[R-390] Audio and BFO

[David Wise]

Have you checked the AF GAIN pot?  Sometimes a carbon pot
will develop a crack in the resistance element.  Output
goes from 0 to 100% as the wiper crosses the crack.
If the endpoints are riveted, that's where it will let go.
Another suspect is feedback resistor(s) which normally
reduce the gain.

Recently my BFO faded out
after being weak for a long time.  (I hadn't noticed because
I use an outboard detector with its own BFO.)  Resistors,
caps, B+, and tube were okay.  Apparently the Q of the BFO coil
has deteriorated over the years and is now too low to sustain
oscillation.  The injection tap is very low impedance - almost
a short - compared to the cathode.  I added an "emitter follower"
transistor circuit to supplement the drive from the tube, and
now it's loud and clear.  This mod is simple and reversible.

Dave Wise 

-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net] On Behalf Of Michael Hardie
Sent: Tuesday, January 05, 2010 9:32 AM
To: R-390 List
Subject: [R-390] Audio and BFO

I'm still hunting down the cause of "over sensitive" audio, with the gain at 
"1" the audio is at a level that should be heard with the control closer to 
"10", and now there's a BFO problem that might be related.  The BFO only 
works with very strong stations, and seems weak then.  Ideas?

Mike VE7MMH