Need help troubleshooting MC2100WAM motor controller

Thread Starter

ymg200

Joined Oct 2, 2015
26
I see a number of discussions about MC2100 in this forum, but I haven't seen one with my symptoms.
My treadmill stopped working. It has MC2100WAM motor controller.
I do not have EE education, but I'm handy with electronics to some extend....
The Console seems to be functional and it moves the incline motor. The main motor, however, refuses to move the belt.
I see no visual indication that any component is burned/damaged.
The diode bridge has 120V AC input and 107V DC output. Is this voltage drop expected from the diode bridge?
Blue wire in HD2 harness (WPM from Console) does carry PWM. I don't have oscilloscope, but my multimeter can measure freq duty cycle, which is from 20% at the lowest speed to 85% at the highest, controlled from the Console.
I have tested the motor by powering it with 12V SLA battery and it works.
LG3 and LG4 terminals (DC output to the motor) give 0V, so it looks like the board doesn't want the motor to move.
MC1900 chip does get 5V (measured btw Pin 10 - Gnd and Pin 20 - V+).
If I connect pin9 to pin10 (Gnd) of MC-1900 chip, then the LED lights up. This has no effect on the motor.
Power resistor 7W 3.3K (on two tall legs) heats up a lot - to the point that it starts smelling in few minutes and I have to turn the power off. The voltage drop across this resistor is 91V with 120Hz frequency, which I assume comes from the diode bridge. Applying Ohm's law I=U/R=91/3300=0.03A. If the current is so small, why does it burn then? What am I missing here?

Troubleshooting instructions say that LED being off indicates no AC voltage to the controller, which is not the case here.
To summarize my symptoms:
1. LED is off
2. Power resistor 3.3K 7W is extremely hot
3. Zero voltage to the motor output (terminals LG3, LG4)

I started looking at components close to the burning power resistor and noticed the output to Pin7 of MC1900 (according to the attached schematics).
Pin7 of MC1900 IC is called "AC power Voltage sense" in the schematics. I've measured it to be 4.9V (btw Pin10 and Pin7), but it seems to be DC, not AC.

What can make the power resistor burn?

Everything else seems to work: PWM from the Console, incline motor, voltage to MC1900 IC.

The troubleshooting guide says Red wire in HD2 harness is 9V, but in my case it is 15V. I assume that I have a different variation of MC2100 board.

Last observation: the LED blinks briefly (1/10sec or less) when I turn the power on. This is the only time the LED lights up.

I appreciate any help with the troubleshooting.

20200212_234319.jpg
 

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MaxHeadRoom

Joined Jul 18, 2013
20,065
What does the RED LED do when the board gets a command? It should flash very steadily.
Red on HD2 is typically 15vdc to 17vdc on those boards.
Coincidentally i just attempted a fix on a friends with the same model as yours, it has the outboard LV power transformer version.
With his the motor only ran when the incline was being lowered, any other time the motor was dead.
I hooked up a PWM simulator into the HD2 and the motor ran every time.
The conclusion was that it was something in the operator control panel.
Max.
 

Thread Starter

ymg200

Joined Oct 2, 2015
26
The Red LED is off all the time, while I do get the correct PWM reading on the blue wire from the Console (HD2 harness). LG3 and LG4 outputs to the motor give 0V all the time. The LED lights up if I short pin10 (GND) to pin9 of MC1900 chip, but still no motor movement.
The LED blinks initially (barely visible, less than 1/10sec) when I turn the power on, then goes dark forever.

Here is my uneducated guess: Since 3.3K 7W power resistor burns hot, something must be shorting it. This short might be pushing the controller into the "safe mode", shutting down the LED and signal to the motor. I see on the schematics that this resistor is right after the diode bridge and connected to the ground via two diodes D7 and D8. My thinking is that these diodes could be bad.

I did diode test with my multimeter.
D7 - Forward biased: 0.59V Reverse biased: 0V
D8 (zener) - Forward biased: 0.57V Reverse biased: 1.87V

Zener diode 1.87V reversed voltage doesn't look right to me, but I've never tested a zener diode before. Note that I did not remove diodes from the board, so other components might affect measurements.

Does this test seem conclusive to you? Does this tell that the zener diode is bad?

If zener diode is in fact bad, would that cause 3.3K power resistor overheating?
 

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MaxHeadRoom

Joined Jul 18, 2013
20,065
The normal operation is at switch on, after a small delay, the LED will light steady, once a PWM command is seen, the the motor should start and the LED will flash about 1/2 sec intervals.
Try a suitable incandescent lamp on the motor terminals, ( motor disconnected).
Max.
 

