[Elecraft] KXPA100 Keying Mod

[riese-k3djc at juno.com]

Heathkit SB220 or at least my 221 keys from the bias voltage,,, ??? 130
vo;ts or so
I did a complete install of all the Harbach updates and this makes for a
very stable amp. The heath line of anps are a deal,,, 3 to 500 bux as
opposed to
3500 for newer amps

Bob K3DJC





On Thu, 9 Jun 2016 10:43:58 -0400 David Robertson <kd1na363 at gmail.com>
writes:
> 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
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