[HBR] Using vintage Carbon Comp resistors

[Don]

I have a few questions regarding carbon composition resistors. The caveat
against using carbon comp resistors come up regularly.

I built a receiver in 2004 that now has several thousand hours using all
carbon comp resistors. I measured each resistor before use and discarded any
resistor outside of approx 10% tolerance. I haven't (knock on wood) had
apparent problems with almost continous use for 6 years.

Is the problem a change in the component value over time while in storage or
does the age reduce the reliability in some way even after installation in a
circuit? Will it continue to degrade in situ?

Marvin suggests restoration from heating prior to installing. I assume from
this and past posts that, once installed, a carbon comp resistors chance of
failure is less likely if the resistor is heated and the measured value is
within tolerance. 

I still have thousands of AB carbon comp but for recent projects I have been
using Metal Oxide Xicon 5% 2W for any resistor calling for a 1/2w rating.
The 2W MO resistors are approx the same size as my AB CC 1/2w.

What application in a receivers circuit are carbon comp resistors superior
to other types i.e. noise, stability, non-reactive, etc.?

Don
K5UOS



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Sent: Saturday, March 28, 2009 8:00 AM
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Subject: HBR Digest, Vol 61, Issue 15

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Today's Topics:

   1. Re: HBR-11; lessons learned to date (match at ece.utah.edu)


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Message: 1
Date: Fri, 27 Mar 2009 11:00:04 -0600
From: match at ece.utah.edu
Subject: Re: [HBR] HBR-11; lessons learned to date
To: hbr at mailman.qth.net
Message-ID: <49CCB1B4.24581.5DA0038 at match.ece.utah.edu>
Content-Type: text/plain; charset=US-ASCII

Tom Nickel wrote:

> Speaking of components, in an attempt to preserve the
> vintage look of the 60's I have been using NOS and recycled carbon
> composition resistors.
> 
> I think I read where the carbon comp
> resistors drift up in value over time?? Well, last night, I pulled
> out a 100 Ohm 10% 1/2-watt resistor from my junk box.? After cleaning
> off the gunk, my DMM measured the unit at about 148 Ohms!

Yes they drift upward. Sometimes you can bring them back down by cooking 
them in your wifes oven for an hour or two at 350 when she's off visiting
her 
Mom.

Another point: If you're using carbon comp resistors, even the NOS ones are 
probably old and should be checked and possibly cooked.

Marvin
KA7TPH



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