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My Precor M9.23 treadmill stopped working, and after reading the troubleshooting manual and looking at the motor control board, the led indicated that the board is bad. Upon removal of the board, I saw that one of the diodes had failed (blown open). I can't see any indications on the diode to know what to replace it with, but attached is a close-up image of the blown diode, a similar circuit board with the diode intact, and a close-up showing the intact diode in question (small black/gray cylindrical, I believe uni-directional).

2 questions, should I attempt to replace the failed diode, or is it's failure likely the result of some other failed component? I'd rather not replace the entire board if possible, it's ~$300 US.

What can I replace it with? I can't make out what the markings mean. It's roughly 10 mm in length.

I've tried looking for the circuit diagram for this board, or better pictures of the TVS diode, but no luck.

Thanks. blown diode

picture of similar circuit board

closeup of TVS diode (intact)

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    \$\begingroup\$ A TVS is just a protection component across teh supply rails ... a circuit will generally work fine without it (until a transient comes along). As your board doesn't work, that suggests something else is wrong. \$\endgroup\$
    – user_1818839
    Mar 10, 2018 at 17:17
  • \$\begingroup\$ What I was thinking was that if a TVS fails open, the rest of the board probably was exposed to a fairly significant high voltage transient. \$\endgroup\$
    – user57037
    Mar 10, 2018 at 17:30
  • \$\begingroup\$ Must have list; a) part numbers for TVS FET2, FET1 b) trace related tracks into a schematic c) a few years experience in repair or reverse engineering or find someone with 20 yrs experience and a crystal ball but the pictures are not enough a) mine says TVS protects FET2 so, at a minimum replace these \$\endgroup\$
    – Tony Stewart EE75
    Mar 10, 2018 at 17:33
  • \$\begingroup\$ Even the service manual is board level not component level sportsmith.net/images/product_support/precor/Service%20Manuals/… But FET1 & 2 are the PWM controls for the treadmill motor. So when it doesn't work, is it just the motor or everything \$\endgroup\$
    – Tony Stewart EE75
    Mar 10, 2018 at 17:33
  • \$\begingroup\$ The console works, but the drive and incline motors don't work. the "MOT DRV" LED lights up, which "indicates a defective drive motor circuit. Typically caused by a component failure on the lower PCA." - per the service manual sportsmith.net/images/product_support/precor/Service%20Manuals/… \$\endgroup\$
    – djwx
    Mar 10, 2018 at 18:47

1 Answer 1

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This failure is the tip of the iceberg. TVS stands for transient voltage suppressor. It's job was probably to clip unusually high voltages to keep the rest of the circuitry safe.

If the above is true, then sufficiently high voltage at sufficiently long duration and low impedance came along to blow out the TVS. This means it no longer served its protective function, so the high voltage most likely proceeded to blow out other components. Very likely, there are several dead components now on that board.

Basically, that board is toast. Unless you have a schematic or a service manual, there is little you can do about it except replace it.

However, before replacing the blown board with a new one, try to figure out what killed the first board. Did you have a nearby lightning strike? Is the unit intended for 120 V and was somehow plugged into 240 V? If you don't identify the problem, you can't know the new board won't be subjected to the same over voltage, and fry as soon as you install it.

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