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post #43 of Old 02-18-2013
Join Date: Mar 2011
Location: Solomons, MD
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Re: Autohelm ST4000 Plus problems

I wonder if they really mean the processor, or if that is just a general statement they make to avoid fixing it. My unit had the low battery problem. Here is what I was able to determine on mine. The processor has a "buzzer" output line which is logic level 5 volt pulses. This drives an inverter which acts as a buffer. That in turn drives a transistor that creates 12 volt pulses from the 5 volt pulses. The 12 volt pulses feed a charge pump/voltage doubler to provide 24 volt bias on one of the power MOSFETs. This MOSFET switches closed to provide 12 volts to power the H-bridge driving the motor and the output clutch circuit (whatever that is used for). In my device, the BAV99 (I think that's the nomenclature, going from memory) dual diode package was bad. The result was that only 12 volts was biasing the MOSFET so that it did not switch closed to power the H-bridge. I was able to replace it and get mine operational again. Before I did that though, I removed that MOSFET and jumpered the 12 volts from 12 volts external to 12 volts internal to check that everything else was operating normally.
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