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Tuesday, 31 March 2020 14:20

150 Watt Power Amplifier for 23cms

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Following my recent amplifier build for QO-100 I decided to scrap the power hungry Spectrian board. It wasn't faulty but I had decided to put the XRF 286S transistors to better use in a 23cm amplifier.

Nervously I fired up my hotplate and raised the temperature to 235 centigrade, the trick is to turn the plate off when you drop the board onto it. If you leave the power on the hotplate it will continue to rise in temperature to a destructive level. Using a pair of ceramic tipped tweezers I removed the transistors very rapidly and put them to one side to cool naturally.

I had heard stories about the XRF 286 being damaged during removal/re-soldering in this way, so I wasn't too surprised when I found one of the three read short circuit gate to source. Luckily the two remaining devices had survived; reading fine when tested with a multimeter. My plan was to use a PCB which I bought from Jim W6PQL, fixing holes were sized in US inches but 3mm screws fitted nicely.

My engineer friend milled some 6mm thick copper for me; he also drilled and tapped all fixing points including those for mounting to the heat-sink. I planned to reuse the aluminium chassis and heat-sink from which I had just removed the Spectrian board. Using a nibbling tool I removed a bit more of the chassis to get the new larger PCB assembly to mate with the heat-sink.

So the assembly sequence is important; once the PCB is screwed to the copper heat soak It's difficult to get enough heat onto the board to solder. Initially the board is fitted to the copper with a few screws, at this point I slid the transistors into position and marked the position for soldering. After soldering the transistors to the copper and checking for any bridges I placed the PCB sections under the drain and gate tabs to check for correct alignment. When satisfied the fit was good I populated the two sections with the rest of the components including power cables, bias feed and coax. Ensuring the bias pots were adjusted for minimum/no bias I screwed the boards onto the copper heat-soak and soldered the gate and drain tabs. These tabs are thin and usually solder to the board quite easily.

Using 6mm copper allowed me to power up the amplifier prior to housing, I was able to check voltages and set the bias without generating much heat. I set the bias at one amp each transistor as this was the setting for the Spectrian amplifier, I'm still pondering whether to experiment further with the setting. Up till now I have run it at over a hundred Watts and a friend monitoring my DATV on a spectrum analyser said it was clean with spectral regrowth 30dB down.

On this occasion I only took these two pictures. You can see a good video showing the hotplate soldering process on YouTube, look for W6PQL.


Read 534 times Last modified on Wednesday, 01 April 2020 15:02
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