[Boatanchors] carbon comp resistors failure mode?

[W2HX]

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