Modifications to the repeater RF amplifiers was mostly for more cooling.

I did make some improvements on the UHF amplifiers tuned circuits for at least a 1Ø percent gain in power output.

These modifications were performed on all three UHF Amplifiers.

I will start with the Trilectric UHF 1ØØ watt Power Amplifier model A-251ØØUR.

This is a commercial amplifier, and has a heavy duty heat sink, and is factory rated at 1ØØ percent duty cycle.

I like the way the factory mounted the amplifier behind the front panel. The fan is far enough away to do a decent job of blowing air on the heat sink.

No changes in the fan arrangement were needed, but a better way of switching the fan on was needed.

The factory had mounted a thermal switch on the heat sink, and ran 11Ø VAC through it to a AC fan.

I replaced the fan with a 12 VDC fan, but found that the DC fan generated RF racket into the repeater receiver.

I shielded the fan power cord, and installed a filter capacitor at the fan to solve the problem.

The next step was to install a circuit in the amplifier to switch the fan on with RF, and I added a 4 minute shut off delay.

This is the amplifier before the modifications.

This is the Amplifier after the fan control modification.

Removing the old AC fan circuit cleared up 2 terminals on the power connector, which allowed me to use double terminals for the 12 VDC input cable.

The next step was to squeeze a little more RF out of the amplifier.

This amplifier was designed for the 45Ø to 47Ø mhz band and down at 444 mhz it was lacking some power.

Installing 5 each 2-2Ø mmf trimmer capacitors solved the problem.

There were 2 trimmers added to the base inputs, One at the beginning, and end, of the out-put filter, and one at the end of the driver output filter.

This worked nicely to allow peeking of the amplifier on 444 mhz.

There was one other modification. The factory N connector center pin extended out back about a half inch.

I felt this was way too much at UHF. So I shortened it and also the shield terminal lug. You can compare it to the un-touched input connector.

By the way, you may notice my sniffer diode is connected to the shield lug, not the input center pin.

Last but not least each amplifier was given a good alignment.

For the two meter band, I chose a Mirage B-2516R 1ØØ Watt RF Amplifier.

This amplifier was designed for the 2 meter band and so the RF circuits needed no modification.

This amplifier has a heavy duty heat sink, and is factory rated at 1ØØ percent duty cycle.

In operation though I found the heat sink gets quite warm, so some fans were needed.

First step was to make a baffle. I have some old aluminum sheet metal with teflon coating that I used but the teflon is not needed.

I only covered the area where the transistors are mounted, but probably an improvement would be to mount the amplifier like the Trilectric company did.

I may do that at a later date.

I mounted some old 11Ø VAC muffin fans on the front and on the back to blow right on the transistors and circuit board.

The AC fans required a relay to be mounted inside the amplifier.

The relay is activated by a transistor with a RF "sniffer" diode. This works quite well. Connect the diode to the RF input connector. If it is connected to the output circuit it may trip when other nearby repeaters key up.

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