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Radio Broadcast Technical Consulting and Sales
 
10032 Island Drive, Brainerd, MN  56401

 


Engineer's Log of Mark W. Persons
High Voltage Blocking Capacitor


January 25, 2010:  Here are the charred remains of a PA tube, a driver tube, and a high voltage blocking capacitor from a CCA FM2500B FM Transmitter.  The normal tube appearance is of nickel plating, not brown/red. 

The blower motor quit, but the transmitter wanted to keep running.  My guess is that the airflow switch did not shut the transmitter down.  The switch worked normally when tested so I can only assume it stuck from being turned on for a long time.

The transmitter normally runs 2200 Watts using a 3CX400A driver tube and 3CX3000A7 PA tube.  The client keeps spare Econco rebuilt tubes on the shelf, but who keeps a spare blocking capacitor?  

Since there was no spare capacitor, the only option to avoid two days of off-air time was to rebuild the original capacitor.  They were not designed for rebuilding, but what else was there to do? 

I took the capacitor to the shop and used a hacksaw to cut through the outer cylinder and into the dielectric between the outer and inner cylinders.

After exposing the original dielectric, I found an arc-over spot. 

 

Plenty of cleaning was needed to get any sharp edges and other blemishes down to prevent future are-overs.
New Kapton dielectric material was wound onto the inside cylinder of the blocking capacitor.  This material is 4-3/4" wide and is used by Continental Electronics in their transmitter blocking capacitors. 

Yes, it has a more reddish-brown color as opposed to the original clear-white, which was probably Teflon.

 

The original dielectric had six turns, but testing showed that was one turn too many with this new dielectric material.  Two stainless steel hose clamps were used to hold the outer cylinder tightly on the dielectric. 
The proof is in the capacitance of the assembly.  Here it measures almost exactly the same as the original 550 pf. 
The final assembly, back in the transmitter, just before the high voltage was turned on.  Yes, the outer parts of the blocking capacitor have an odd/oxidized appearance.  They are silver plated and get to look that way regardless of being overheated or not.  The oxidation does not hurt their performance.

Success...no arcs and the operating parameters came back to normal. 

This is a permanent fix for the problem and is more repairable should this happen again in the future.


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

  Questions?  Email Mark Persons:  teki@mwpersons.com    

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page last edited 02/24/2016