[Collins] 51S1 project correction/need help

[Dr.Gerald Johnson]

Unless the tube is used only for that crystal oscillator, the problem is
more likely the crystal.

Ways for testing local oscillators:

1. Another receiver. The local oscillators always leak a little. That's
the nemesis of the superhet receiver in war time. And why many military
HF receivers have two RF stages. Its not that they need the RF gain, its
to cut the LO signal leakage to the antenna. And why TRF receivers were
used through WW2. A leaky LO to the antenna is as good as a beacon for
the enemy finding you.

2. You could use an oscilloscope with 20 MHZ or greater bandwidth to
look for that signal.

3. You could use an RF VTVM probe to look for that signal.

4. You could use a grid dipper in heterodyne mode or in absorption wave
meter mode to look for that signal.

5. You could use a signal generator or the grid dipper to inject a 17.5
MHz signal to see if the receiver receives with that signal added to
prove its lack of the local oscillator that kills the receiver.

Now you need a crystal test oscillator to determine if its the crystal
or the circuit/tube. You could take a 6" test lead with minigator or
microgator clips on the ends, form it into a coil of 4 or 5 turns and
connect the ends to the crystal. Then test with the grid dipper. Going
upward in frequency VERY slowly, you should get a deep dip and if you
were listening to the frequency with a receiver you should notice the
dipper locking to the crystal frequency for most of that dip. Or with
signal generator and scope you can terminate the signal generator and
scope each with a 50 ohm resistor and connect one lead of the crystal to
the generator and the other to the scope (generator and scope grounds
connected) and tune to 17.5 MHz to see a closely spaced peak and null in
the transfer of energy. If there's no response on the dipper or the
signal generator/scope, then the crystal is indeed bad. Its handy to
practice these tests with a working crystal to better know their
characteristics. My HP-5100/5110 synthesizer is especially handy for
such tests.

If the crystal is good and oscillates in other oscillators then there
could be tube or circuit problems, including leaky screen by pass
capacitor, open grid and cathode capacitors and the like to upset the
oscillator. Even parts could be missing having been grabbed to fix
another radio.

73, Jerry, K0CQ, Technical Advisor to the CRA
-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.