[Elecraft] KPA100 problems

[Tom Hammond]

Carl:

>Yes, it certainly would be nice to kill this [output power fluctuation] bug.

Subject: K2 Application Note: OPTIONAL power control mod
Date:    Thu, 5 Aug 1999 12:18:50 -0700
From:    Wayne Burdick <n6kr@elecraft.com>

THIS APPLIES TO ALL K2s
(esp. those with amps on the other end)

BACKGROUND:

When pressing TUNE, the power displayed typically varies
over a range of about +/- 0.5 W at the 5 W level. The actual
variance depends on power level, band, load, and supply
voltage. The variance is generally wider on 40 and 30
meters.

There are two reasons for the power not staying at exactly
the level you specify. First, some heating of the final
transistors occurs that may cause power to drift upwards a
small amount during TUNE; this is normal. Second, power is
controlled by an 8-bit D-to-A converter, which has limited
resolution. In other words, each DAC step may translate to
as much as a few tenths of a watt on some bands at 5W.

This small variance in TUNE power level is not significant
in terms of actual radiated signal. The difference between
4.5 W and 5 W, for example, is less than 0.5 dB. However, it
may be possible on a particular K2 to reduce the variance
with a simple component change.


WHAT TO DO:

NOTE: This change is completely OPTIONAL! It may result in a
       reduction of the maximum power that you can put out on
       the highest bands, especially under high-SWR
       conditions.

1. Test TUNE at 5 W working into a 50-ohm dummy load on 40m.
    If pressing TUNE repeatedly results in a power output
    reasonably close to 5 W each time, there's no need to do
    anything.

2. If you have a Field-test K2 (S/N 1-100), the next step is
    to replace R99 on the RF board with 270 ohms (if you
    haven't already done so). This resistor was 470 ohms on
    the field-test units.

3. Temporarily insert a 1K potentiometer at R98 (use leads
    of 1" or less). Set the pot to mid-range (500 ohms).

4. Switch to 10 meters and set power for 10 W (or your
    desired maximum power output on 10 meters).

5. Your goal is to find the largest value of R98 (up to 1 K)
    that allows for the desired maximum power output on 10
    meters using TUNE. Try various R98 settings until you
    find this value.

6. Once you've found the highest usable value for R98,
    switch back to 40 m and see if this has improved power
    setting resolution using TUNE at 2 W, 5 W, etc.

7. Replace the pot at R98 with a fixed resistor. Use a
    standard resistance value that is equal to or lower than
    your final pot value.