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Technical Tips from Mark W. Persons|
RF Power Amplifier Repair

Friday, August 17, 2012:  A Silicon Valley 500 Watt FM RF Amplifier Module (model 0101-500CH) came into the shop for repair.  It is used in the 88 to 108 MHz FM broadcast band as a driver for a tube stage in a large transmitter or to go directly to an antenna for a low power FM station.  Silicon Valley is now made by Delta RF Technology. 

This unit had two failed MRF151G power FET transistors.        

This is the "after" photo.  It isn't exactly as it was built.

See below.  

Some 16 surface mount bias resistors on the gates of the transistors had overheated and failed.  This happens when the user tries to drive the amplifier too hard to get more output power.  The original burned resistors left black spots where they had been. 

Those resistors were 22 ohms at 1/8 watt each.  In a series parallel combination, four of them were 22 ohms total into each transistor gate.  My solution was to use a 2-watt metal film resistor to replace four resistors at a time.  I also replaced the surface mounted .01 mfd ceramic bypass capacitors because they were falling off the board too.

In the end, the amplifier came back to life making a full 500 watts of output power with just 12 watts of drive power.     

September 13, 2012 e-mail:  Hi Mark, Just dropping you a note to let you know how much I enjoy your web stories and Tech Tips.  Its full of great info.  I am also a broadcast engineer (retired) and ham operator.  Nice job on that Silicon Valley 0101-500CH module.

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 02/21/2016