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There were several different versions of the 1296 board that Jim produced. I spoke with Jim on my board a while back when I was building mine and revised the later rev boards to have two trimmers on each input vs one fixed value and one trimmer. Depending on which rev board you may obtain success by replacing the fixed caps with another trimmer of around same value on the input side of the amp. I was having the same challenge with my board, not reaching the rated RF output of 160 to 180 Watts with 10W drive . Mike (K7MDL) and I tried various values on the input with no success. Based on my ongoing discussion with Jim for my Rev board (Rev 7.3) from 2012, I incorporated those changes and after my amplifier was producing and easy 170 to 180 watts with 9 to 11 watts drive.
If you would like to talk about this off line or I can share anything else of my experience I had with mine getting it finally working, I be happy to share.
Thanks for the info Paul. I am not crazy about replacing input caps with
trimmers but I certainly could try replacing them with different values
of fixed caps. Using trimmers adds more inductance and it seems to me
will make them trickier to adjust having 2 variable caps per stage which will interact with
each other. I don't see any of his boards with extra trimmers - i
looked through multiple versions.
Your point #2 is very interesting. I suspect your board is
different than mine which uses branchline couplers on input and output -
not sure where you are referring to as the output coax on mine is
connected directly to the branchline coupler port. I don't see any
breaks or bridges. I have attached a photo of mine. My board is rev
Here is a 7.1 version board. Only 1 cap on the input.
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Correction, only 1 fixed cap on the input, the other is a trimmer to ground. I do not know the values.
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