I have a sole treadmill with this lower board - Alatech 6300a. When the deadman tether switch magnet is connected on the console, the treadmill immediately ramps up to full speed. But, the problem isn’t the upper console. It’s the lower board...

There’s a white 12 pin connector on the lower right of image that has the cable that connects the lower board to upper board. I disconnect that cable from the upper board, and I can jump a couple of the pins of the cable together to simulate the deadman magnet being connected and again the treadmill immediately goes full speed (the deadman tether magnet is seen as connected when you jump pin 1 s/w black and pin 4 vcc orange).

Connecting the tether when the console is connected or jumping those pins typically just clicks the relay on the lower board and makes it ready to receive a speed signal (the speed up signal on the 12 pin would be a repeated momentary connection of pins 5 and 7). Any idea what’s shorted on the lower board causing the race condition? Any way I can test or treat?

 

Check the semiconductors mounted to the heat-sink.
Max.

 

Check the semiconductors mounted to the heat-sink.
Max.

Thank you. One of those three semiconductors was bad. A couple of follow up questions if you don’t mind. 1)What made you think it was these on the heat sink? 2) (apologies in advance for not getting the terminology correct) any ideas what caused the burnout? Was it more likely too much gate current or too much output? Thanks again.

 

The heat sink mounted components are usually the ones operating at higher current/temp rating, so these are the first likely suspects.
It could be they may have been stressed by a lack of lubrication on the moving parts, dirty belt etc.
Max.