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Technical Tips from Mark W. Persons

Harris 300 Watt Preamplifier Modification

September 28, 2013:  This is a Harris 992-7073-001 IPA module from a Harris HT-5 FM Transmitter.  Its job is to take 7 watts of exciter power and amplify it up to as much as 300 watts.  Only about 100 watts is required to drive this transmitter at the 4400 watt level.  As you can see, four 2-watt carbon composition resistors overheated and failed.  I have seen this before and the cure was to replace the resistors with larger ones and the amplifier came back to life.  Not this time.  The transistor failed too.  That transistor is no longer available.  Harris has a replacement module for sale at about $3000. 
One answer to replace the transistor amplifier card with a Broadcast Concepts BLF278 300 watt pallet amplifier.  The new amplifier bolted down nicely on top of the original heat sink.  Some low-inductance resistors were added on the input side to reduce 7 watts of exciter RF power to about 3 watts.  The amplifier is designed to achieve 300 watts of output when the incoming power is 48 VDC.  In this case, the power supply regulator in the transmitter was set to 30 volts to achieve sufficient drive to the transmitter's tube-type power amplifier.   

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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