On Thu, Apr 15, 2010 at 7:47 AM, mark drillock <mdrillock@cox.net> wrote:

> It is a fine theory to say replace things in advance of their failure
> but experience shows that the replacement parts are often what fails
> these days. Whether that is due to declining parts quality or lack of
> workmanship in the replacing, I'd say some of both.
>

Mark has a good point here.  In the 1950's the US Navy did a study on the
value of 'preventative maintenance' on electrical and electronic devices.
This was back before printed circuits and solid state electronics.  Most
control and switching circuits were relays and many of the relays were open,
not encased, like in cross bar switches.  And regs at the time called for
periodic cleaning of the relay contacts.  They found that their down time
and failure rate dropped dramatically when they quit doing the cleaning,
only cleaned (reads 'messed with') things when there was a failure or
malfunction.  The process of doing the preventive maintenance was causing
more problems than it was preventing.  Later studies confirmed this for
other electrical and electronic devices.  They also arrived at the
conclusion that 80% or better of failures in electronics (especially after
the introduction of solid state devices) was mechanical in nature, bad
connectors, corroded connectors, broken pins, poor contact, etc.
Now, how many times on this list have we read about problems arising right
after work has been done or parts replaced?  And how often have various
problems been diagnosed as loose connections, corroded wires and plugs and
connectors, corroded gronds, etc?  The mechanical part of the equation.