Recurring Transmitter Failure - Disconnected Transcript

Recurring Transmitter Failure - Disconnected Transcript
A radio installation on a hilltop suddenly stopped working reliably. What was causing the transmitter to periodically fail?
Mysteries of Science


A large radio installation on a hilltop was used to dispatch trucks, equipment and personnel for an industrial site. The repeater was powered by batteries that were charged using solar panels.

The transceiver's final amplifier which boosts the power of the radio signal was replaced the previous year. Suddenly factory communications came to a halt. The receiver worked fine but there was zero power going to the antennae.

A few minutes later the transmitter started working again. After a few transmissions the output again dropped to zero. Five minutes later the transmitter worked again, but only for about thirty seconds.

What was causing the transmitter to periodically fail? Why would the transmitter suddenly start working but only for a short time?  

Here's the rest of the story.

Maybe it was Murphy's Law, or perhaps the higher voltage from the truck's alternator had blown the fuse. A new fuse did the trick and this time the unit kept running.

So while the symptoms were very similar, the causes where quite different.


A customer with an IMTS mobile vehicular phone (a primitive forerunner of cellular phones) complained that the unit would go dead after about 30 seconds of transmitting, then would come back on in ten seconds or so.

I traced the problem to the main power connection, where a previous technician had replaced the recommend 7A fast-blow fuse with a 5A slow-blow. The transmitter current was evidently just enough to cause the slow-blow element to separate, and then to "heal" when it cooled down.

It took some time to convince the customer that the root cause of the problem was an intermittent fuse.

Michael Moran, CET
Sounds like overheat in the PA stage. Someone most likely forgot to attach a fan connector or a cooling plate.
Robert Oppenheimer, LEP

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