Elecraft KPA3 Manual - Manual (Page 21)

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Circuit Description
The KPA3 option consists of three subassemblies: the KPA3 power amplifier, a fan panel to provide cooling,
and the KPAIO3 interface module.
KPAIO3
The KPAIO3 acts as an interface between the RF board and the KPA3 power amplifier. It includes a relay that
allows the KPA3 to be bypassed when it is not needed during low power operation. The cooling fans are also
controlled by circuitry on the KPAIO3. There are four fan speeds, plus OFF, selected automatically based on
measurement of the KPA3 heat sink temperature.
KPA3 Fan Panel
The KPA3 Fan Panel replaces the blank panel equipped with the K3/10. Two fans are mounted on this panel,
along with a circuit breaker to protect the KPA3 from accidental short circuits. The fan panel is connected to the
KPAIO3 fan connectors and to the heavy 12-volt connections of the KPA3 module.
KPA3 Power Amplifier
The KPA3 Power Amplifier consists of two RF power transistors operating as a broadband push-pull amplifier;
a PIN diode TR switch; and associated bias and control circuitry. The KPA3 provides the necessary gain to
increase the power output of the K3 from 10 watts to 100 watts from 1.8 MHz and 54 MHz. A heat sink is
provided that, together with the cooling fans, provides the necessary cooling to maintain the operating
temperature of the output transistors at a safe level.
The KPA3 is mounted in the K3 with the circuit side of the board down so that only the heat sink is visible after
removing the top cover. Each KPA3 is fully tested at the factory and there are no end user adjustments needed
during installation.