120 Watt home brew FM transmitter repair

Today’s little task was to repair a 120 Watt FM 88-108 Mhz test transmitter. Home brew box using some odd modules and little buck power supplies etc. The owner of the radio stations uses this to check location and coverage when looking to establish a repeater site. This box was left on a premises, where the owner decided to put it in a cardboard box with no ventilation. She didn’t like to look of it. Think the feeling was mutual. Given that the ambient temperature is around 35-40 dungarees at the moment, things were not going to turn out well.

The output stage has failed with the main MOSFET MRF101AN going all unhappy and bitter and twisted. Also have a fault with the fan driver circuit which would not have helped matters. All sad parts repaired, we had a nice little transmitter up and running. Set it up at about 90% of it’s rated 120 Watts output. Given the final stage has no final filtering as such, it was very clean as far as harmonics go. Was also quite surprised that, from a power point of view, it was more that 50% efficient. Test for a few hours, until the workshop was wafting with a smell of hot transformer oil from the dummy load.


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