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Technical Tips from Mark W. Persons

Harris FM-20K Transmitter PA Door Problem

October 8, 2008:  Here is your intrepid engineer Mark Persons working on a Harris FM-20K FM Broadcast Transmitter. 

In the back, the PA cavity door frame has been eaten away by a poor connection on a door fastener.  It doesn't take much RF resistance for things to start going wrong resulting in an RF arc, which will eat metal away.       
There were burn marks on the PA door as well.  Here, Scott Schmeling, from the Linder Broadcast Group, is seen cleaning the door with an electric grinder.
The original RF braid was removed and the cavity door lip was cleaned using mineral spirits and a 3M Scotch Brite scouring pad.
New RF braid was installed around the lip of the cavity and soldered together.  It is important that very little solder be used so it does not prevent the door from compressing most of the braid.

The Harris part number for RF braid strip is 813-9363-001.  It is sold by the foot!   

Here is a replacement nut in place of the original nut that used two holes for mounting.  You can see the original holes just above and below the nut.  The nut is available from Harris as is the RF braid. 


The stories go on and on.  Stop in again sometime.  I'll leave the soldering iron on for you. 
Mark W. Persons   ham W0MH      

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