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

Thu Jul 17 22:45:51 EDT 2014

My memory may be failing me but if I am not recalling incorrectly, there
also were some specific testing conditions required - possibly voltage
related for specific resistance ranges...

 I've been in the electronic components industry for decades.  I even had to
address the "bake before testing" requirements with military equipment
manufacturers rejecting "out of tolerance" resistors who should have known
about it .

Quick 1970s and later history and status of Carbon Composition Resistors:

Allen Bradley exited the market due to lack of demand.

TRW-IRC exited  market because their manufacturing equipment produced in the
early 20th century was no longer maintainable and it didn't make economic
sense to retool it.
Between them, it was the end of Mil Spec carbon comp resistors.

TRW then brokered a commercial composition product manufactured in Europe
(IBT Series - and no 1/8th Watt or 2 Watt products).  The resistive portion
of the company was sold to TT Electronics in 1990 and the IBT products are
still available today.

Stackpole had dropped out of that business earlier but their overall quality
was so poor that most companies wouldn't buy their products.   I presume
they don't have quality issues today as they still exist.

Ohmite added and still has a Carbon Comp offering.
Stackpole added commercial Carbon Comp resistors back into their product
line at some point.
RCD added Carbon Comp resistors to their product line at some point

Offering 1/4W, 1/2W and 1W Carbon Comp resistors today:

TT/IRC, Kamaya, Ohmite, RCD Components (may only be selling 1W products
now), and Stackpole

With world-wide demand very limited, it is likely all are sold under private
label from a single source manufacturing source.

Now I just need to find time to repair my Cubic R-3030A..... And this is the
one problem it won't have     :)

-
Greg

-----Original Message-----
From: Premium-Rx [mailto:premium-rx-bounces at mailman.qth.net] On Behalf Of
jrusgrove at comcast.net
Sent: Thursday, July 17, 2014 6:16 PM
To: premium-rx at mailman.qth.net
Subject: Re: [Premium-Rx] Experiment: How to readjust a drifted carbon
compresistor

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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