PCS CyberMax 800 Watt transmitter repair

Today’s mission Jim, should you decide to accept it, is to get the PCS CyberMax 800 watt FM broadcast transmitter up and running again. When this was commissioned, it was left adjusted to 600 Watts output, never like leaving a unit at full power when you are not in full control of the installation. The complaint was no or very little power output.

PCS market some quite good modular systems, enabling you to build systems as you wish. They also sell assembled units as a complete transmitter, ready to go. However, I often find that a degree of tweaking is needed, prior to on site commissioning. This one was a typical case where the pilot stereo modulation was far too high, causing poor audio.  Anyway, upon test, showed that the power adjustment had been wound to maximum. Customer fiddling I think. The PA output transistor was damaged and had to be replaced. These devices are not cheap, well in excess of 150€. PCS are good at selling spares luckily.

The replacement transistor, drop in equivalent id rated at 1500 Watts. So with some careful adjustment of the final stage bias, I can ensure that the maximum rated output of 800 Watts cannot be exceeded.

All reassemble and a test for a few hours showed that all seems to be well. So, off back to the station and a check on the antenna for SWR etc…

When I look back 10 and 20 years ago at similar power rated transmitters, there is no comparison. What one can now get from one dual transistor a couple of inches square is amazing. I have another 1000 Watt transmitter in the shop for repair at present. It consists of a massive PSU with two 500 watt stages combined together and needs 2 of the “Worlds Strongest Men” plus surgical supports to lift it onto a reinforced bench.


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