I posted this in another response but it fits here. Beside using a
GDO as an indicating wave meter or as a grid dip oscillator it is
possible to use it as a hetrodyne detector. In order to do this it must
have a headphone connection across the meter. The advantage of using it
this way is that the sensitivity is much greater than in the other
arrangements. One listens to the headphones and tunes for a beat. The
coupling need not be nearly as tight as in the other applications.
However, I tested this using my Millen GDO to listen for the LO of
an S-40A. Western Electric very high impedance headphones. could not
hear a beat when coupling to the antenna lead. The LO output there is
very low so its quite possible coupling more closely to the LO might be
audible. However, too tight coupling will shift the oscillator or even
kill it.
Using a separate receiver is a better way to hear the LO. Should
work from radiation out the antenna terminal or from the tube. Keep in
mind that the LO will be displaced from the signal frequency by the IF
frequency. For lower bands nearly always above the signal but in many
all wave receivers the highest band or two bands will have the LO below
the signal.
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On 3/5/2025 9:41 PM, Jim Whartenby via groups.io wrote:
Tom
Depending on your model of GDO you can either use it to generate a
signal and look for a dip in "grid" voltage at resonance as power is
absorbed by the LO tank circuit or you can use it as a "receiver" and
monitor whatever frequency is generated by the LO.? In either case the
GDO coil has to be closely coupled to the oscillator coil under test.
If the LO tuned circuit is good then you should be able to verify that
the LO tank circuit is adsorbing power form the GDO.? If you have not
verified this then I would first suspect that the distance between the
GDO coil and the LO coil is too great so no coupling is taking place.? I
have a GDO but have never used it.? I prefer to use a working receiver
as a tester since it is much more sensitive.
There is also the slight possibility that when you replaced components,
you may have mis-wired something or the dreaded previous owner did the
deed.? This is something that we have all done at one time or another,
especially if we have taken a break halfway through the process.? I
believe Jacques asked if you had double checked the LO circuit wiring
but I don't remember your answer.
An open LO coil is easy to ohm-out but an open tank or coupling
capacitor is a bit more of a problem.? Then there is the issue of a
broken or shorted wire to the tuning capacitor oscillator section or a
faulty band switch segment.? I would closely eyeball things in the LO
before resorting to any more digging into the chassis.? Even verify the
replacement component values.? I have sometimes transposed resistor
stripes so it helps to verify resistors values with the VOM or DMM.
Regards,
Jim
--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998
