The AM Forum

My NC303 suddenly lost the ability to hear CW and SSB. Still has good AM ability and nice audio. The S meter shows SSB signals being received but no audio.

I have checked all of the tubes and find them all good. Also, tried two other 6BE6 in the BFO/product detector circuit with no luck. The mode switch looks ok as well.

It appears the PD output no longer reaches the audio circuitry.

Any ideas?

Thanks, Rich

Hi Rich,

That should be an easy problem to troubleshoot since you narrowed it down and it appears to be a broken path.

The S meter is probably measuring AGC in the I.F., so that area is working OK.  First inject an appropriate steady signal into the I.F. (or RF input)    Then use a scope to check the product detector input and output as well as the diode detector and BFO to be sure they are generating signals and mixing properly.  

If so, then it's just a matter of finding out why the path from the detectors to the audio input is broken.  Might be a bad connection, bad cap, etc.  Sometimes using a capacitor to jump across suspected open parts will find it quickly. Trace it out with a scope probe.

Let us know.

T

 

First I'd check to see if the BFO is running, and on the right frequency, look at pin 2 of V8 with a X10 scope probe. It may pull the oscillator a few cycles but it should still sweep through the IF frequency when you rotate the CWO knob.   I would think it would either be "on" frequency, way off frequency, or not running.

Then make sure you have IF signal on pin 7 of V8.   If nothing on pin 7, look at pin 5 of V6 and see if the signal is there.

AVC/S meter signal is generated through a regular old 6al5 envelope detector.
 

A good source.

http://wireless-girl.com/Projects/NationalNC303/