Building Old Kits??

[Scott Johnson]

Not to belabor this thread, but here are my observations:
1.  Carbon composition resistors change value primarily due to absorbtion of
moisture.  New units that have not been through heating/cooling cycles
typically do not exhibit this phenomena unless they were stored in very damp
conditions.  Value changes seem to be most profound in resistors that are
run near their dissipation ratings.  Heath used primarily high quality Dale
or Allen Bradley resistors, and I have rarely had one go out of spec.  I did
say rarely.  Early carbon comps are much more prone to value changes that
those made in the last forty years, and the established reliability mil
units with the fifth yellow band are even more reliable.

2.  non polar caps fail primarily due to dielectric breakdown, and again
this is moisture (as well as time X applied voltage) related, with the
exception of the Back beauties, which mostly fail due to a chemical
incompatibility issue with the encapsulation (although they could just as
easily be fine, however, just like a cat, I just don't trust 'em)  I have
also found that the red domino style mica caps will develop high resistance
leakage which can cause fits in grid circuits!

3. Electrolyics can be reformed,but I have had limited luck with low voltage
electrolytics,and on top of that,low voltage electrolytics will exhibit an
increasing ESR without necessarily changing value.  This is a problem in
bulk filtering and high current AC coupling,but not necessarily in timing
applications

I would concur with testing the parts before use,but if we were betting on
the most common NOS parts to be problematic, my money would be on any low
voltage electrolytics.

Scott
----- Original Message -----
From: "pjhend at ameritech.net" <pjhend at AMERITECH.NET>
To: <BOATANCHORS at LISTSERV.TEMPE.GOV>
Sent: Thursday, May 29, 2003 7:35 AM
Subject: Re: Building Old Kits??


Thanks to everyone who reponded to my question! It seems there is
widespread agreement that following should be done:

*  Replace Electrolytics or reform them w/ a variac
*  Test all resisters and paper caps for possible changes in value
*  Possibly replace tubular caps as well
*  Thouroughly clean all component leads of any oxidation for better solder
connections

Thanks again!
73
Paul
W9BBR







Hi Folks,

I'm getting ready build a few old unassembled heathkit's and other kits
from the 1960's and 1970's. I was wondering if I have to worry about the
original components degrading with time, or, should they still be ok if
never used in a circuit? Particularly, the capacitors and resistors.
Anything to worry about? Thanks for the help!
73
Paul
W9BBR




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