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Dead 7x12 Minilathe. Lovely plumage, the chinese minilathe.

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  • Dead 7x12 Minilathe. Lovely plumage, the chinese minilathe.

    7x12 Chinese minilathe.

    Managed to strip a tooth or six off the hi/low gearbox. Got replacement gears, stripped it all down, started putting it back together. Fitted the motor, connected it back up to control box, switched on.

    Slight "PHHHUUTT" from somewhere inside the motor or control box. Control display lights up, mianbs light on. Turning the chuck by hand makes the speed display register. Seems to be no volts on the motor. Check continuity through the motor, ohmmeter across the brushes flicks about when you turn the motor shaft. Otherwise motor ignores control box and does nothing

    found this thread on here
    http://bbs.homeshopmachinist.net/showthread.php?t=41676

    Thats it, thats exactly the controller. Changed the SCR's and Rectifiers as described, nothing.

    It the only step now a new controller board at £85 or is there a plan B i can try ?

  • #2
    I would check all connections at the switch and motor. My 9 x20 had a similar problem and was no more than loose,very loose, terminals within the switch housing. A good tightening and all is well for many years to date. A similar problem in the house turned out to be a poor electrical connection in a bedroom outlet. A push in type of wiring as opposed to winding the wire under the screw.The house was built in 1984 and the connection failed some 25 yrs later. Bob.

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    • #3
      I've had lots of experience with the Chinese 7 X 12" lathe.
      I've never had any problem with the two-speed gearbox on it, but, I only use the low speed anyway. Eventually, I would imagine, I'll have to get in there and replace the gears. I completely removed the two-speed gears from my milling machine, and installed the LMS belt drive conversion. And, I did manage to wear out the spindle bearings in the mill. (that was a saga that needs to be told around another campfire)

      What it sounds like, to me, is the MOSFETs (or SCR's) in the motor controller board have blown from an overload or being incorrectly hooked up. I'm on my third control board. I bought the second one from LMS, And sent the first one to a fellow in Oregon who replaced the MOSFETs and tested the board for $80. The third control board that blew, I substituted my repaired board for it, and replaced the MOSFETS myself, with heavy duty ones from "Digikey Electronics". Two simple MOSFETS, three leads on each one, and about ten minutes with a good soldering station for electronics.
      BTW, the MOSFETs on my mill also blew, so, I replaced them with "Digikey" MOSFETs and haven't had a problem since.
      No, my controller doesn't look like the one in your lathe. Of course, mine is meant to be used in the US where we only have 120 Volts AC.
      I think the SCR controlled boards are a bit more expensive than the MOSFET controls...
      No good deed goes unpunished.

      Comment


      • #4
        Originally posted by saltmine
        I've had lots of experience with the Chinese 7 X 12" lathe.
        I've never had any problem with the two-speed gearbox on it, but, I only use the low speed anyway. Eventually, I would imagine, I'll have to get in there and replace the gears. I completely removed the two-speed gears from my milling machine, and installed the LMS belt drive conversion. And, I did manage to wear out the spindle bearings in the mill. (that was a saga that needs to be told around another campfire)

        What it sounds like, to me, is the MOSFETs (or SCR's) in the motor controller board have blown from an overload or being incorrectly hooked up. I'm on my third control board. I bought the second one from LMS, And sent the first one to a fellow in Oregon who replaced the MOSFETs and tested the board for $80. The third control board that blew, I substituted my repaired board for it, and replaced the MOSFETS myself, with heavy duty ones from "Digikey Electronics". Two simple MOSFETS, three leads on each one, and about ten minutes with a good soldering station for electronics.
        BTW, the MOSFETs on my mill also blew, so, I replaced them with "Digikey" MOSFETs and haven't had a problem since.
        No, my controller doesn't look like the one in your lathe. Of course, mine is meant to be used in the US where we only have 120 Volts AC.
        I think the SCR controlled boards are a bit more expensive than the MOSFET controls...
        This is the KB board as per the other thread,



        it has no MOSFETS, just two SCR's and three Rectifiers in isolated TO220 packs, which ive replaced. Still no volts on the motor. I can only concluded one of the 6/8 pin IC's has blown as well, which ofc you have no chance of obtaining, or even replacing, there tiny
        Last edited by DeadLatheOwner; 05-11-2012, 09:16 PM.

