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Technical Tips from Mark W. Persons

Repair of a Tube Socket

This is the top side of a tube socket for a 4CX15,000A Tube in an RCA BTF-20E 20 KW FM Transmitter.  You can never have too many tools when doing this work.  That adjustable wrench is just 4 inches long.

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Here is the socket with the screen contact ring removed.  Something looks a little funny on the left side of the right-hand screen bypass capacitor. 

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Sure enough, the dielectric material has failed causing a short circuit in the screen circuit.  You can see the arc-through near the top.  Fortunately there was a spare part at the site.

In this transmitter repair adventure, that screen bypass was not the only problem.

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The entire tube socket had to be removed for bench repair.  Note the scattering of parts in the bottom of the PA cabinet.  A piece of cardboard was placed over the incoming air duct to keep parts from falling into the cooling fan.

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On the bench, a nylon spacer was discovered as the cause to why control grid voltage went to near zero at times causing sudden PA overloads.  You can see it discolored brown and black on the right side of the socket.  Since those are no longer readily available, I used a porcelain insulator as a replacement.

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After the spacer was replaced, the tube socket needed to be reassembled.  It is extremely important to get the socket contact rings aligned properly.  Failure to do so could cause undue physical stress on the tube causing an air leak at one of the seals and tube failure. 

The best way to get the alignment right is to use a tube.  Tighten the hardware as much as possible, then remove the tube and do the final tightening.  Recheck to make sure the tube fits easily into the socket.

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Here the socket insulators and bypass capacitors are being tested with a Sencore Z Meter to see if there is any leakage at 600 Volts.

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Back in the transmitter, the screen ring had to be installed and tightened after the rest of the socket was in place.  Again, the tube was used as a jig to get the alignment right.  Also, you can never have too many tools.  A right-angle screwdriver with ratchet was ideal for final assembly. 

Note the black line on the socket and transmitter cavity.  That helped get parts back in the original orientation.  You can get into real trouble in a hurry if you don't think it all through first and mark the way as you go.

This kind of project is not for the faint of heart.  

Story from October 2009 at KMFY Radio in Grand Rapids, MN. 

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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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