MC-60 treadmill motor controller, fault.

Thread Starter

electronis whiz

Joined Jul 29, 2010
512
So i ended up with an old junk treadmill figured be fun to tinker with, part out or maybe try and use. The main board had issue with the elevation motor i found an electrolytic cap had failed swapped that it went all the way up then down then back up slightly which it did not do before. I was told that incline was just stuck at like 2.X. Also told it did run but after some time it would trip the internal breaker. I figured maybe weak breaker, but still not sure they seemed to think maybe needed oil and load on motor was issue. However i can't even get it the tread to spin. I set in in a running mode, looked at the LED the power/logic board had MWM LED pulsing and so did this MC-60. MC 60 also had 12V LED on, but SCR trig was totally dead. Current limit LED was off too.

I used a DIMM and checked for open resistors, diodes, etc all looks ok. I traced around the PWM LED thinking that is probably tapped off whatever drives the SCRs. I see an opto isolator, some resistors, etc. one lead off of an ST LM324m to that part of the circuit. The schematic sheet inside the thing just says replace the MC60, but i don't really have much interest spending much on this as main board could still have issues, motor could be near toast, etc.

I did notice though that something marked RPS and looks like resistor 42 is missing. I see no burn marks on the PCB but thought maybe that failed and fell off. But seeing some other pics this seems to be cut off too. From the schematics i have seen that is part of the current limit circuit. I initially thought maybe if that skews it too much even no load motor being connected would be overcurrent but now that i think about then the overcurrent LED should be on. Short of testing the optoisolator or the ICs on this i don't see what issue would be.

I am also finding it odd that this has ICs at all as the PWM is feed into this from the console PCB which then passes through the power PCB and to this one. And to me the H and L terminal look like maybe logic power from the other PCB and the W is the PWM signaling. I see a trimpot with some rust on top of it but continuity it seemed fine. I see no sign of any other issue like other blown caps corrosion, burned, electrical smell, etc.

I have many DIMMS, i have a fluke scope although i do not know a ton about using it yet. I have stash of old caps, resistors, etc so probably have something i can put in if needed. I am just kind of lost at this point short of desoldering parts and testing open circuit, etc which if it is going to be that bad i would just assume part the thing out. Just seems to me there should be some simple piece that goes between the PWM input coming in and the SCR trigger, i honestly thought it would just go right to an SCR, but it appears there is more to this thing.

I guess wondering if there are any ideas where i should look, things i could probe, i also saw something about this board could be controlled by a potentiometer would that or something else be any good to test this. The PWM LED lights on it so I assume that input is working so thinking issue lies in the middle somewhere?

20200506_234534[1].jpg

20200506_234509[1].jpg

20200506_234830[1].jpg
 

cortiz1216

Joined May 7, 2020
1
I am new to learning Circuit boards but it looks like Resistor R42 is missing next to the LED light. You can see there was something soldered there. The little sticks are still there just missing the resistor.
 

Thread Starter

electronis whiz

Joined Jul 29, 2010
512
I saw that too and see in a schematic it should be a 5.4K ohm resistor, but many other online places have this exact same board and same thing that resistor is like cot off for some reason. I guess i can replace and see if works, but that is part of current limit ciruit so not sure what that would have to do in the end with SCR trigger issue.
 

MaxHeadRoom

Joined Jul 18, 2013
20,707
These boards had 'Option' resistors fitted, the one refered to is one of these,( see the box lines printed around it) it may have been removed purposely.
There is a schematic here and many other places if you search, It shows the pot wires connected, normally a 5k to 10k linear pot.
The MC-60 is a SCR bridge type controller with 0-10v pot on the input, IOW no PWM involved.
Max.
 

Thread Starter

electronis whiz

Joined Jul 29, 2010
512
I saw a few schematics but this was driven by a PWM from the control board on the treadmill i know that part works due to the PWM LED on the other PCB and this one pulsing. I saw info on using these with a pot which at this point i just want to use the existing PWM, etc. I am not real sure how PWM works in this case.

I found this schematic https://electronics.stackexchange.com/questions/82354/need-help-with-90vdc-pm-motor-speed-control-circuit

After looking it seems that the opto isolator ground is passed through that SCR trigger LED. I am assuming if that LED was dead there would then be no ground for the this side of the opto isolator so this would prevent trigger of the SCRs? I ohm checked the opto was kind of high resistance but intact, i checked the LED and it seems to be open circuit when checked with ohm meter, checked the 12V LED and it had resistance. So i am thinking that LED may be bad. can try and swap it but unsure what kind of voltage, AFIK all led are like 3V so would think any LED would work, but if this is part of the SCR trigger i think wrong voltage or too much resistance change may cause issues?
 

MaxHeadRoom

Joined Jul 18, 2013
20,707
I have a link for some PWM boards for the MC2100, they use a 20Hz PWM signal.
The MC2100 uses a opto isolater for the PWM signal it has a low resistance on the input to the opto, so another resistor has to be added to suit the level of PWM level being input.
Never seen PWM control on a MC-60 however?
Although Studying the link, it seems that instead of using the usual control input, the opto drive to the SCR is directly input with a PWM, if so, by passing the pot circuit, this to me is not all that of a good idea,
Seems it is just to get around the necessity of turning the pot to zero to restart. This has now been overcome by just clipping a resistor in the pot circuit.
Max.
 

Thread Starter

electronis whiz

Joined Jul 29, 2010
512
I am not real sure what the purpose of that clipped resistor was as it seems to be part of the current limit circuit. The terminals for the speed control pot seem to be the interface to the console for telling this what to do. I get PWM reduces power by pulsating, but i do not see how that equates to a potentiometer.

It seems there are two variants of input circuit on these. Looking the low left corner you see VCC12V a resistor, LED, transistor. I think that the 3 wires for that PWM are 12V+, GND and a signal wire. The speed led was flashing but not the SCR trigger so think has to be something in that string in the middle where is passes through the ICs, resistors, etc and finally the opto and the SCR trig LED. Sense the current limit LED is not lit i am thinking that is not the issue where it ties to U1D, i am not sure what U2C and U2D do it looks to me like something out of a switching mode setup where it is a feedback or where it like gets a loopback signal to trigger itself again.
 

MaxHeadRoom

Joined Jul 18, 2013
20,707
Here is details including the 'clipped' resistor chart.
I also have some notes by the original reverse engineered version as to details of the various parts of the circuits.
Max.
 

Attachments

Thread Starter

electronis whiz

Joined Jul 29, 2010
512
I re-did a couple solders on the large transistor/SCR that looked like had a crack on the top side, then went over with a heatgun trying to maybe reflow some of them. Put in machine it ran for a few minutes then quit, unplugged it checked temp on the heatsink was slightly warm, but expected. I let set like 10-15 min try again nothing. I got this i was told it tripped the little internal breaker, so far not had that just runs or does not. Motor seemed fine, machine itself is kind of noisy, junky sounding so may just part out. Still be cool to have the SCR thing work then could use motor on something but may just use motor for a generator so not hughe. Fact it worked then failed makes me think bad solder or maybe a broken trace in PCB heat, etc expands it and then it dies. Best idea i can come up with from what i can see.
 
Top