Hi everyone,

I am trying to breath life back into our treadmill. It's a ProForm J8Li
Sears Model 831.297990. This model has the cushioned running platform
and the constant torque motor. Needless to say it is a nice machine. We
bought it new in 1999 (for a $1000.00 on sale. I think it was $1499.99
retail). Anyway, we purchased the service plan and two years later we
had Sears replace the Motor Controller Board.

Now, the Motor Controller Board went out again. Current Symptoms:
Turn on Machine
Incline motor sounds like soft relay chatter.
Then came that burning smell ... so I unplugged it.

Then:
I pulled the cover.
Took a look ... nothing obviously burnt.
Plugged the machine back in and fired it up.
Power supply board LEDs illuminated for all of the voltages (except PWM
LED). I assumed for now that everything was ok.
The Incline motor growled.
I metered the pins to the Incline motor (120V two hots and a neutral.
One hot raises, other hot lowers. I am assuming they are controlling
motor direction via the split capacitor attached to the drive unit that
these wires terminate into. When I metered this wire set, both hots had
120V.

I remember DC motors having a brake option and I wondered if this is
how they "hold" the incline at a given level. So, I "hotwired" the
incline motor to set an incline on the treadmill bed. When I set an
incline, I removed the 120V power. The weight of the bed alone causes
the unit to "backspin" and the bed returns to the 0 incline position.
So maybe that part is working ok. I am not sure.

What never illuminated was the PWM light on the Power Supply board. I
remembered that the incline used to go through a 'quick cycle' before
the display board lit up. So, I looked at the notched, counting disk on
the incline motor and "mimicked" movement. The display beeped,
illuminated and so I pushed the start button. The PWM light began
blinking on the Power Supply Board.

It was the motor controller board --again. --Sigh.

So I pulled the Controller Board.

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

Anyway, to make a long story short. I was wondering if I was on the
right track. This diode has the weirdest part number I have ever seen.
And I don't remember what the approximates are for checking for a leaky
FET. I went to Icon's website and they want about $170.00 for a new
board (with the same crappy parts?). So I thought I would see if anyone
has 'beefed these up'. So, If anyone has got some experience with these
controllers, or even a schematic, then I would really appreciate it.

Thanks in advance,
John

John:
I repaired a motor controller board for a Sears treadmil a few months
ago. I don't
think it was the same model. My board had about 5 SCRs on a heat sink
that wrapped around the board. Two of the SCRs were bad; replaced them
and that fixed the problem. Some of the SCRs were used as diodes to
convert the input AC to DC to provide the running voltages. I had to
get
the exact same SCRs for the unit to work. They were in TO-220 cases.
Hope this helps..
John G

Hi John G.,

I believe you are describing the MC-60 board. I can tell you that,
visually and electronically, it is a completely different animal. My
board has only one heat sink that goes down one side of the board.
Screwed to that decent radiator is a single FET and a High Powered
Diode of some sort.

Thanks for the help, but repairing an MC-70 with notes on an MC-60 will
be helpful, but it's probably not going to get me to the finish line.

The big Identifier of an MC-70 is two large black ELNA electrolytics
sitting next to a one big mother of a brick-cased rectifier, and a heat
sink that runs the length of one side of the board.

Here is a link to a page displaying the MC-60 and the MC-70 boards.
Maybe this will help others help me.

http://www.treadmilldoctorstore.com/...-Electronics_5

Thanks to John G. for trying,
John

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.

Dang! You're right Franc, that doesn't sound right. What the heck was I
thinking? The FET has to be fed by a smaller amplifier driver, or
directly from the output of a PWM circuit. I remember building a
variable PWM circuit with a 555 timer (1st semester freshman year in
digital design 101).

I'll get the part number for the diode. D1 has only two leads.

What the heck is the relay for? Oh ... duhh. It's for the bridge
rectifier ... that's a DC motor! So the Motor Controller Board has it's
own power supply that is completely separate from the Power Supply
Board. The Power Supply board just powers the treadmill computer and
sensors. Yeah, that makes sense. They must have done that to keep the
noise out of the computer electronic circuits.

All right. I'll go stare at the board again.