[Boatanchors] Relay Repair - Demagnetization Question

[Jim Haynes]

On Tue, 2 Feb 2010, mac wrote:
>>
>> Vacuum relays as used on amps sometimes appear dead on receive.
>> Reversing
>> polarity sometimes works.
>
> Relay contacts that are not carrying any current (like the receive
> side of a TR relay)
> will tend to do this.  The arcing that takes place when contacts make
> and break current tends
> to have a cleansing action on the contact surfaces.  Doesn't take much
> and heavy arcing will
> burn contacts,  of course.  The vacuum relay in my ART-13 does this
> and a few dozen cycles
> making and breaking a half an amp or so will restore the receive side.
>
It surprises me that vacuum relays have that problem, but your experience
shows they do.

When the use of hermetically-sealed relays first started, especially
in the military, there was a surprising development of contact failures.
After all, you'd think that a sealed relay couldn't get dirty contacts.
It turned out that organic substances outgassing from the winding
would, with the help of a little sparking, form an insulating goo on
the relay contacts.  So for a while there was a doctrine that you
don't use a sealed relay unless the situation absolutely requires it;
that a simple dust cover is much better for contact reliability.

There is an awful lot of lore about contacts and their problems and
solutions.