The AM Forum

My R390A  has developed a problem.  The BFO won't null.  I can zero beat a carrier, but there seems to be a secondary tone that stays present.  Even with no antenna connected, it's still there.  I suspect something in there is oscillating.  I tried the easiest thing first....changing the BFO tube, the first 3 amplifier tubes, the AGC/Time constant tube, and no difference.  The pitch control does change the tone, but it never goes away.  Not a huge problem for AM reception as I'm still able to zero beat the signal, but sure makes it impossible for CW or SSB reception.  Anyone ever experience this that may give me some guidance?  I did make a mistake when muting the RF gain terminals on the rear...I first connected an external relay incorrectly that may have sent 120v AC into there.  The RX mutes fine with the connections to the break-in relay and RF gain terminals connected, although the RF gain pot is very sensitive (I also changed the pot as it shorted out).  I'm using the Diode output to a stereo amlifier (sounds great). I appreciate any comments...73.

Jay, I thought Ray, WD2AFJ told you to bring it to his place and he would try to help you with it? It almost sounds if one of your IF's is off slightly. This is a pretty common occurance that I have found with other receivers. If I can do it, anyone can.

Betchya one of the IF toobs is oscillating, maybe due to a bad bypass cap. 

Try this before you yoink the IF chassis - first make sure all the toob shields are good and tight and all the toobs are firmly planted in the sockets.  Rock each toob a bit to make sure it's making good contact.

Then, getchyer scope out and make a coil of stiff wire, maybe 5-10 turns just big enough to slip over a 6BA6.  Connect your scope probe to either end of the wire, set the scope for max sensitivity, then remove the IF toob shields one by one and slip the coil over each toob.  Go back to front, IE 4th IF to 3rd IF etc until you find where you don't see the oscillation.  The stage you just checked is the one that's oscillating, concentrate your repair efforts there.

If you don't see any oscillation with the above method, you'll hafta pull the IF chassis out and probe directly (or you can use one of those toob extenders with the terminals on the perimeter).

73 John

You could also tune the IF  transformer a bit on each stage and listen if the tone changes. Start at the last stage and work upstream.