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Rotor Trouble - Again!



Hey, Rick. Regarding your message...

>>I have a Yaesu G-5400B. I repaired the elevation rotor, replacing the pot, ...
>>
>>Rick 
>>W2JAZ
>>----

Please contact me off-list if you want to discuss this more. My contact info is good on QRZ.

I followed your thread on the original repair. My elevation feedback pot failed suddenly when I foolishly shorted a motor terminal to the pot wiper terminal on the back of the controller. Then I pushed the "up" button! By the time I noticed the elevation meter trying to follow A.C., it was too late (I could hear the little needle hitting the pegs - click-ick-ick-ick). Lucky for me the ground leg of the pot opened, but the wiper & top of pot have continuity through the whole elevation range (45 - 500 ohms)! The lucky part was that it happened with the pot in the full 'down' position. 

Well (after cussing every word I knew and missing a good AO-7 pass where K3SZH worked rare DX), it seemed like a lot of trouble to take down the rotor when it had a perfectly good variable resistance for feedback, just no ground reference. I contemplated an OpAmp circuit, but I got around it by putting a bias resistor to -5v on the wiper terminal, thusly providing the missing 'ground' level voltage for input to the controller. The bias resistor effectively completes a 'voltage divider' circuit with the pot, creating a 'ghost' ground reference. Recalibration of the elevation meter & analog input to tracker was required. I got the tracker to read right from 0 to 90 anyhow, but there is still a little 'offset' in the analog meter reading. Faced with the prospect of hauling down the array, the offset is highly tolerable. I usually read the elevation off the computer screen anyways (especially now).

If your feedback pot is open, is it possible that there is an intermittant short circuit somewhere that is shunting motor current through the feedback pot? This short could be anything from a solder bridge at a connector, to a frayed conductor in the cable, and could survive R&R of the rotor. 

Please, don't give up the fight! This may be the last problem between you and many years of trouble-free satellite operations (or not... hi hi).

73,
Ed - N8BBQ
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