Some transmitters must adjust their power at different times of day, to prevent signals travelling too far at night. As this transmitter is apparently producing 4.5 kW, presumably each amplifier is contributing 900W. Each amplifier has its gain controlled by the "power modulation" control, combined with a signal fed back from the antenna (so that if something goes wrong at the antenna, the reflected power is kept under control).In other transmitters there may be signals from different studios carrying different channels, but apparently not here. The first stage of the power amp is a signal splitter (or divider, which probably explains the odd symbol on it!) to feed the same signal to five actual amplifier modules.Again, this changeover is probably automatic and its status is probably shown by the LEDs above. The changeover switch connects one exciter into the power amplifier, and the other one into a resistor called a "dummy load" to absorb the power frrom the other one.The pi network may also provide attenuation between the exciter and the later stages. You may not have got there yet, but radio signals tend to "bounce" off the wrong impedance, and either open or short circuits can reflect signal back into a transmitter, and overheat it. Each exciter feeds a "pi network" of three resistors, into a changeover switch.This isn't unusual transmitters are designed for redundancy if one exciter fails, the other can be started immediately to keep you on air (and this may even be automatic) And the exciters are probably the blue handled boxes below, of which only one is currently in use. An exciter is actually a complete low power transmitter : probably 8 Watts judging by the top display. Two input sockets, labelled "Input exciter 1,2".However it can be used to aid a first level diagnosis of problems, and which circuit board or black box to replace when something fails. It's not a circuit diagram, but a block diagram, which means it can't be used to understand the transmitter at the component level.
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