25 Jul
2012
25 Jul
'12
5:24 p.m.
On 25 Jul 2012 at 18:36, Tothwolf wrote:
With those sockets, you have gold contacts against tin
plated IC
leads. This can cause all sorts of problems where the gold isn't hard
enough to break through the tin oxide. New tin oxide will also begin
to transfer to the surface of the gold contacts. We saw this a lot
back with 30-pin and 72-pin SIMMs when people would put modules with
gold-plated contacts in sockets with tin contacts.
Given that these are machine-pin sockets, I don't think that will be
a problem. Generally, you get a gas-tight connection with these.
After all, silver oxidizes/sulfides like crazy, yet there are acres
of old wire-wrap (silver plating over copper wire wrapped around gold-
plated pins) that are still working just fine.
Wiping-contact sockets, SIMMs, edge connectors, etc., I could see a
problem.
Then there are those carbon comp resistors... We have
no way of
knowing the vintage and storage history of those resistors. Carbon
comp resistors in general really, really don't like humidity and will
drift in value over time as humidity begins to affect the carbon. This
is why modern metal film resistors are much more reliable then carbon
comps in general purpose applications. Hopefully he happened to use
old stock AB resistors since their carbon comp resistors were somewhat
better sealed and tended to age /much/ better.
Tolerances on carbon comp resistors matter. If you think that by
buying 1000 20% carbon resistors you'll get 100 that lie within 5% of
the marked value, you'll be sore disappointed. It seems counter-
intuitive, but comp resistors were "cherry picked" by value during
the manufacturing process. With film, that's not a problem, as
values can be laser-trimmed.
--Chuck