[Collins] 51J / R-388 Crystal ???

[Dr. Gerald N. Johnson, electrical engineer]

Any 500 KHz crystal will do better than no crystal though it might be
optimized with different circuit impedances.

Can it have been that the crystal filter was sacrificed when mechanical
filters were added to your 51J?

Though if the original crystal failed the radio would have worked poorly
so removal probably kept the receiver working.

The venerable Radiotron Designer's Handbook has a section, 26.6, on the
design of variable bandwidth crystal filters. It shows how to measure
the crystal constants and then to design the filter around those
constants. Some of the variables include the transformer resonant
impedance, the neutralizing capacitor (no differential capacitor in this
circuit), and the load impedance for the filter.

73, Jerry, K0CQ
-- 
Entire content copyright Dr. Gerald N. Johnson. Reproduction by
permission only.