[Amps] Output Coupling Capacitors: Failure Mode?

[Fern Rivard]

Eddy:

    If you have a padder capacitor that is switched in when going to 160 meters, I would suggest that it's gone bad. Also make sure that you use a mica capacitor of the correct value rather than a ceramic cap as the ceramic caps are known to drift in value when used on 160 meters.
    Fern


----- Original Message ----- 
From: "EP Swynar" <gswynar at durham.net>
To: <amps at contesting.com>
Sent: Wednesday, October 31, 2007 12:02 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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