SB-300 sensitivity

[Michael Waldrop]

Hello everyone,
   
  Recently I aquired a nice SB-300 receiver to match one of my SB-400
  transmitters. The receiver is in excellent physical condition although I
  did have to replace all of the electrolytics, no big deal that's expected
  for a receiver as old as the SB-300 is. However, I did notice the sensitivity
  of the SB-300 is rather low. Using an dual trace O'Scope I looked at the 
  signal output of the first mixer stage, the input to the sideband crystal
  filter, and the output of the crystal filter that feeds the first IF amp. The
  display showed a strong signal at the filter input. However, at the filter's
  output the signal was not near as strong as the input of the filter.
   
  Has anyone else compared the input and output signal levels on their
  SB-300? It appears there is allot of signal loss in the filter which, in
  my opinion, would make the receiver less sensitive. The input
  to the first IF amp's grid has a no signal grid voltage of about
  -.2 volts DC. With a weak signal the first IF's grid voltage increases to
  about -.5 and higher depending on the signal strength, normal AGC action.
  Both CR1 and CR2 are good, they don't appear to be failing or leaking.
  I have also compared the signal strength on the SB-300 to my SB-303 and
  there's approximately 4 to 6 db difference between the two receivers, the
  SB-300 is less sensitive than the SB-303. I have a Heathkit Mohawk RX-1
  that's allot more sensitive than the SB-300. I changed the input capacitor on
  the first IF amplifier from .001ufd to .01ufd and that did improve the receivers
  overall sensitivity but not by much. All rotory switch contacts have been
  cleaned and tubes tested and, when needed, replaced with known good
  tubes. Any comments and/or suggestions?
   
   
  73's,
   
  Mike
  W5RKL
   

		
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