On 30 Nov 2006 05:18:21 -0800, "John" put finger
to keyboard and composed:

I am trying to breath life back into our treadmill. It's a ProForm J8Li
Sears Model 831.297990

Now, the Motor Controller Board went out again.

From my research it is the infamous MC-70, Rev A, GE motor controller.
I checked the Zeners, power diodes and such. D1 is a TO-220 package ...

Part number?

... (I think that is the size ... it's been a few years since college and I am
more electrically-oriented than electronically). I checked it with my
Fluke 87 and I got 0.4V forward biased and 1.2V reverse biased
(measured in circuit). There might be enough of the junction left to
meter at low voltage, but could it be failing under full voltage? THEN
I noticed the case of D1 was cracked ... rather blatantly cracked, so I
am assuming I have found at least one problem. I can't remember the
approximate R-values for a leaky FET. This board has a IRF250 Fet
(that's an N-channel, right?). Looks like the gate is driven off of a
relay ...

I don't have any experience with your treadmill, but that doesn't
sound right.

... and the D1 diode is [snubbing?] the stored energy of the drive
motor to protect the FET, right?

The specs of the IRF250 are 200V, 30A, 150W, RDSon = 0.085 ohm. You
might try to find a MOSFET with a lower RDSon. That would reduce its
power dissipation.

- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.