[Hallicrafters] SR-400A audio

[Jim Liles]

Just finished restoring an SR-400A.  After completely aligning this radio, 
found it to have poor audio.  Sounded fuzzy like it had some 60Hz overtones.
Checked for ripple on the audio and found 4 millivolts of ripple and no 
flattening.  This indicated that it was not an amplitude problem.  
Started injecting pure sine into the audio amp and backward.  Sounded 
great until the second receive mixer where the VFO is injected.  The fuzzy 
reappeared.  Put a scope on the VFO output and found no ripple meaning it
had to be an FM problem.  Put the VFO output on a spectrum analyzer 
and found the waveform to look like a saw blade was superimposed.  This 
was a vcap problem.  The audio was best when the calibrate or RIT pots 
were not centered and it was always better when using RIT.  Note that on 
the SR-400A, the calibrate pot is 50K and the RIT 25K and the SR-2000
both are 25K.  Added a 2ufd cap to ground on the center of both pots 
and the waveform was pristine as well as the audio.  What was the 
difference?  The SMeter on this SR-400A reads S3 on a 1uv and S1 
on a .3 uv signal. Before the change, I could copy a 1 uv or above 
sideband signal and after, one that would barely move the needle 
and the background noise was perceptibly lower.  If you have an 
SR-400A, test by tuning through a cw signal with cal pot centered 
and then with it fully clockwise.  If it sounds fuzzy centered and 
better fully clockwise, put a 2uf cap from the center of the pot to 
ground.  Have a great day --- Kindest regards Jim K9AXN    
 Phone 847-913-8576