Thread Starter

ymg200

Joined Oct 2, 2015
26
I measured the output to the motor with the multimeter. It's steady zero.
The 3.3K 7W resistor heats up to 200F in 6 min (I measured with the IR thermometer).
What do you think about that zener diode D8 that connects this resistor to the ground? Did my test prove that the diode is bad, or the test is inconclusive because I didn't remove the diode from the board before testing it?
 

Thread Starter

ymg200

Joined Oct 2, 2015
26
>> The normal operation is at switch on, after a small delay, the LED will light steady
I my case the LED barely blinks on the power on (with no delay), then goes dark.
 

Thread Starter

ymg200

Joined Oct 2, 2015
26
When power is on, the voltage drop on D7 diode is virtually zero, but the voltage drop on D8 zener diode is 15V, measured both directions. Can this tell anything whether diodes are good?
 

MaxHeadRoom

Joined Jul 18, 2013
20,065
I measured the output to the motor with the multimeter. It's steady zero.
The 3.3K 7W resistor heats up to 200F in 6 min (I measured with the IR thermometer).
With motor disconnected does the &w resistor heat up still?
It is begining to sound like one of the power components, Mosfet or Diode on the H.S.
Max.
 

Thread Starter

ymg200

Joined Oct 2, 2015
26
Yes, with the motor disconnected 3.3K resistor heats up the same. 200F in 6 min.
Can I diagnose those power components without removing them from the board? What do I need to do?
 

MaxHeadRoom

Joined Jul 18, 2013
20,065
It would be reasonable to conclude that if the 7w resistor is heating up then there must be something else also passing a lot of current.
What I would do is remove the screws from the semi's on the heatsink and slide the PCT board out of the chassis, with the motor disconnected, plug in and when it heats up, SWITCH OFF and test (feel) the power components that were heat sunk, for over heating.
They should normally all be cold.
When re assembling, ensure that the H.S. insulators are in place.
Max.
 

Thread Starter

ymg200

Joined Oct 2, 2015
26
I took the board out of the H.S. and reconnected (except LG3 and LG4 to the motor). I pushed the power resistor to over 200F, but did not notice that any other components heat up. I checked with IR thermometer and by touch. Some components were slightly warmer, but not too warm. Power components were less than 75F. Somewhere in the center of the board thermometer registered 80F, but I couldn't tell by touch that any component was warmer than others.
What would be the next step to diagnose? Why you think that the culprit is not D8 zener that leads to the power resistor that's running hot?
 

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MaxHeadRoom

Joined Jul 18, 2013
20,065
Unfortunately I wont have access to the model of board you have, at this time.
It may take some patient reverse engineering of the circuit by a physical trace and determine the circuit down stream of the resistor.
Max.
 

Thread Starter

ymg200

Joined Oct 2, 2015
26
Max,
Somebody already reverse-engineered this board. I have attached the schematics in my first post. This schematics seems accurate to me. Whatever components I traced, the schematics matches. It shows that diodes D7 and D8 connect the resistor to the ground, that's why I was asking about those diodes. Did you see the schematics?
 

MaxHeadRoom

Joined Jul 18, 2013
20,065
I was one of those that originally posted the schematic. I have not confirmed that it is all accurate, it will be a week before I can get my hands on that particular model of board.
Did you check across the zener D8 for 10vdc?
The resistor is just to drop the voltage for the 10v supply via D7 and D8.
If no 10v, then I suspect either the zener or the 100uf cap, C4.
Max.
 

MaxHeadRoom

Joined Jul 18, 2013
20,065
Incidentally I measured the volt drop across the 3.3k 7w resistor on another version of the board and it measure 94vdc. No excessive heat above normal.
Max.
 

Thread Starter

ymg200

Joined Oct 2, 2015
26
Incidentally I measured the volt drop across the 3.3k 7w resistor on another version of the board and it measure 94vdc. No excessive heat above normal.
Max.
I measured mine at 92V, but it heats up like crazy, Does yours stay cool?
The voltage drop across my D8 zener is 15V. It's not 10V as specified, but doesn't look critical to me. Is 15V across zener acceptable?
Output from the diode bridge: 107V.
Resistor and zener are in series between "+" and ground and it adds up: 92V resistor + 15zener = 107V from the diode bridge.

I also checked C4 cap, it measures 100uF, just like it should. I've removed the cap from the board to do the measurement.

I'm puzzled how come both of our resistors drop the same voltage, but mine is burning hot.....
Maybe it should be this hot and I'm looking in the wrong direction? How hot do you think your resistor is?
 

MaxHeadRoom

Joined Jul 18, 2013
20,065
The heating is what I would expect from a 7w resistor, I can touch it after 10min and it does not exceed what I would expect.
The output of the bridge is 110v.
Max.
 
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