[Collins] 75A-2 NBFM plug in unit

[Dr.Gerald Johnson]

It would be called a NBFM adapter. Circuit surely to be in QSTin the 50s, 
maybe late 40s, also CQ, and certainly in the Editors and Engineer's 
Radio Handbooks of that era. Yup, 11th edition from 1948 on page 274. 
Uses two tubes, a 6AU6 limiter and a 6AL5 discriminator plus a 
discriminator transformer.

Or you could copy the schematic from most any tube vintage Motorola 
FM rig. They nearly all used 455 kHz IFs. Or take some receiver like a 
Sensicon, chop away all but the last two IF stages (first and second 
limiter), the discriminator and audio up to the volume control (to be sure 
to get the deemphasis network) and package the remains as a NBFM 
adapter. Or leave all the parts in the chassis, just use those parts 
needed. I'm sure I have schematic books for many vintage Motorola 
radios, more schematics than I have radios. But I may still have some 
workable Sensicon receiver strips in garage, barn, or corn crib.

The use of tube NBFM adapters was abandoned long before internet. 
Though there could be solid state devices, using some common ICs, or 
at least what used to be common.

Or a DSB product detector with a carrier adapter that extracts the 
carrier then shifts it 90 degrees for the local oscillator for the product 
detector will make a NBFM adapter.

73, Jerry, K0CQ, Technical Advisor to the CRA

-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.