HW and SB series carrier out of control

[Michael Waldrop]

Hello everyone,
   
  Recently, while restoring an SB-400, I ran across 
a problem that had me puzzled. The problem is when
tuning or keying the transmitter into a dummy load
either in TUNE or CW modes, the MIC/CW level control
on the front panel had no control over the drive regardless
what position the MIC/CW level control was at. After
reviewing the circuit in the manual the fix was obvious.
   
  Most of the HW and SB series transmitters and transceivers
have the same basic balance modulator circuit. The carrier
null pot, 200 ohms, and the carrier null variable capacitor.
  I noticed when the balance modulator carrier null control
was out of adjustment, the carrier in TUNE or CW modes 
could not be controlled by the front panel MIC/CW level control.
This is caused by the balance modulator null control pot not
  being adjusted properly, allowed carrier to be present at the 
  primary balance modulator transformer and ultimately at
  the input of the final amplifier grids.
   
  The fix for this problem is to clean the carrier null control
pot, I did it 3 times, then null the carrier in sideband mode. Once
that is done, the front panel MIC/CW level control regained control
  of the carrier output like it's designed to do. 
   
  This fix has been working now for a couple of weeks. I'm aware of
  the problems the null contol pot in the balance modulator have
  caused. More than likely I will end up having to replace this pot.
  However, for now, it's working fine.
   
  73's,
  Mike
W5RKL


		
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