Just a few days ago, I had an experience which demonstrated what I had
thought was quite probably true for many years.
The question is, "Why does a carbon composition resistor change values?"
The correct answer is, "Because it is a carbon composition resistor."
In my consulting with a particular long-standing electronics firm, I have
access to a stockroom which has been on site for some 40 years. The site is
closing, and the more valuable inventory is gone, and they are allowing
employees (I've been so frequently at the site that they consider me one of
theirs) to clear out the rest. Amongst the remaining stock are carbon
composition resistors from 1/2 watt to 5 watts. Several vendors are
represented, and the age of the inventory is from one year to over 20 years.
A few of us ham types decided to measure the resistors to see how close to
their original 5%, 10% and 20% tolerances they truly were. The result is
that fewer than 1/5 of them are in tolerance. The vast majority have
drifted higher in value. Some of them known to be less than five years old
now exhibit a value of more than four times what they should be. And all of
them passed through the gaze of QA when they came into the building.
Evidence suggests that none of the resistors we measured have ever been in a
circuit. Most typical were values of 50 - 80% higher than nominal.
Pushing the resistors hard, as is done in the classic suppressors,
inevitably cannot help the situation. I remain convinced, however, that a
carbon composition resistor will change in value over time even if it is not
heated.
Peter, you may have struck on something here. I wonder how many suppressors
have been 'upgraded' when all they needed was replacement of the resistor.
Another item to add to the maintenance checksheet then: Annual furnace
filter cleaning, automobile oil changing, and suppressor resistor
replacement. ;)
FF
Peter wrote:
>
>I have a nasty suspicion that one of the problems with the classic
>suppressor
>is that the carbon comp resistor is pushed pretty hard in terms of
>dissipation,
>especially on the higher bands, by the fundamental current. Add to that
>fact
>that the resistor is in a high temperature environment as well, all of
>which
>isn't good for reliability. The result is that the resistor value goes
>high, and
>a suppressor that worked becomes a suppressor that doesn't work anymore.
>
>73
>
>Peter G3RZP
>
>
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