[Boatanchors] Carbon Comp Resistors: The Darkness Gathers?

[Glenn Little WB4UIV]

A carbon composition resistor is mage of a composite of a binding agent 
and carbon.
The carbon granules size determines the range of resistance
The larger granules for the lower value resistance and the smaller 
granules for the higher resistance. (I may have this backwards).
The compression of the carbon in the binding agent determines the actual 
resistance.
When the binding agent absorbs moisture, it expands and decreases the 
distance between the carbon granules.
This causes the resistance to go up.
Baking will drive out the moisture, through the same crack in the 
housing that the moisture came in through.
The crack may be at a seem or where the wire leads leave the resistor body.
If the moisture got in once, unless you do something to reseal the 
resistor, the moisture will get in again.
To me, baking the resistor to dry it out is a fool's mission, unless 
positive steps are taken to prevent the moisture from reentering.
It took a long time for the moisture to migrate into the resistor.
When you bake it, you are driving the moisture out at a much faster rate.
This faster exit rate probably increases the size if the crack causing 
the resistor to absorb moisture at a higher rate when cooled.

This is my take of many experiments to restore older carbon composition 
resistors.
I have seen resistors, possibly made by Ohmite, that did not change value.
The advantage of carbon composition resistors is that they can handle 
surges much better than film resistors.
Film resistors do not change with age in the same manner as carbon 
composition resistors do, they have a much tighter tolerance and they 
are thermally quieter.

YMMV
73
Glenn
WB4UIV

On 7/2/2018 6:45 PM, Francesco Ledda wrote:
> You are spot on! A night cooking in the oven at 200F will do the trick!
>
> -----Original Message-----
> From: boatanchors-bounces at mailman.qth.net
> [mailto:boatanchors-bounces at mailman.qth.net] On Behalf Of Marvin Match
> Sent: Monday, July 2, 2018 5:43 PM
> To: boatanchors at mailman.qth.net
> Subject: Re: [Boatanchors] Carbon Comp Resistors: The Darkness Gathers?
>
>   From memory, which is not always reliable at my age:
>
> I seem to remember that carbon comps are hygoscopic, meaning they absorb
> moisture. If you have a stock of NOS carbon comps that have drifted
> high, try baking them in the oven. 300 for a couple hours ought to do it.
>
> Marvin KA7TPH
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-- 
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Glenn Little                ARRL Technical Specialist   QCWA  LM 28417
Amateur Callsign:  WB4UIV            wb4uiv at arrl.net    AMSAT LM 2178
QTH:  Goose Creek, SC USA (EM92xx)  USSVI LM   NRA LM   SBE ARRL TAPR
"It is not the class of license that the Amateur holds but the class
of the Amateur that holds the license"