[Amps] AM Downward Modulation

[Joe Isabella]

Make sure you turn the carrier down to no more than 25W (less, if you can -- I used to run 20W carrier max on my 100W rig).  If you run more than that and 100% modulate, the PA doesn't have enough headroom for your peaks.  You'll distort, splatter, or both.  Running AM on modern gear can be done quite successfully, but you do have to be careful.  I suggest you use an O-Scope and watch the envelope.  Power meters (even so-called "Peak Reading" types) can be deceiving while running AM.  A scope trace showing your carrier and nice clean modulation (with no flat-topping or overlapping at the zero point) is the best tool to set up your rig for AM.  Borrow one if you have to.
   
  Tip 1: On the Kenwood, leave the power adjustment at max, and use the carrier adjustment to lower it to <25W.
   
  Tip 2: Try to adjust your scope so your carrier covers one square above and one below the zero line.  When you modulate , 100% is one additional square above the carrier in each direction (2 squares up, two down).  You may not get 100%, but get as close as possible with out crossing zero or flat-topping (your negative modulation should just reach the zero line, but no more).
   
  Joe, N3JI
  

harold b mandel <ka1xo at juno.com> wrote:
  In my screen-modulated days when the tetrodes in the exciter
were not neutralized, and not tuned correctly, and when
the screen supply was unstable my exciter would exhibit
downward modulation, whereby the anode current would
deflect downward on voice peaks from the quiet carrier
current.

What does it mean when a Kenwood TS-450s/AT
downward modulates in AM?

The RF Power actually decreases in both average and peak 
settings.

The waveform fills with speech, but the envelope shrinks.

Anyone ever see this or know if it is normal?

All other modes transmit normally.

Thanks,

Hal
W4HBM
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