SX100 intermittently deaf on 75M

[wa2dtw]

I am using a Hallicrafters SX100 paired with a Johnson Ranger/Thunderbolt combination on 75 meters.  A Johnson electronic TR switch is in between these and the antenna.  The SX100 has been going intermittently deaf, mostly on 75M.   It seems to work on other bands, and sometimes tuning the receiver to the lower end of 80 meters restores the sensitivity very temporarily.   I have tried changing most of the capacitors in the front end of the SX100, many times thinking the problem was fixed, only to have it revert back to deafness the next time I tried to get on 75 with this combo.   Does this ring a bell for anyone?  Could RF be leaking through the TR switch and killing the front end of the SX100?  (the TR switch seems to work, though).
Thanks and 73 and Happy New Year to everyone

Steve WA2DTW

[W7XXX]

My guess is the electronic TR switch, but hook the SX-100 straight to the antenna and let it run for awhile and this should answer that question.

[WQ9E]

Steve,

If it sometimes resumes operation when you tune to the low end of the range it sounds like it could be a problem with marginal operation of the 1'st conversion oscillator circuit.  If it "intermits" fairly often put an external receiver tuned to the 1'st oscillator (dial frequency + 1650 Khz.) and see if the signal drops or disappears in the external receiver when you lose sensitivity.  Most likely cause would be a marginal 6C4 tube (V3) but it could also be one of the fixed capacitors associated with this range or R-35 and/or R-36 has drifted greatly in value causing V-3 to be operating near the edge of oscillation.

Rodger WQ9E

[wa2dtw]

Many thanks.  So far, I have changed the  1st RF tube (6CB6) although it tested OK.  Attached the antenna directly to the receiver.  It seemed to work fine.  Re-attached to the TR switch.. Seems to work fine.
Next, will try listening for the oscillator frequency (thanks, Roger for great idea) and changing V3).
Hopefully we're getting close to the problem at hand.
73
Steve