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Tuesday, September 4, 2012

Technical considerations about Yaesu G-250

The Yaesu G-250 is a very common rotator in small size VHF and UHF stations but this rotator has a little problem: Sometimes, the rotator starts to oscillate quickly left and right from the settled position in the indoor unit. Those oscillations are very dangerous, for the rotator itself and for the antennas. The only cure to this is to turn off the indoor unit when not in use.

The indoor unit uses a hysteresis voltage comparator based on a 741 op-amp. The red resistor is the one who adjust this hysteresis. It is a 330K resistor. The solution is to replace that resistor with a potentiometer (330K or even 470K is ok).

Just set the potentiometer to a 330K value and look for a position where the rotator starts to oscillate. Then, move the potentiometer until the oscillation stops. Now play a bit with the potentiometer and the indoor unit's knob to ensure the rotator do not oscillate any more.

As a rule of thumb, you must be able to do (very) small knob movements without any rotor movement, this is, you must obtain a new bigger hysteresis.

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