[1000mp] MP RX TR relay repaired

[W0UN -- John Brosnahan]

Pete's problem of poor performance in RX-only relays is something
I can address from experience.  In the 1970s I was the engineer for
the University of Colorado Radio Astronomy Observatory doing HF
and low-VHF radio astronomy.

The main array consisted of a two-element interferometer with
many delay lines that were switched in by standard Dow-Key
coaxial relays.  These have gold contacts and were installed indoors
in a benign environment at 8800 ft elevation is the pristine Colorado
air.

But oxidation caused a lot of problems.  Even before the signal began
to drop in amplitude a phase shift was generated by the capacitance
of the contacts as they became insulated from each other due to a
oxide layer only molecules thick.  The Dow-Key relays have virtually
no wiping action and even the gold would oxidize.   We started using
thin strips of file-card stock to wipe between the contacts which would
cure the phase shift for a week or two.

But the final solution required that the preamp power be inserted BEFORE
the phase shifters rather than after the phase shifters.  This change allowed
12 VDC to be run through the relays as well as the RF signal and
eliminated the problem of contact oxidation.

An important thing to note is that these relays would check out OK
with an ohmmeter.  The test voltage of the meter was enough to
break down the oxidation.   So the relays always looked fine with
a meter and would start to work electrically after testing but in fact
were very problematic.


73--John   W0UN


>It's interesting that some relays seem really prone to this sort of 
>problem, while others are not.  I recently rebuilt my 80m array relay box, 
>using nice small sealed PC board relays instead of the big honkin Radio 
>shack plug-ins used before.  since then I've found that every few days 
>when I first turn the array to a given direction I need to "blip" the 
>relay with a little RF before receiving sensitivity is normal.  This never 
>happened with the old, big relays.
>
>I think I recall someone (w8ji, maybe), talking about relay designs that 
>avoid this problem by wiping the contacts against each other to rub off 
>the molecules-thick oxide layers that form, or else using contact material 
>that doesn't oxidize so readily.  Maybe it's just a contact material issue 
>(and another black eye for Yaesu's engineers).
>
>Since this is RX-only, I wonder if a modification to feed a little DC 
>across the NC contacts wouldn't be the sure solution.
>
>
>73, Pete N4ZR
>The World HF Contest Station Database
>is back, at www.pvrc.org/WCSD/WCSDsearch.htm
>
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