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  #1  
Old 08-24-2010, 05:19 PM
thearrow thearrow is offline
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Default MC-60 Motor Controller Repair (Treadmill)

Background:
Let me begin by saying I'm a tinkerer with a basic, limited understanding of electronics.
Long story short, my treadmill got some liquid spilled on it and probably shorted something on the motor control board. Immediately after the spill, it tripped the GFI on the outlet. I let it dry, and can now plug it into the wall without it tripping the GFI.

Symptoms:
The treadmill runs normally, except no power goes to the motor.
I've tested the motor with a cordless drill battery, it works fine.
The power supply seems to be working, outputting 12V, and the PWM light flashes.
Very little voltage (1V?) is output at the motor spades on the control board.
After disassembly, the resistors at RPS 1 and RPS 2 were broken. I tried re-soldering them to no effect.
Now, the "CUR LIM" LED is lit, which it is never supposed to be:
"D16- Labeled CUR LIM- This light should never come on. If this light does come on, it means that the treadmill is exceeding the current limit of the controller. Continued use in this condition typically will cause repeat failure of the controller."


I've followed all the steps here:
http://www.treadmilldoctor.com/Sympt...kmHy!307018887


I've found this old thread:
http://forum.allaboutcircuits.com/sh...ad.php?t=18895
with a nice hand-drawn schematic from trolley1 in the second post.


Now I'm at a loss. As soon as power is presented to the treadmill, all four LEDs on the controller board light up. How can I determine which component is causing the CUR LIM to light? Is this due to a short or a faulty component? How can I fix this to regain a working treadmill?

I blew the fuse on my multimeter today by being an idiot, so I need to get a replacement before I can use it. I can take more pictures if needed.


Here are some pictures (Click for huge size):
Overview of the board



RPS 1 (out of focus, dur....)


RPS 2


CUR LIM LED



Thanks!

Last edited by thearrow; 08-24-2010 at 05:58 PM.
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  #2  
Old 08-24-2010, 06:04 PM
tom66 tom66 is offline
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It's possible the output FETs/BJTs (the three large ICs on the heatsink) are failed, which usually leads to them becoming shorted. Can you read the part numbers?

If the fuse on your multimeter is blown, you can still measure voltage, only current is unavailable.
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  #3  
Old 08-24-2010, 06:28 PM
thearrow thearrow is offline
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Quote:
If the fuse on your multimeter is blown, you can still measure voltage, only current is unavailable.
That makes sense .... I must've fried something else then, because it no longer reads anything? ...... damn


Quote:
It's possible the output FETs/BJTs (the three large ICs on the heatsink) are failed, which usually leads to them becoming shorted. Can you read the part numbers?
These guys?


The outer ones read S4020L with 677 vertical down the side.
The middle one reads D4020L with 113 vertical down the side.


There are an additional 2x D4020L on the opposite side of the board.

Thanks for your reply!
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  #4  
Old 08-24-2010, 07:53 PM
tom66 tom66 is offline
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Tough luck on the meter. However you can pick up a basic multimeter for less than $10 which will do everything you need and more for this project. Cheap meters do tend to fail suddenly, but I still like them because they do the job fine for their price.

After closer inspection that is intriguing.

It turns out those S4020L are actually SCRs (silicon controlled rectifiers). And the D4020 are diodes, which matches up with their part number. Obviously this board is designed for AC motors, or an AC power source. It is not outputting a DC voltage normally, right? SCRs aren't designed for DC; not usually, at least. I suspect it is just fancy PWM control for an AC motor. What voltage is the motor rated at?

Once you get a meter, you can test the diodes. Set to diode test, place the terminals on any combination of pins both backwards and forwards. If you always get "1 ." or overload, then the diodes are fried, and if you get "0" (or a close value, less than 100) the diodes are fried. Let us know.
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  #5  
Old 08-24-2010, 08:27 PM
thearrow thearrow is offline
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Thanks so much for your help.

I should have another multimeter by Thursday. I'll try testing the diodes then.

It appears to be AC power supply -> controller board -> DC motor.
I don't know how I measured DC 12V over the blue and white wires coming from the power board, which appear to be carrying AC.... lol (maybe this kind of stuff explains my dead multimeter )

Maybe this sheet I found will give you a better picture of what's going on :

*EDIT: fixed the image

Last edited by thearrow; 08-24-2010 at 09:17 PM.
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  #6  
Old 08-24-2010, 08:40 PM
tom66 tom66 is offline
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Well, if there's a DC motor it probably is DC. I don't know though. My suspicions point to one of the diodes failing open. This leads to no current and hence your symptom. It could also explain the symptom when the motor was locked at full speed: the SCR had failed, and this was pumping all the current through the motor, which overheated the diode.
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Old 08-24-2010, 09:21 PM
thearrow thearrow is offline
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Fixed the image in my previous post.....

Would one of these diodes failing cause the CUR LIM light to come on, or is that a different problem?

Thanks!
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  #8  
Old 08-24-2010, 09:28 PM
tom66 tom66 is offline
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If the diode shorted out, it could. However, the CUR LIM probably comes from measuring the motor or diode drop voltage; if it is too low, the light will illuminate, because the motor is dropping too much voltage. There is no current shunt, so I cannot see any other way it could work.
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  #9  
Old 08-25-2010, 02:57 AM
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marshallf3 marshallf3 is offline
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Still white stuff on one of the ICs in the pictures. Also, sadly, sometimes water damage can be worse than "just drying it out" will fix.

As to the guy with the multimeter - some do just put the fuse in series with one of the leads so it can kill the entire meter, try replacing it first.
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  #10  
Old 08-25-2010, 09:37 PM
thearrow thearrow is offline
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White stuff on ICs is just leftover residue from a label.

Bad (good?) news:
All 3 of the D4020 show .47 in the diode test mode. Reversing the leads gives 0.
Both the S4020L show .3

Does this mean the diodes are still good? What other components should I test?


Last edited by thearrow; 08-25-2010 at 10:08 PM.
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