[R-390] R-390A 800 cps tone in cal below 8mh bfo off

[Larry H]

Thanks for the suggestion, Roy.  I used my daily r390a to listen to the 17 mc osc and it is clean.  I listened to it at 2 points, J221 and 1st mixer cathode.  I listened to the calibrator cathode follower at V205 pin 8.  Very interesting results here.  I listened to numerous points above and below 8 mh.  The results are very clean signals with no modulation on all cal frequencies as long as the 1st crystal osc is off (megacycles set to above 8 mh).  As soon as I turn it on (by switching the megacycle to below 8 mh) I hear the 800 cps tone easily on ALL cal frequencies above and below 8 mh.
I've tried 3 different tubes in the 3 related sockets, the 2 calibrator 5814 tubes and the 17 mh osc 6ak5.  I tried a different 17 mc crystal.  I also substituted the RF amp and the 1st mixer.  Unfortunately, the fix is not that simple.  One weird thing though, when testing, I installed a socket extender in V205 so as to get easier access to the tube pins, and the problem went away.  So I tried a different extender with the same results.  Take it out and tone returns.  I'm sure it's not a socket contact issue as I've deoxed it and the tubes are tight in the socket and moving the tube around causes no change or static.
Question - how can a 17 mh osc cause an 800 cps tone modulation on every cal frequency?
Any suggestions greatly appreciated.
Regards, Larry
      From: Roy Morgan <k1lky68 at gmail.com>
 To: Larry H <dinlarh at att.net>; 390 list <r-390 at mailman.qth.net> 
 Sent: Tuesday, September 8, 2015 7:40 PM
 Subject: Re: [R-390] R-390A 800 cps tone in cal below 8mh bfo off
   



On Sep 8, 2015, at 8:00 PM, Larry H <dinlarh at att.net> wrote:

> ...when I'm using the calibrate signal below 8 mh, I hear about an 800 cycle tone with the bfo OFF. It is weak, but it's not right.

I suspect something odd about the 17 mc crystal signal that is used below 8 mc.

I doubt it’s the BFO or the calibrator.

Get another receiver and listen to the 17 mc signal, also listen to the calibrator signal with the other receiver.  If you can, listen to the BFO signal with the other receiver.  I suspect you will soon find out where that 800 cycle signal is coming from.

Roy

 
Roy Morgan
k1lky68 at gmail.com
K1LKY Since 1958