[R-390] BFO Problem/Question for a newly restored R-390A --- Problem Resolved

[Chuck Curran]

Hello All:

 

I just wanted to let you know that the BFO is now running at 100%.  Thanks
to all who provided suggestions and comments.  I received quite a number of
direct e-mails.  Lots of good discussion and some interesting stories about
similar problems too.

 

The first item I had was about the number of available turns, I did find out
that the BFO shaft does have about 3 turns.  I needed about 1 ½ to bring it
right back on the money.  I used the second method of BFO calibration to get
it right on the money, as shown on page 159 of the TM-11-856A manual.  

 

I am not knowledgeable enough to appreciate why the Plate resistor R530
going from 22K to 33K also changed the resonant frequency, causing the last
45 years worth of service people to keep twisting it further and further to
get a zero beat.  The inductance and 3 mica caps in the can really establish
the point for resonance, I would have expected that a lowering of the plate
voltage would have decreased the output signal amplitude, but left the
frequency alone.  What am I missing in this Hartley oscillator operational
theory?

 

It’s nice to now have the calibrator functional for more than 8-10 seconds
and a decent BFO for the rest of the time.  I am running out of problems –
time to get a different radio?

 

Chuck