[Boatanchors] carbon comp resistors failure mode?

[D C *Mac* Macdonald]

I'd hazard a guess that a "leaky" filter capacitor
in the bias power supply circuitry could do that.
The increased AC ripple that results will cause
an overall reduction in the effective DC output.
 
73 - Mac, K2GKK/5
OKlahoma City, OK


* In my many years, I have come to the conclusion that one useless *
* man is a shame, two is a law firm and three or more is Congress. *

* -- John Adams (1735- 1826) *


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> From: w2hx at w2hx.com
> To: boatanchors at mailman.qth.net
> Date: Wed, 15 Apr 2009 22:27:45 -0400
> Subject: [Boatanchors] carbon comp resistors failure mode?
>
> Hello all,
>
> I am trying to troubleshoot some odd behavior in a bias supply/keying
> circuit of an amp. I am trying to figure out why the bias supply slowly
> becomes less negative as the system warms up when it should not. This is
> causing the finals to begin conducting when they should be in cutoff even
> though the system isn't keyed!
>
> It is quite a complicated little system, but I was thinking that one
> possible cause of this could be a bad resistor. There is a 2W carbon
> composition resistor in such a place in the circuit as to be suspect.
>
> But the question is, is it a common failure mode for a carbon comp to LOWER
> in value as it warms up? Has anyone seen that behavior? Or should I bark up
> another tree?
>
> Thanks
> Eugene w2hx