Remember that almost every voltage inside this linear amplifier is lethal, including the voltage levels present on this board. Turn the amplifier off, unplug it, and short out power supply rails before you touch anything. Always apply the “one hand in the pocket” principle when working on anything above 24V! Refer to the Safety chapter in the ARRL Handbook for further information on proper procedures to use when working on equipment such as this amplifier.
An earlier version of the board is shown below. In Version 2, per the schematic above, the 10K 5W resistor and relay immediately above were removed.
The shunt regulator design is based on High Voltage Shunt Screen Stabilizer at: http://www.deserthound.com/amateur-radio/ts520-820.html