[Hallicrafters] Re: GB> FW: SX-101A

[ne1s]

Langston, Mike writes:
> ________________________________________ 
> 
> Now it has this "problem".  
> 
> It's going to be hard to describe. It is like a steady pulsing
> interruption to the signal, The interruption is very short (much shorter
> than a "dit") and the rate is about 3-4 per second.  
> 
> I've heard the term "motor-boating" used before.......... 
> 

Now this may not be your problem at all, but less than 2 weeks ago I was 
working on an NC-183 with the same symptom. In this case, the "motorboating" 
occurred only with the RF gain pot between ~90 - 100% of full-tilt boogie. 
Below ~90%, it stopped, all was OK except that I could not take advantage of 
the full gain the rcvr was capable of. The RF gain pot controlled the gain 
to (IIRC) 2 of the 3 IF stages (by adjusting the grid bias to these stages). 
This implied that there was IF oscillation, but I was hearing an oscillation 
at an audio rate. I was scratching my head for a while on this, and tried 
paralleling large decoupling caps to the IF and audio stages, one at a time, 
to no avail. Then I discovered the motorboating turned into a regular beat 
note when I switched from AVC to MVC - aha! Now, in MVC, we had the symptom 
of an oscillating IF. To make a long story somewhat shorter, what was 
happening with the AVC enabled was that the IF would start oscillating, 
generate an AVC voltage which would then reduce the IF gain enough to stop 
the oscillation, at which point the AVC voltage would go away, and the 
oscillation would then start up again, ad infinitum, at an audio rate (set 
by the AVC decay time constant, which muast be on the order of tenths of 
seconds). I found the offending IF stage with the aid of my 'scope and by 
selectively yanking IF tubes. The cause? The plate lead was too close to the 
grid lead in the 2nd IF. Pushed it just a tad farther away with a 
screwdriver, and everything was fine. 

73/GL,
 -Larry/NE1S