[Boatanchors] carbon comp resistors failure mode?

[W2HX]

Thanks Mac. As a matter of fact, there is ONE and only one filter cap in
this bias supply. And as soon as I locate it, I will replace it just for
good measure. It is a 47uF 200V filter cap. Because even though this Zener
is bad, it doesn't fully explain the behavior. A bad cap as you point out,
could better explain it. I wonder if both are bad.

Thanks for the suggestion.

Eugene w2hx

-----Original Message-----
From: D C *Mac* Macdonald [mailto:k2gkk at hotmail.com] 
Sent: Wednesday, April 15, 2009 11:20 PM
To: Eugene Hertz; boatanchors at mailman.qth.net
Subject: RE: [Boatanchors] carbon comp resistors failure mode?


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) *


----------------------------------------
> 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