Solved! -SB-101 AVC Hang/SLow Recovery

[John P.]

Hi ALL,
 
    After many, many hours of sleuthing found the problem with the SB-101 AVC hang problem.  When keying the transmitter, with no output (mike gain @ zero) for about 2 to 3 minutes, upon returning to receive the S-Meter would kick up to S-9 and slowly decay back to normal over 40 to 60 seconds.  I applied ALL of the factory and non-factory fixes to no avail.  THis includes checking voltages and resistances.
 
   Finally Len (SM5DFF) eMailed me via the Yahoo group about hisr fix which was to ground the screen grids of the 6146's during receive.  I found this curious and investigated.
 
   What I found was...  That during transmit the screens of the 6146's were at 300 volts, but during transmit the were "floated" in the circuit - at about 0 volts (per the book).
 
   Well, I found that for some reason that the screen grid circuit was holding at about 150 volts and SLOWLY decaying.  This circuit HAPPENS to be supplying the screens of the isolation amplifier which is just after the balanced modulator.  This isolation amplifier (6AU6) should be cut off too during receive but alas, it was seeing the decaying positive voltage too as I watched it and its affect on the AVC - as the residual screen grid voltage on this tube decayed the AVC would slowly recover.  
  
    ANY residual signal from the balanced modulator (very very small) was "leaking" thru to the "IF" stage and causing the stage to be desensitized by the AVC.
 
  I tested this theory that it was the 6146's by removing the 800V plate voltage and just keying the transmitter and there was NO PROBLEM with the positive voltage or AVC.
 
  The solution is rather simple now. Add a relay to ground the screens of the 6146's during receive.  I did CAREFULLY wire in a switch to manually simulate this grounding action and the problem was eliminated.
 
  BTW, I also tried another pair of 6146's before trying the no-plate-voltage test. Also tried replacing MANY different tubes.
 
  In many years of doing this it is a new one on me and something I would NEVER expect, but then again I shall never know everything
 
  I suspect that there may be some space charge or residual and very small leakage (micro amps??) that is taking place from the hot plates of the 6146's that is causing this problem.
 
  Maybe this is a design quirk/flaw in the SB-101.  I do not know.
 
  As Sherlock Holmes would say - "After eliminating all the possibilities, whatever is left over, no matter how improbable or impossible, is the answer"  (with appologies to Sir Arthur Cannon Doyle).
 
  Thanks for everyone's input and time !!!!


      

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