My carl lewis treadmill MOT18 will not start..the displays work but the tread doesnt move.

The MOT18 power circuit appears to be a variable pulse width DC motor speed control. I don't have a circuit for this particular version and I doubt if you will be able to buy one. The manufacturer has taken care to erase or mask component details so that his circuit would be difficult for another manufacturer to copy.


I have however repaired one of these treadmills and you may find my notes useful.




I was presented with a completely dead machine but the display activated when the start button was pressed. I was told that the machine ran very fast and could only be stopped by switching off at the mains. Once stopped it would not start again.




The display panel appeared to working normally suggesting that the display battery did not need replacing.




Since the safety key had failed to stop the belt I removed the display panel lid (undo all the screws in the back except those which screw into the support rail).




Safety key insertion closes the contacts of a micro-switch which was found to be operating correctly.




The speed control was checked. It is a 10kOhm pot and was found to be OK.




One of the battery contacts was found to be loose and was refitted. The display panel was replaced.




Checking under the motor cover top panel (four screws, two at the front on each side and two more within recesses at the rear) I found a board mounted fuse had blown. This is a 10amp fast (quick) blow which is 20mm long by 5mm diameter. It's a very common type available from most on-line electronic stores or retail outlets.




This blown fuse confirmed my original thought that the speed control power semiconductor had gone into meltdown and hence the uncontrollable high speed. Switch off and next time you try to switch on the fuse blows because nothing is limiting the surge current through the stalled motor.




Checked for nasty smell characteristic of wire insulation in burnt out motor. Checked top motor brush for burning or pitting but all seemed well.




I made a careful written note of the connections to the power board (match wire colour to the termination designations printed on the circuit board) and removed the small sub-chassis carrying the printed circuit board (three screws but the one near the small transformer is the only one to remove completely as the others are in slots in the sub-chassis).




With the power board on the bench I realised that this speed controller had two power semiconductors. One was a K2837 power switching transistor. The other was a 20V60S fast recovery power diode which was wired as a freewheeling diode designed to prevent the motor from generating high voltage spikes when the transistor switched off. The transistor is obviously being driven by a variable width pulse generator. The pulse generator probably lurks under the metal can which is earthed to prevent radio frequency interference from the fast rise and fall pulses of a square wave.




I assumed that all was good with the pulse generator circuit since it is not fed by the blown fuse.




The most obvious source of the problem was a defective K2837 MOSFET transistor. The reason it failed was most probably a degraded 20V60S diode - the resulting high voltage spikes would be sudden death to the transistor.




I replaced both and Eureka - one working treadmill.




When I replaced the diode I had difficulty in buying the original 20V60S so I replaced it with a Fairchild Semiconductors device type RHRP1560. In some respects it's better than the original and with any luck I won't have this back on my workbench.