This content has been archived. It may no longer be relevant
A club member contacted me with a problem with a 2 metre linear amplifier (a Microwave modules 100 watt model) no RF output was the complaint.
After bringing it home and dismantling it in the workshop first thing I noticed was a burnt out resistor.A study of the schematic (kindly loaned by the owner) revealed that this is in the relay switching cicuit for tx/rx changeover.
I replaced it with a 100 ohm, (as the schematic) the relay switching then worked fine when the amp was fed from a 2 metre radio (my FT290) but, as the owner said, no RF output, however I noticed that the amp was drawing about 15 amps from my PSU when keyed, also the tuner/amplifier in my workshop which I have on for backround music produced a loud buzz when the amp was keyed indicating that lots of RF was being produced but this wasn’t getting to the output.
A careful check of the circuitry between the PA transistor and the output relay revealed that a coil in the low pass filter was missing(??)
A look round and i found it wedged under one of the sockets on the rear.
I had a careful look round and also found a 100 ohm resistor and a trimming capacitor which was also from the output circuit, I also noticed a 47nf capacitor soldered to the base of the output transistor, with one lead disconnected and “floating” this told me where the 100 ohm resistor went (Which wasn’t on the schematic!)
I refitted both the coil, the trimmer and the resistor and also had a good look round the pcb for any other components “about to fall off” and resoldered a few suspect joints. The amp then worked fine.
It remains a bit of a mystery why the components had fallen off the board though (constant heating and cooling or poor soldering during manfacture perhaps).
The amp was then passed back to it’s owner.
Tony g0mqg