From: "Dr. Barry L. Ornitz" <ornitz@usa.net>
Newsgroups: rec.antiques.radio+phono
Subject: Re: Shelf Life of Resistors
Date: Fri, 19 Jan 2001 01:34:36 -0500

<mdemeo@frontiernet.net> wrote in message
news:3a67ae3e.1578511@news.frontiernet.net...
> A very good friend of mine gave me thousands of resistors
> that he has had for many years when he repaired 2 way
> radio.  A lot are in small file type drawers from
> Motorola, NOS of every conceivable value.  I would think
> that, unlike capacitors, they should last indefinitely.

Metal film resistors are quite stable with age, carbon film
resistors age slightly, but carbon composition resistors
tend to go up in value as others have noted.  This is due to
how they are constructed.  They are a mixture of graphite
and clay that is compressed into a rod and then end caps are
attached.  [Some manufacturers molded the composition around
the leads, however.]  With the gradual ingress of moisture,
the mixture swells slightly, reducing the contact between
the graphite grains and increasing the resistance.  The
effect is permanent.

        Dr. Barry L. Ornitz     WA4VZQ     ornitz@usa.net

From: "Dr. Barry L. Ornitz" <ornitz@usa.net>
Newsgroups: rec.antiques.radio+phono
Subject: Resistors Shorting, was Re: Shelf Life of Resistors
Date: Sat, 20 Jan 2001 00:45:05 -0500

"Dick Carroll" <dixie@townsqr.com> wrote in message
news:94b37i$aut$0@208.206.142.99...
>   I once repaired a old GE Master Pro VHF base station which had blown
> the fuse in the plate HV supply where the fault turned out to be a 100K
> ohm, 2 watt carbon composition bleeder resistor across the filter cap
> that had changed value to 30 ohms. Yes, 30 ohms. Quite the exception to
> the norm, as you have observed. Most of this change surely took place in
> a short time since the transmitter was operating up until the fuse blew.
>   Do you know how this drastic change, in the reverse to the norm, might
> have taken place? I know I double and triple checked both my meter
> setting and the resistor before believing it, and replacing the resistor
> cleared the fault.

This one is very simple.  It overheated and charred the organic binder.
This produces conductive carbon lowering the resistance.  Eventually these
usually overheat so badly that the phenolic encapsulation even chars and
becomes conductive.

Resistors have always had a maximum voltage rating - independent of the
power rating.
It was not uncommon for older equipment to abuse these ratings.  Today most
resistors have an even lower maximum voltage rating (250 volts to 350
volts).  Heath learned an expensive lesson in their color TV kits when some
rookie engineer used single high-megohm resistors in the focus divider.
While well within its power rating, the resistor had several kilovolts
across it.  The resistor would blow taking the CRT with it.  I had a friend
who went through three CRT's at Heath's expense before someone understood
why the failures were happening.

It is very possible the original 100K resistor may have been running close
to its rated dissipation and maximum voltage rating.

        73,  Barry     WA4VZQ     ornitz@usa.net