MC-60 from treadmill, broken resistors

Discussion in 'The Projects Forum' started by DomMc, Nov 9, 2014.

  1. DomMc

    Thread Starter New Member

    Nov 9, 2014
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    Hi,
    I'm new to this forum. Seems like a lot of people with experience of the MC60 power board.
    Mine is from a treadmill and all works except for the belt.

    When I took out the board I could see that one of the resistors had a broken leg. (R37)
    Then I noticed a second one also broken. R38.

    Is this normal?! I thought resistors could blow but not break.. or is this what happens when they blow in a normal operating condition.
    Excuse the newbie question......

    Anyway, after using resistor color code calculator I've calculated that the resistors are:
    R37 - 794 ohm (1% tolerance)
    R38 - 6.2M ohm (5% tolerance)

    I problem is that R37 does not match the value in the schematic I found on this site.
    It has R37 as a 230k Ohm resistor - but when I check the color code for that it doesn't seem to match my board.

    Help!
     
  2. DomMc

    Thread Starter New Member

    Nov 9, 2014
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    IMG_0247.JPG

    Hopefully you can see this ok... its from my phone through a magnifying glass.
     
  3. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The resistors do not seem cooked or overheated, but the lead seems to have been cut almost?
    Max.
     
  4. DomMc

    Thread Starter New Member

    Nov 9, 2014
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    That's exactly what I thought.
    It was working though and then just stopped. Can only assume that they snapped when warm, through vibration.
    But that's a total guess.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Although there appears to be a dual option label over the top which may a case of the resistor is snipped for one of these options?
    Max.
     
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  6. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    It's quite common for circuits to include one or more components which are deliberately disconnected according to model/mode of operation.
     
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  7. DomMc

    Thread Starter New Member

    Nov 9, 2014
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    Oh really? I wasn't aware of that.
    Ok, so the broken resistors could be a total red herring. At least that explains the clean breaks.

    So I guess the first thing to check is the PWM chip? or would you guys recommend anything?
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The one reverse-engineered copy I have 0f the MC-60 shows a SCR bridge method of control, this is phase angle control and not PWM, Look at the power devices and get the part number off of them and determine which you have.
    Then use the schematic you have to trace logically for the right voltages etc.
    Max.
     
  9. DomMc

    Thread Starter New Member

    Nov 9, 2014
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    Thanks!
    And final newbie question.
    Is it possible to troubleshoot the board without connecting to mains?

    For two reasons, one as the treadmill is not at my house.
    And secondly, I'd be much happier testing components on the board with a much lower voltage and current.

    I read that resistors can't be tested without breaking one of the legs as it will not give a correct value due to the rest of the board effectively being in parallel.

    But can I test the other components without power to the board?
    i.e. check the diodes using continuity testing on my multimeter?

    Again, apologies if these are stupid questions.
     
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You would have to consult a schematic if you have, it is the same for testing S.S. devices, there may be other components in circuit that affect a reading, they often have to be unsoldered from the board to test.
    In something like this type of circuit, the first thing to suspect is any part of the power input/control.
    Max.
     
  11. DomMc

    Thread Starter New Member

    Nov 9, 2014
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    Just an update.
    After spending some more time looking at the board, I found that there is a blown component.

    This is a couple of pics of the components which are screwed to the heatsync frame.
    The damaged one is identified as Q3 in the board, and is a S4020L according to the schematic.

    FullSizeRender.jpg FullSizeRender2.jpg

    Seems to be on the the path of the (0-120v DC) A+ line, according to this schematic, so fingers crossed it's the problem!
     
  12. MaxHeadRoom

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  13. IamJatinah

    Member

    Oct 22, 2014
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    Correct! Many assemblies have what designers call "Tuning Sets", which may be a few resistors in parallel so they can be cut/trimmed during testing to dial in "current" levels, ASIC levels the MCB will see under normal use, etc ;o) Do not replace these "Cut" resistors, you may end up with wrong calibrations for speed and or lift...
     
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  14. IamJatinah

    Member

    Oct 22, 2014
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    Max is right, these are SCR's with that part number you gave, being used to rectify input AC~ One other note, MC-60 units do NOT have a "hold up" or storage capacitor on them. This is most unusual, and leads one to the notion that there is an additional PCB connected between the MC-60 and the actual motor, which would have capacitors on it, and/or a hefty inductor. I have repaired and reverse engineered much of that unit, and found it can "NOT" be used to power a motor without adding a serial inductor of say 1-3mh and/or an added hold-up capacitor. The original treadmill design would have added this extra current capacity into the system, as this PCB cannot drive a motor as it sits by itself, there is simply not enough balls in the unit without the added capacitance on the motor drive rail. I can explain futher if needed, but please be aware, "any" motor, to overcome it's inertia and begin spinning, can consume up to 600x it's expected current for an instant(<50msec) to overcome inertia. This completely starves this unit, and draws high input AC to try to drive things. ;o)
     
  15. MaxHeadRoom

    Expert

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    Not strictly true, SCR bridge control rarely, if ever use a capacitor on the output of the bridge, the main player in this field for some years is the KB drives, also re-labled by Baldor, and non of their SCR drives ever use a capacitance after the bridge.
    Not the case of course for their PWM versions which all use capacitor smoothing.
    Max.
     
  16. DomMc

    Thread Starter New Member

    Nov 9, 2014
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    Ok, am gutted.
    Thought that SCR/thyristor was definitely the problem but it obviously wasn't the only one.

    Put the board back in - slid the dial - and only 3 LEDs light. (the one with CUR LIM does not light)
    The belt doesn't move.
     
  17. DomMc

    Thread Starter New Member

    Nov 9, 2014
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    So, if the SCR trig LED is illuminating, can I assume (with some degree of confidence) that the circled part of the circuit is probably ok - and that either the MOC3052 is toast or something in the bridge/rectifier is fried?
    [​IMG]
    (although I would have guessed that if for example, Q3 was dead - there would still be some power to the motor as half the bridge/rectifier circuit would still be operational?)

    Is there any way to bypass the MOC3052 to isolate the issue?


    I suppose I should confirm a couple of my assumptions too. Being new to all this....
    - Q2 and Q3 will not let any voltage/current through unless they have a signal on their gates, which are connected in parallel to U3 (MOC3052).
    - I assume that SW (s1) maps the to slider on the console of the treadmill.
     
  18. MaxHeadRoom

    Expert

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    At this point the best route would be to trace the signal through under power and see if it can be determined where it ends?
    Max.
     
  19. DomMc

    Thread Starter New Member

    Nov 9, 2014
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    It's kind of tricky because the treadmill is not at my house. I can go and connect and test, but obviously I want to arrive with as much info and options as possible.
    Have you any idea how I can simulate the gate signal from the MOC3052 to the SCRs?

    In terms of checking, would you ground one side of a multimeter and check the output of D11 then Q2, then D10, Q3?
     
  20. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Here is a couple of PDF's I think your DWG shows the mOC3052 wrong.
    Also shows clipping the resistors change the model #.
    Max.
     
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