Elecraft KXPA100 Manual - Manual (Page 20)

20
Connections to an External Amplifier
If you want to drive an external high-powered amplifier with your KXPA100, you must be careful to avoid
exceeding the voltage and current limits of the KXPA100
PA KEY circuit. Figure 6 shows the PA Key
connections to an external amplifier when using the KXPA100 connected to a transceiver with and without a
control cable.
Typically external amplifiers have a voltage on their key input. The amplifier is enabled for transmit when the
key input is grounded. The open-circuit voltage and the current that flows when the key input is grounded must
not exceed the limits the KXPA100 can handle to avoid damaging the KXPA100. When no control cable is
used, this voltage is also applied to the transceiver.
The Elecraft KX3 can accept up to 30 Vdc on the PA Key line and sink up to 100 mA when in transmit.
The KXPA100 PA Key limits depend upon the serial number:
 Up to serial number 700, do not apply more than 5 Vdc.
 Serial number 700 and above, do not apply more than 30 Vdc.
Figure 6. PA Key Connections to an External Amplifier.
Measuring the External Amplifier Key Line Voltage and Current
You can measure the voltage and current with a DMM. Be sure power is applied to the amplifier as it would be
in normal operation. In receive (not transmitting) mode, measure the voltage across the key line to ensure it does
not exceed the above values. Switch your DMM to current to measure the current that flows in transmit mode
(your amplifier will switch to transmit mode when you make this measurement).