13th August
written by Todd Harrison

This is Part 5. You may want to read (part1), (part2), (part3) & (part4) first.

Today I continue my rainy day power supply repair project. I have checked everything twice and even took a shot in the dark by replacing all the ICs on the board. I still don’t know why this circuit is not working. The power transistors on this board should supply a ground path for the motor when activated by a plus on their gates. The pulse is not appearing at the gates so I know at the heart of the problem the pulse width modulation chip (PWM) is not doing its job. I have replaced the PWM chip but that didn’t help.

In this video I dig a bit deeper using my oscilloscope to look at some select voltage signals going and coming from the PWM chip. I even bypass the PWM and inject a pulse train from my function generator to test the power conditioning and power transistors. I can’t simply replace the existing PWM chip with another manual controlled PWM chip because it currently uses current sense feedback as well as signals from adjacent control circuitry to not only start the plus train but also control its duty cycle. These are critical features for a treadmill or metal lathe motor control circuit in order to maintain a selected motor speed under load.

Just posted part 6 of 6 which is the final post.

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