General
The main board handles T/R switching, RF gain, low-pass filtering, FWD/REV power sensing and current sensing. The board is wired to the control module and to the RF amplifier module with flat cables. There is also a pin header connector (J3) for the F-sense module which is used in PA100-D digital model.

Transmit/Receive control
There are two methods for T/R switching (PTT control). In case of JUMA exciters the T/R switching is fed via the RF coaxial cable as DC current from exciter to the PA100. Thus no extra PTT cable is needed with JUMA exciters. In case of other exciters a PTT cable is needed between an exciter and PA100. It is a grounding signal (open collector, open drain or relay signal) to the tip of the KEY IN/OUT jack socket J2. The required sink current is less than 2 mA. TR1 handles this KEY IN detections. The final T/R switching signal (KEY => +TX) is routed via the control board which controls the T/R relay RL1 by means of the MOSFET TR3. The exciter RF is passed by directly to the ANT connector in STBY state. During TX state the RF is fed to the input of the PA100 amplifier and the amplifier output is connected to the ANT connector.

Input attenuator
The relays RL2 and RL3 and the related resistors form a RF input attenuator. This allows operation with various exciter power levels (3W...10W). There are four selectable gain levels in 2 dB steps.

Low-pass filters
There are 6 low-pass filter blocks on the board which will attenuate the harmonics. The filters are switched with the relays R4...R9 by means of the relay driver IC2. Each lower band (1.8/3.5/7/10 MHz) is using an own low-pass filter and the upper bands have one filter block for 14/18 MHz and another for 21/24/28 MHz. The RF coils are constructed using iron powder toroidal cores. See winding instructions.

Current sense
IC1 is a high side current sensing circuit. The circuit is sensing the differential voltage across the 0.005 ohm shunt resistor R7 in the RF amplifier module. The circuit provides current dependent voltage of 0.1V/A for the control board. The current sensing circuit also includes a latching type over current detector. The over current level (default 24 A) is set with the resistors R19 and R20. The current signal value and over current state is handled by the control board.

Forward and reverse power sensing (SWR bridge)
The toroidal transformers T1 and T2 are measuring RF voltage and RF current. The transformers are combined for relative forward and relative reverse outputs. These signals are rectified with the diodes D11 and D12 and fed to the control module.

There are no adjustments in the main board.