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Technical Tips from Mark W. Persons
BE FM35 Transmitter work

November 3, 2010:  Your intrepid engineer Mark Persons, now retired, worked on a Broadcast Electronics FM35A Transmitter at KCLD Radio in St. Cloud, MN.  The symptom was a PA screen power supply breaker trip when the screen voltage was raised above about 200 VDC.  The usual ohmmeter checks revealed no problems.  Mark used a Sencore Z-Meter to put 600 volts on the screen circuit in the tube socket.  Ah..ha.  An arc appeared on one of the screen bypass capacitors.  It lit up very well and was then easy to spot.              

Each of the four capacitor assemblies consists of two Kapton insulator strips which were plated with metal on both sides, separated by an aluminum block.  To the left of this burned one is a piece of Kapton, which is often used as a dielectric material in plate blocking capacitors.  That material was used in place of the burned capacitor while a replacement was ordered and shipped from Broadcast Electronics.            

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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