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        • #5
          Originally posted by Bob Fisher
          I would check all connections at the switch and motor. My 9 x20 had a similar problem and was no more than loose,very loose, terminals within the switch housing. A good tightening and all is well for many years to date. A similar problem in the house turned out to be a poor electrical connection in a bedroom outlet. A push in type of wiring as opposed to winding the wire under the screw.The house was built in 1984 and the connection failed some 25 yrs later. Bob.

          The large board is powered up, proved with a voltmeter. the daughter board has power, because it lights the display which then registers the rotation of the chuck if you give it a spin.

          A circuit diagram with some voltages on would be really useful at this point in time !!!

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          • #6
            Check the motor brushes

            Check the motor brushes. If they aren't free to move then the control board can't make the motor move. (And it a whole lot cheaper to check...)

            Comment


            • #7
              IIRC the one you pointed to as smoked, is the reverse emf diode, usually that one does not blow, unless one of the bridge pair is shorted.
              The other 2 pairs form the bridge, 2 SCR's and two Diodes.
              In the past I have mounted a larger capacity set off board for a larger motor.
              You can check it with a 120v lamp first before hooking up the motor.
              Max.

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              • #8
                Plan B would be to carefully check for burnt pc board traces, and check those two larger resistors- black one on the left, and the ceramic grey thing on the right. Both should have some values printed on them.

                If swarf caused the malfunction, then a problem could have developed in the small circuitry. Like you say, it may be a write-off at this point. One short for even a small fraction of a second would definitely kill a board if it was in the wrong spot. There are lots of wrong spots in small circuitry, and IC circuitry.
                I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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                • #9
                  Your country of location may help with where to direct you for spares or advise? With your reference to £ I'm assuming RHS of the big pond so Farnells may be a bits supplier.

                  This may help http://www.axiscontrols.co.uk/shop/kbic-range/39.aspx or perhaps something in this http://www.homemodelenginemachinist....;topicseen#new

                  Regards Ian.
                  You might not like what I say,but that doesn't mean I'm wrong.

                  Comment


                  • #10
                    Originally posted by Circlip
                    Your country of location may help with where to direct you for spares or advise? With your reference to £ I'm assuming RHS of the big pond so Farnells may be a bits supplier.

                    This may help http://www.axiscontrols.co.uk/shop/kbic-range/39.aspx or perhaps something in this http://www.homemodelenginemachinist....;topicseen#new

                    Regards Ian.
                    Im in the UK.

                    To recap:

                    The board has power, at various points, except the motor connector. The display boards works, display lights and responds to chuck being turned by hand. Motor has continuity by an ohmmeter across brushes and across the connector plug when shaft is rotated. The two SCR's and three power rectifiers have been replaced by S8010 and D8010 devices (800v 10A isolated tag TO220 devices). You can here a relay clicking when you flick the reverse switch.

                    So it looks to me like the microelectronics on the control board is kaput for reasons unknown, apart from a very quiet small 'phhuuut" from the region of the motor or board when i first turned on, no smoke, no sparks, and no sign of swarf on board. Very, very baffling.

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                    • #11
                      If there is a short in the motor itself, the MOSFET's could have instantly died again once you powered on.

                      The motor is probably a DC motor. I would confirm that the motor works independently by applying the rated voltage to it. You said you measured continuity in the motor windings, but it isn't certain if this continuity is a short or a low resistance (as it should be).

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                      • #12
                        I had the plug in resistor go open on one, it was not visible to the naked eye.
                        Also a quick check of the motor is to run off of a Automotive battery, check both directions.
                        Max.

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                        • #13
                          Nah, it's just pinin' for the fjords!

                          Yeah, I just had to say that :-)

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