Frequency Stability modification for Hamtronics® TA-451 UHF Exciters
This modification eliminates frequency drift when first applying the B+ keyed voltage to the exciter.
Drift is eliminated by applying constant B+ voltage to the oscillator, modulator, and first tripler circuits, keyed voltage still operates the succeeding stages allowing the transmitter to be keyed from zero power to full power.
Simply disconnect R-18 where it junctions E1 and connect the removed end of the resistor to E4. E4 is the terminal where constant B+ is applied to power the crystal oven (strongly recommended on the UHF band). R-18 is the bias resistor for the regulator diode VR1. Refer to the original schematic diagram to help understand this modification. Also, remember to connect constant B+ to terminal E4 if you are not using an oven and had not previously connected constant B+ to this terminal. This modification is helpful when you are using a TA-451 for linking signals back to the main repeater site from satellite receive sites. The satellite receivers may quickly key and unkey if an operator is marginal, making good voting difficult. The stability (both RF and audio) obtained will make the voting process more reliable.
The only drawback (if you can call it one) is that you will hear the exciter at all times when you are close to it. Sometimes when you are working on it, it is hard to tell if the exciter is keyed to full power or not. Connecting a wattmeter, or viewing an ammeter will help till you get used to it.
Kevin K Custer W3KKC
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