This is my most challenging attempt at a repair to-date.
The John Deere 200LC is a 45,000 pound excavator. Buried in the center of the massive hydraulic and mechanical systems is the 200LC engine controller.
This particular controller developed a fault when the wires to the servo which controlled the engine throttle shorted together.
I have access to the service manual for the unit which indicates which pins are which on the engine controller, but I don't have a schematic for the engine controller, and therein lies the challenge.
The controller is a completely sealed unit and the pass transistors are heat-sinked to the case around the edge. After powering up the unit with 24 volts (it's a 24-volt system) I checked the voltage on the pins to the motor servo and traced out a circuit for servo motor.
What I found was a pair of reasonable-looking H-bridge circuits. The voltage to the base and gates of the associated components didn't make sense, however.
Finding parts has been a little tricky, too. The controller is 90's vintage, and as such none of the parts are current but they do have substitutes easily enough available. Had to order some parts from utsource in Singapore which took a while.
Wed Aug 29 15:44:40 CDT 2012
After the first pass of replacing a bad transistor and MOSFET in the servo motor H-bridge, the unit went back into service but didn't work. There was absolutely no power to the unit at all.
After tracing the circuit I found an 18V Zener diode on the first regulator that had failed short. Replacing with another 18V Zener at least got the first pass transistor working and the unit was making +5V at all of the chips.
But that was about it. The unit still didn't control the motor servo. Unfortunately without a unit schematic further debugging would become exceedingly difficult.
One of the techniques I did find useful on a board like this was to use a common laser pointer to ensure that I was able to correctly identify a via that crossed from the front to the back of the board. Just hold the pointer over the via in question and find the red dot on the back.
I guess I can't win 'em all.
This page last modified Wed Aug 29 15:44:46 CDT 2012 by timc!
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