[Collins] COLLINS R-388/51J-3 FREQUENCY ACCURACY & CALIBRATION

[Dr. Gerald N. Johnson, electrical engineer]

Probably you need some crystals, but care in alignment may improve the
situation.

First you need to set the crystal calibrator with WWV to be as close to
perfect as possible (slow swishing sound of the beat note, well below 10
Hz is possible).

Then go to a low frequency multiple of the IF frequency, say 2 MHz.
Without antenna, turn on the BFO and calibrator and adjust the BFO
frequency for zero beat. If the line on the BFO knob is not upright, set
it so. Do the IF alignment with the test signal zero beat with the BFO.
Paragraph 5.3.6 in the Collins 51J-4 manual.

Check the HF crystals now. Some are used for high side injection, and
then for low side injection.

Paragraph 5.3.3 mentions a ceramic trimmer, C-167, labeled XTAL on the
chassis that affects ALL the HF crystals. Once you have set the BFO
precisely, then you may find a setting of the XTAL trimmer that improves
the average frequency error, or you may find you need to replace some
crystals. 5.3.3 says the circuit capacitance should be set to 32 pf with
the XTAL trimmer, so new crystals need to be calibrated for 32 pf
circuit load.

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