[Amps] Need a Capacitor- help

[Jeffrey Madore]

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 - ><>