[Milsurplus] Really Weird BFO

[David Stinson]

Restored a Bendix 555 portable Marine DF rig.
https://goo.gl/photos/T7rd2ET4Gp4D9bXb7
Open IF transformer, battery corrosion, 
"golden screwdriver" stuff etc.  Fun little rig.
Surprisingly sensitive and the direction finder
works quite well on LF and MF.

This thing has the weirdest "BFO" circuit I've seen.
There are two boards inside- Front-end and 
IF/Audio.  Here is a diagram of the IF/Audio
board:
https://goo.gl/photos/jaPgpm3Tjd22MTYr9

The RF from the front-end inputs on Pin 4 
on the red buss to First IF transistor TR-4.  
RF is also fed down to an 800 cycle 
*audio oscillator,* TR-5, which, when 
"tone" mode is selected, is barely biased on until
RF appears on the red buss.  This RF fully drives
the audio oscillator on, which outputs on the green
buss, goes to the Front-End board where it 
goes through a "gain balance" pot- one for each band.
It comes back from the Front-end and is sent to 
the Audio gain pot.   Strange!  
Actually works well for CW.  Of course, it's not 
useful for copying Sideband as it only outputs 
800 cycles.  Here's a demo of  it copying CW,
first the audio oscillator off, then on.
https://goo.gl/photos/LS8uFLzoXKYxfZyX6

How about that for a "BFO?"

73 DE Dave AB5S