[AMRadio] AMRadio Digest, Vol 73, Issue 11


JAMES HANLON knjhanlon at msn.com
Fri Feb 12 12:35:48 EST 2010



  Jack Schrader wrote:
  > I hope you remember me, Jack with the ht20.
  >
  > Got the ht20 hooked up and have god output in cw but not am. I hooked up a 
  > 100w 210v test light to the antenna output on the ht20 but when I speak into 
  > the mic on am the light gets dimmer not brighter. Any idea about that?
  >
  > Tnx    Jack 
  >
  >   
  Possibilities:

  1) audio out-of-phase
  2) insufficient drive to grid of final
  3) final not properly loaded
  4) insufficient capacitance in filter of HV supply to final/modulators
  5) improper impedance match in mod tranny

  If after checking the obvious and it still occurs, just put it on the 
  air. I have one rig that does this but still sounds quite acceptable. 
  You can look deeper into it later, but have some fun with it in the mean 
  time...

  Does the modulation envelope look OK on the scope?

  73,
  -Larry/NE1S


  Jack,

  First off, I have an HT-20 on the air and it does well on AM, so the problem is not generic to the transmitter design.  I have not used a light bulb for a dummy antenna, at least not recently, but my average-reading watt meter does not bobble during modulation.  I would expect a light bulb to get brighter during modulation, not dimmer, which is something you also know.

  From the list of things that Larry suggested, above, I would not think that #1 would not be a cause of this particular problem.  Human voice audio is not symmetric, so you can achieve more full modulation by selecting the better phasing connection, but neither phasing connection should cause "downward modulation."  #5 is also not likely to be the problem if the rig has the original modulation transformer.  The other suggestions are all likely causes.  Another one might be a very flat (low gain, low emission) 4D32 final amplifier tube.  

  Let me know how much final grid drive you are running and what the loaded plate current is for the final amp.  The manual recommends 260 ma plate current for CW and 320 ma for phone, but I don't run any more than 200 ma for either phone or CW.  4D32's are too expensive!  It also recommends 10 ma for grid current.  The ARRL Handbook ratings for a 4D32 are a maximum of 240 ma for CW and 220 ma for phone, with 12 ma of grid current for the CW condition and 10 ma for phone.  I can compare your operating parameters to the ones I'm running currently on my rig.  Mine is tuned up on 14286 right now in anticipation of the Classic Exchange AM event on Sunday.  

  Good luck,

  Jim Hanlon, W8KGI


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