[Premium-Rx] Experiment: How to readjust a drifted carbon compresistor

[jrusgrove at comcast.net]

Ran into moisture absorption / value drift about 10 years ago when developing resistively loaded 
collection antennas for a mil project. At the time not very many suppliers were stocking carbon 
comps as most customers had moved on to the newer resistor types. One supplier had NOS resistors in 
the original sleeves / plastic. Needed 40 different resistor values but only a handful of each 
value. They were sold in full sleeves so there were plenty to sort through. Almost all had drifted 
out of tolerance to the high side - some several times the tolerance. A call to the supplier turned 
up MIL-R-39008C. Of particular interest was section 6.9 on page 30:

http://www.w1vd.com/MIL-R-39008Cpage30.pdf

After baking for the allotted time almost all of the resistors were back in tolerance ... most were 
quite close to the indicated value. The antenna circuit boards and resistors were conformal coated 
and only minimal resistance change was noted over the course of the next year - the last time I had 
access to the antennas.

I've seen many high reading carbon comp resistors while restoring tube type receivers. The worst 
offenders seem to be screen-dropping resistors. These dissipate a fair amount of power and may be 
more 'stressed' than resistors elsewhere in the receiver. Leaky 'black beauty' (or similar) bypass 
capacitor from screen to ground add to the resistor dissipation.

Interestingly, new manufacture carbon comps appear quite glossy - perhaps manufacturers are using an 
improved coating to stave off moisture absorption. Would be interesting to get confirmation on this.

Jay W1VD



----- Original Message ----- 
From: "Tisha Hayes" <tisha.hayes at gmail.com>
To: <premium-rx at mailman.qth.net>
Sent: Thursday, July 17, 2014 6:34 PM
Subject: [Premium-Rx] Experiment: How to readjust a drifted carbon compresistor


> Usually what causes carbon comp resistors to deteriorate to where the
> resistance rises is that the carbon granules gradually suck up moisture.
> The heating drives out the moisture so you are seeing the values falling
> closer to where they originally were from manufacturing.
>
> Since the lacquer coating on the resistors is going to be deteriorated or
> cooked off I would suggest re-sealing the resistor with a lacquer (or clear
> fingernail polish) after they have cooled. Notice that the values over the
> next few hours and days begin to creep upwards again on the resistors that
> were cooked. If you can keep the moisture from re-entering the resistor you
> have a better chance of keeping it stable.
>
> Slower, longer duration heat is going to be better than the rapid
> treatment. If you had a constant current supply you could estimate the
> current draw at the desired resistance and let the resistors run warm. You
> want to try to avoid thermal shocks as it is going to embrittle the carbon
> granules and make the resistor mechanically fragile (it will crack and
> break). Simple wattage calculations (E*I=W) will let you calculate the
> thermal wattage.
>
> -- 
> Ms. Tisha Hayes. AA4HA
>
> *""In this denial of the right to participate in government, not merely the
> degradation of woman and the perpetuation of a great injustice happens, but
> the maiming and repudiation of one-half of the moral and intellectual power
> of the government of the world." -- Fredrick Douglass"*
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