[Hallicrafters] SX-111 Sound Raspiness Solved

[Bill Gerhold]

Those of who saw my previous message about a raspiness or roughness on
received audio whenever a SSB signal was S9 or stronger well, I solved the
problem.  I suspected an AVC problem and since I had some previous
experience modifying Collins KWM-2A, I went directly to the decay cap on the
AVC line C12 a .005uf disk cap.

A .005uf cap will provide a very rapid rise time and a very short decay
time.  When receiving an SSB signal, the signal would cause the receiver AVC
to "Pump" the gain rapidly with corresponding "S" meter bounce.  I assumed
this could be giving me the roughness I was experiencing on strong signals
since it mimicked the sound of the AVC being switch off.  If you ever
switched the AVC off on a rig, you know how the audio sounds on strong
signals - distorted.
 
Going into the rig and locating C12, provided a surprise.  Apparently a
previous owner had already put a .22uf cap in parallel with the .005uf cap!
So, the original pumping action must have been much worse than what I was
hearing when only the .005 cap was in place.  Pulling out my capacitor
substation box, I found that placing a .33uf in parallel with both caps
provided very pleasant listening.  Total capacitance for C12 at this point
is .55uf.

Well, the roughness or raspiness on strong signals is gone, and the "S"
meter hangs up there nicely, not bouncing up and down rapidly with each
syllable.  The SX-111 sounds great at this point, and I have put it back
into service.  

................... Bill (K2WH) ..........................