I've built several GDO's over the years. My favorite circuit is the old
standby using a split stator variable capacitor and a shunt fed coplets
oscillator. This capacitor can be 100-250 pf per section. My favorite
tube is the 6CW4 nuvistor, though a 6DV4 might
be a better choice at VHF. I put a TO-5 transistor heat sink over the
tube fearing it might run hot! I use a cathode resistor of about
150-330 ohms (un-bypassed) and connect a bnc connector to the cathode
via a 1000pf capacitor to steal a little signal to drive my frequency
counter. With the counter set for low level input I get good drive up
to about 100mhz or so. My coil socket is an RCA (phono) connector and
the coils use a matching plug. A better idea might be to use a 3 pin
DIN plug and socket, this allows for a ground connection to go to a coil
center tap. If you add the center tap the circuit will work below
1.5mhz with proper coils, otherwise it will not oscillate below this
frequency! (Having coils that go to 455khz would be nice to test if
stages). Power supply used two 6.3v 300ma filament transformers back to
back with a bridge rectifier. You can find transformers small enough to
fit the supply into a box small enough to house the instrument, yet not
too big to be hand hold-able. The coils can be wound on 1/4" or 3/8"
plastic water line intended for ice maker use. These will fit into
small places. Use of peaking chokes in the plate and filament lines can
help eliminate "drop outs" in frequency coverage. Sensitivity can be
adjusted with a plate pot, or put the pot in the meter circuit. I have
used sub-mini meters salvaged from old jap tape recorders (vu-meters).
These are typically around 500ua sensitivity. See my web site at
www.qsl.net/wa2mze.