[Elecraft] KXPA100 Keying Mod

[David Robertson]

Everyone.

I had an interesting experience recently. I have a KX3 and KXPA100 ( #519 )
operating in my workshop in my basement. The rig drives my Heathkit SB220
which has the Harbach soft key mod which has less then 1 volt keying
voltage. This allows me to run up to 1000 watts no problem.

A couple of weeks ago a friend brought me his home brew linear to test.
Before connecting his linear to my KPA100 output I checked the schematic
and measured the keying voltage on his linear which was 24 volts dc at less
then 100 ma. When i connected the key line from his linear to the RCA jack
on the KXPA100 interface cable the linear went to key down without any
transmit command from the KX3. I then disconnected the KXPA100 from my KX3
and using the Elecraft keying interface cable to the linear directly from
the KX3, the linear keyed up upon transmit commands from the KX3 normally.
Well I finished testing my friends linear with 10 watts drive and
everything worked fine despite reduced output power.

Later I emailed Elecraft Support and asked them about the KXPA100 keying
problem.
I quickly got a response and a solution to the problem.

It seems that the first manufactured KXPA100 linears below serial number
700 were designed to operate the KPA500 linear which has only a 5 volt
keying voltage. Evidently,There was some problems with keying linear
amplifiers with higher keying voltages so after KXPA100 serial number 700
and above Elecraft modified the keying circuit to accommodate linear amps
with keying voltages up to 30 volts.

I either missed the memo or Elecraft didn't contact those of us about this
problem. Elecraft now has a mod kit that solved the keying problem. The kit
part number is KXPAMDKT2 and is free. It consists of a 4.7K 1/4 watt
ressistor (E500158) and a diode BV19WSTPMS diode (E560076). The diode is
surface mount and the resistor is a leaded resistor. The  instructions are
clear but some may have problems in removing a surface mount resistor (R28)
and installing the diode in its place. The resistor in the kit solders to
existing components.

The reason I am putting out this information is because I didn't know of
the limitations of the KXPA 100's keying input circuit and could have
easily blown that circuit out. Thanks for the bandwidth.
73

Dave Robertson KD1NA