[Amps] Output Coupling Capacitors: Failure Mode?

[EP Swynar]

Hi Again All,

Please excuse the confusing terminology --- when I said "buffer" capacitor,
I meant "PADDER" capacitor...

Sorry about that...!

~73~ Eddy VE3CUI - VE3XZ


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----- Original Message ----- 
From: "EP Swynar" <gswynar at durham.net>
To: <amps at contesting.com>
Sent: Wednesday, October 31, 2007 1:02 PM
Subject: [Amps] Output Coupling Capacitors: Failure Mode?


> Good Afternoon All,
>
> A few weeks ago there was a thread meandering its way through the
reflector re. the failure mode of the output coupling capacitor in linear
amplifiers --- I wish that I'd paid better attention, as I suspect my
coupling cap here is entering the proverbial "...zone of no return" here...
>
> Specifically, when I make a long transmission on 160-meters, the RF output
from my 2x813 kilowatt starts to creep downward. To-day I took the top cover
off the amplifier, and fired-up into a Heath Cantenna. The output dropped
RAPIDLY as I keyed the rig on & off, ending-up at just over 100-watts in a
few minutes!
>
> Re-peaking the plate tuning capacitor helps some --- adding more "C" at
the plate loading capacitor helps some, too...but it's all short-lived. In
no time at all, RF drops again.
>
> I've felt the door knob output coupling capacitor for excessive heat (with
the B+ off, of course!), but it feels normal. I've substituted the plate
tuning buffer capacitor with another similar fixed vacuum unit, with no
difference. Ditto the buffer cap for the loading side.
>
> Interestingly enough, the rig puts out a healthy, steady, 600-plus watts
on other bands --- the problem seems limited to 160 (my favourite one, of
course!).
>
> Can output coupling caps go south like this, but on a frequency /
band-specific basis...? My next step is to replace mine with a 10KV glass
capacitor (0.002-ufd.), but it's gonna be tricky getting that deep inside
the bowels of this thing!
>
> Any & all thoughts appreciated, with thanks in advance...
>
> ~73!~ Eddy VE3CUI - VE3XZ
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