This amplifier was used as a FM brodacst transmitter, but was taken out of service. It is build from 4 similar pushpull stages each delivering 250W. Two puspull stages are combined on the PCB and 2×500 w is delivered to the two antenna connectors, so to get 1KW an external combiner is needed.

2x500w seen from cooling finn side
Close look at one pushpull stage

There is no schematic available but i found one which is very close, using slightly different components

The amplifier was delivered with a matching power suppy. But it needs some modification before it can be used. First the 3 pin mains connector must be removed. I soldered the mains cable direcly to the supply. The output from the supply is going via PCB terminals to the RF amplifier, these terminals was probably connected internal in a rack mounted setup, but as the rack was not supplied the power supply and RF board was connected with thick wires soldere directly to the PCB terminals:

The thin red wire is an enable input (PTT) , it is at 12V not connected. put it to gnd/minus for transmit, current is appx 0.5mA. The the LED is power on indicator.

Then i was ready to test one half of the amplifier (500W) in a dummy load . For testing a used my IC-7300 , slowy increasing input power i had more than 400w output power, even if the amplifier was designed for the FM brodcast band , it works fine on the 70Mhz band .

Next step is to get it to work in class A or AB so it is linear and can be used with SSB. When you examin the schematic it is quite easy to add an external bias circuit, by just removing the resistors from base to ground, or supplying bias from a low impedance source. But the transistors are silicon bipolar transistors, and they or not designed to work in linear mode, so risk off termal run away is high. In any case you must design a seperate bias circuit for each transistor which monitors device temperature and compensate for temperature rise. I have designed a bias circuit , but have never completed testing.

The PA itself can be used for CW or FM if we sometimes get permission to run high power on 4m.

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