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.


  1. Coda1

    Is this a power supply repair for a Seig mini-lathe? Although the motor you’re working with looks like a sur plus tread mill motor, I’m not familiar with the power supply. I’m having a problem with my motor Seig mini-lathe controller and would like to post pics, if I can, to better explain my “similar” problem.

  2. 21/02/2012

    @Coda1, The motor and power supply slash control board all come from an old tread mill. The mini lathe is even a hack with a chuck and some bearings. Nothing commercial really. You can send photos to my email but I don’t think I could help with just photos.

  3. Coda1

    Yeah…I was going through your power supply repair series just now and realized you mention where the motor and power supply came from…a tread mill, next time I’ll look before I leap. Yeah, I can send pics but also an explanation of what’s troubling my mini-lathe motor controller. it’s basically the ame problem this guy is having:


    and how he repairs it:


    I tried everything he did and found it’s pretty much the same problem (maybe a typical one) but when I replaced the same troubling IC’s he replaced in his video(s) my controller still doesn’t work. I don’t have sophisticated electronic tools to test the components…hell I just picked up a DMM from radio shack today.

  4. 21/02/2012

    @Coda1, I would say you shouldn’t try to fix this. you will be exposed to high voltage mains power which will kill you if you make a mistake. If you must work on this then make sure to use an isolation transformer and a light bulb like garant111 did. Start by tracing all the supply voltages and chip voltages. Did you test your mosfet and opto coupler like garant111 did in his first video? If replacing those chips didn’t work and you can’t find any voltage supply problems you most likely will not be in a position to further test and repair this board yourself. DO NOT WORK ON THIS IF YOU DON’T HAVE AN ISOLATION TRANSFORMER! It would be 1000X smarter to just buy another mini lathe from HF.

  5. 22/02/2012

    @Coda1, If you get the trip-lite !!!You have to remove the grounding strap on the secondary coil before using these as tech bench isolation transformers!!!
    Watch my video and read comments on my blog and YouTube channel for this video:

    These trip-lites are for power line conditioning not ground isolation. As a tech working on equipment you want ground isolation.

    Jameco sales some true tech ground isolation transformers:
    For currents in the 2.5A range use:

    For currents in the 4 amp range you could bump up to the:

    If all you’re going to power is a light bulb the smaller one from Jameco would be just fine.

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