[TenTec] Zero beating aids?

[Dale Saukerson]

Way back about 1978 or so....I built a couple of different circuits 
from either QST or 73.

One was a simple audio filter tuned to 750 Hz. A LED would indicated 
a received tone equal to 750 hz.

The other was an audio frequency generator set to 750 Hz. You would 
audibly beat that against a received tone. If the tones matched, you 
had only one tone to listen to. If they did not match, you would get 
both sum and diff and an audible warbling.

I know I used them with my HW-101. Don't remember if I used them with 
my Corsair or '745.

Any of you remember something like this? (I no longer have my 
archived magazines).
They were probably OP-Amp circuits.

Thanks,
Dale Saukerson
Ex, soon again, N0AOZ