March 2011
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The burned contactor fingers

This is a set of burned contactor fingers on a Harris HS-4P 30 amp RF contactor:

Harris HS-4P RF contactor

Harris HS-4P RF contactor with burned finger stock

The back story is this:

The contactor in question is at the base of Tower #3 of the WBNR (1260 KHz, Beacon, NY) antenna array.  This is the tallest of all the towers, at 405 feet.  As such, it gets struck by lightning often.  There was at least one occasion where one of the inductors in the ATU got “sucked in” due to the huge magnetic field of a high current strike.  It is not at all surprising to me to find other component issues in this ATU.  Because of the burned contacts, I’d suspect that the station was switching modes under power, but I didn’t see that happening today.

The problem manifested itself in very high SWR after changing over from day pattern to night pattern.  This did not occur every time, in fact, it only occurred once in a great while at first.  Then, over the last couple of months it began occurring more and more often.  Since the snow drifts are now down to a manageable six to eight inches, it was a good day to go out and do some exploring.

First of all, I put the station into night time mode just to confirm that there is still an issue.  The transmitter, a Broadcast Electronics AM1A showed very high SWR and carrier fold back.  Left it in night pattern, but turned it off and took a walk, not a drive, to Tower #4 which is all the way at the bottom of a hill, near the old City of Beacon landfill.  I figured that I would check that one first, then look at Tower #3 on the way back.  When I got to Tower #3, I found the issue right away.

Fortunately, I was able to salvage a set of contact and contactor bar from another relay in the same ATU that was not using them.

Burned RF contactor bar

Burned RF contactor bar

The night pattern is only 400 watts, but these are tall towers, 225 degrees, therefore current and voltage are high at the base.  In fact, the slightest change at the base of the night time towers will greatly upset things.

Burned RF contactor fingers

Burned RF contactor fingers

Harris HS-4P contactor repaired

Harris HS-4P contactor repaired

This is the repaired contactor.  I will say, the EF Johnson RF contactors are easier to work on.  Those are the ones with the big rocker bar across the top and two solenoids on either side.  All of the wiring, status switches and contacts are exposed and easy to get to.  This one, not so much.  This is the BE AM1A transmitter

Broadcast Electronics AM1A transmitter

Broadcast Electronics AM1A transmitter

It is not a bad unit, compact, sounds good, reliable, etc.  In order to work on the power supply or anything in that top cabinet, the whole thing needs to be removed from the rack and taken down.  I suppose that is my only gripe about the thing.


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