[Elecraft] KPA100 hot resistor...?

[Don Wilhelm]

Nate,

R13 will likely get hot, and will be uncomfortable to touch, but not so much
as to cause burns.  This resistor is the +12 volt feed to 1/2 of the T/R
switch, and a fair amount of current is required to cause D13 to continue to
be forward biased when presented with full power RF.  Even so, the
dissapation R13 should not be more than 2 watts - even if your supply
voltage is at 14 volts and one end of R13 is at ground level (it actually
should be a diode drop or so above ground).  The resistor is rated at 3
watts which allows a safety margin of at least 1.5.

If you feel it is getting a lot hotter than it should, you might want to
remove one end and measure its value (100 ohms).  A lower value would cause
it to heat more as would an excessively high supply voltage.  The KPA100 T/R
circuit is not active until the power is moved to above 11 watts, so it
should not heat at all at low power settings.

73,
Don W3FPR

----- Original Message ----- 
From: "Michael D. Heit KD7YLA" <mheit@asisna.com>

| My first thoughts is that it may be normal.

---------------------------
| Nate Lewis wrote:
|
| >I was just doing the various last calibration steps to a new KPA100 when
I
| >noticed that R13 was uncomfortably hot.  This is the 3W resistor right in
| >front of the fan.  When I move the K2 power from low power to 11w, it
takes
| >about 30 seconds for R13 to go from room temperature to painful to touch.
| >Is this typical, or have I screwed up somewhere?  This is after doing
bias
| >adjustment, though I first noticed it during fan testing.
| >