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Technical Tips from Mark W. Persons

Custom 100 Watt FM RF Amplifier

Friday, October 11, 2013: A 950 MHz STL RF amplifier came into the shop for checkout.  Well, the two amplifier modules were toast and it really could not be repaired economically.  What to do with the left-over parts?
Then the phone rang from someone looking for an FM band RF amplifier to drive a BE 5 KW FM Transmitter.  His transmitter was missing its IPA (Intermediate Power Amplifier) section entirely.  A 50 watt exciter was capable of driving his transmitter to only about 30% power.  We need "more power Scotty!"
The original failed amplifiers were taken out.  An FM300 pallet amplifier and 250 watt low-pass filter from Broadcast Concepts were installed.   This amplifier is capable of 300 watts when the power supply is 48 VDC.  In this case, the power supply provided 30 VDC and that made the amplifier chassis capable of 100 watts or so.  The existing power supply was just right for this modification. 

A 10 watt exciter signal enters from the lower left.  It goes into a resistive 6 dB pad consisting of four 2-watt resistors and two chassis mounted low-inductance resistors.  This amplifier feeds the low pass filter, on the upper right, which in turn feeds into the original directional coupler. The 100 watt RF output is on the upper left.  A black jacketed cable was added as a shorted quarter wave stub, cut to the customer's frequency.  This should prevent the amplifier from failing because of an arc-over in the transmitter's tube power amplifier stage.

In the end, it came out looking like what you see above. 
An over-temperature switch, which will cut the amplifier power back to 10 watts if cooling fans quit.  The Airpax 67F080 will short, like a mechanical switch, when its temperature goes above 80 degrees Centigrade (176 degrees Fahrenheit).  The place to do this was on the RF input pad. 

Happiness is getting it all working right.

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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