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Technical Tips from Mark W. Persons
Harris 150 Watt Preamplifier Modification

This is a Harris 992-6842-001 150 Watt Preamplifier, which is used in the Harris FM25K1, FM20/25K2, HT20/25, FM30/35K, and HT30/35FM transmitter designs for 88 to 108 MHz.  I think that "preamplifier" is a bad choice of names.  In my opinion, it should be called the 1st IPA (Intermediate Power Amplifier).  The rear circuit card is a power supply voltage regulator. 

Transistors on the front RF module are no longer made.  Harris wants $3200 for a replacement unit.  It is of a more recent design which includes the power supply regulator and heat sink.

An alternative is to use a Broadcast Concepts BLF278 RF Pallet Amplifier, sometimes referred to as the FM300 FM Broadcast Amplifier.  This more recent amplifier design is about $250 and is physically smaller even though it is capable of producing 300 watts of RF output with 48 VDC power.  In this case, the Harris voltage regulator keeps the supply to just 32 volts maximum.  This is just right for 150 watts output. 

The new pallet amplifier has more gain so a 6 dB RF pad at its input is required to reduce 8 watts of RF to 2 watts, which is what the amplifier wants to see.

The BNC input and output jacks were removed and mounted to the Harris heat sink to accommodate the new module.  5/12      

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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