I have a 100w, 144Mhz solid state PA board that I am repairing. I back-burnered
it for a while but really want to get it completed. I only need one component;
an smt capacitor that is in the "antenna load" position of the pi network.
The original was less than 1/8" square. I was able to obtain a 47pf cap which
worked, but was about 10pf to high. Unfortunately, after soldering and
unsoldering it a couple times I killed it.
I tried a 500v rated silver mica cap with the leads cut very short and it
worked, but heated up quickly.
Then I tried your recommendation, Steve, and shaved the silver mica cap down to
a chip. Yes, I ruined the first one but the second one came out fairly decent.
Great idea! After shaving it down, I tested it and all seemed ok. Though it
was physically much larger, I was able to solder it in place. The circuit would
then not resonate at all (there is a 15pf trimmer in parallel with the cap) and
showed no sign of coming near resonance as I tweaked. I'm not sure whether the
end bands of the cap increased the L or the physical size caused additional
capacitive coupling to ground. I did try standing it up on the board and there
was no change. The tank circuit L consists of a U shape piece of 1/4" flat
stock roughly 3/4" high and 1 1/2" long. Just looking at the physical make-up
of the inductor tells me that the inductance is very small and the additional
inductance might have a significant effect on the overall L.
I then tried a small 27pf ceramic 1Kv rated cap and obtained full output with
the trimmer plates fully meshed. The capacitor however, showed signs of heating.
So it appears that I need a very high Q, small smt chip capacitor rated at 500v
to 1000v, and I probably should get a couple different values so that I can
select the right one. I would guess that something around 37pf would give me
what I need.
Can anyone help me obtain these? All sources that I have checked sell smt caps
with much lower voltage ratings. Sources that had what I need required that I
buy a significant quantity. I really don't know what to do and would appreciate
any suggestions.
Thank you,
Jeff - K1LE - CT - ><>
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