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

Mon Jul 28 22:52:03 EDT 2014

I was told by a physicist that when high value metal film resistors are exposed to high voltage the electrostatic forces redeposit the conductive material and raise the resistance.  I wonder if that could be what happens in high value carbon composition resistors?

Gary WA0SPM

Sent from my iPhone

> On Jul 28, 2014, at 5:48 PM, "George Georgevits" <georgg at bigpond.net.au> wrote:
> 
> Just to add my $0.02 worth to this interesting discussion, I seem to recall
> that low value carbon composition resistors failed by drifting out of
> tolerance on the low side.
> 
> High value ones e.g. typically 100K or higher failed by drifting out of
> tolerance on the high side. And to my knowledge, there is no way of bringing
> these back.
> 
> Can anyone explain this??
> 
> Cheers,
> George Georgevits
> VK2KGG
> 
> -----Original Message-----
> From: Premium-Rx [mailto:premium-rx-bounces at mailman.qth.net]On Behalf Of
> Glenn Little
> Sent: Tuesday, 29 July 2014 9:36 AM
> To: jrusgrove at comcast.net; premium-rx at mailman.qth.net
> Subject: Re: [Premium-Rx] Experiment: How to readjust a drifted carbon
> compresistor
> 
> 
> Does anyone address the reason for the resistors drifting high?
> 
> I have some resistors removed from WWII equipment that are in their
> 5% tolerance.
> 
> I suspect that something changed in the way that the resistors were
> sealed allowing moisture to the clay/carbon composite.
> I also suspect that the resistor value was determined by the amount
> of carbon added to the binder.
> As the moisture got into the clay composite, the binder swelled,
> decreasing the compression of the clay causing the resistance to go up.
> This is my thoughts with nothing concrete to confirm this failure mode.
> 
> Any thoughts?
> 
> Might help to predict the failure of our receivers.
> 
> 73
> Glenn
> WB4UIV
> 
> 
> 
> At 09:16 PM 7/17/2014, jrusgrove at comcast.net wrote:
>> 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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