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- Category: Tony Tip's
- David Palmer By
Tonys Tips 10
- Linear Amplifier repair
A club member brought a VHF linear amplifier for 2 metres for repair with the fault “no H.T” (this amplifier was a Dressler which uses a 4CX350A valve)Investigation showed that the top cover was interlocked with a small micro switch which had failed. This switch prevents the H.T. From being turned on if the top cover is removed, this being necessary as the H.T. Voltage is approximately 2000 volts.
I obtained and fitted a replacement switch, this got the amp. Working but as soon as I tried to test it (by feeding it with a 2 metre radio) the H.T. arced over in the P.A. compartment and blew the H.T. Fuse. Oh dear I thought, however, fortunately I had some 4CX350A valves so I fitted a replacement. Next question, what am I going to do for a replacement high voltage fuse. The answer came to me when I was in the kitchen making a coffee and looked at the microwave oven.
High voltage fuses are used in microwaves so I got a replacement one and fitted it (they are even made with the same current rating) next thing I must do though is get a high power slug (about 500 or 1000 watts) for my Bird watt meter, that will cover 2 metres.
It should be noted that the HT voltage used in valve linear amplifiers is usually about 2000 volts so is lethal so DO NOT open one of these unless you know what you are doing!Tony g0mqg