DC Shunt Servo Driver for mill Troubleshooting/Ideas?

Discussion in 'The Projects Forum' started by franzschluter, May 30, 2011.

  1. franzschluter

    Thread Starter Active Member

    Jun 1, 2009
    Hi guys. I have an outdated conventional mill at my shop. The servo drive board doesn't seem to work and I have no idea how these things work. There is no spare part or manual for the machine. It is a Maho MH600 and the company doesn't provide anymore spare parts for this antique.


    Motor DC-Shunt Type:
    190vdc shunt and field. 6 wire output. 2 wires tach, 2 wires field, 2 wires armature. 4,7amps and 0,33amps respectively for its traversing table. The motor thankfully only moves in 1 direction. It reverse and does its limits mechanically.

    On the boards output pins there are AB JK. When measuring JK I measure 200VDC. Pin AB has zero volts. I guess this is why the mill table won't move. I am not sure if JK is armature or field voltage? Maybe someone could answer this for me. AB is 2.5 ohms, JK is 800~900 ohms (i'll recheck this)

    The board:
    Pins AB and JK run to the board. Pins AB goes to two SCRs, and 2 big rectifier diodes. The capacitors were fried. I tried replacing them with their exact values. However the MP 0.1uF cap was replaced by a mylar 0.1uF rated at 250VAC (don't know if the type of cap is critical in this case). After replacing all components there was still no output voltage on Pin AB.

    To be frank I have no idea or reference how this board should work. There should be a DC voltage on pins AB but it is only connected to two SCR and two diodes. How is it suppose to rectify AC to DC in this case? There are a couple of IC 8pin op-amps that I guess compares the reference to the tach and then will trigger the gate of SCR to vary the DC output? Am I mistaken? If so then why isn't the SCR triggering? Could someone provide a circuit example how this should work? Perhaps understanding the board better will give me more insight.

    Special note:
    There was a small box shapped capacitor with 6 pins which I desoldered which is connected somewhat to the Gate of the SCR (parallel). I desoldered it because I read 0 ohms/ 0 voltage drop between its pin and thought it isn't suppose to be shorted.
    The part name for this cap is:
    ZKB 407/043
    -02-PF 213
    Question: Does this mean 3x 250pF? I have no idea how to interpret this. I also cannot find this cap in local supply shops. Is this cap really necessary for the board to work?

    The way I see it:
    Maybe there are some products out there that can satisfy these criteria?
    It needs the ffg specs:
    - 190VDC shunt motor at least 6 amps. compatibility
    - tach input
    - Pot input for speed control
    -220AC input
    Any bright ideas where I could get such a driver board?

    Some pics of the machine and circuit board
    http://s747.photobucket.com/albums/xx117/olftdm/maho 600/

    I need the following help if you can:
    -Provide me any references how an SCR could work in a servo drive setup.
    -Provide me more insight what these pins could mean.
    -A turnkey solution. Perhaps a product that could fit into this category or as a retrofit

    Crazy thoughts:
    Maybe I should just plug 100VDC in pins AB (Bypassed) and see what happens? Maybe AB is the armature and JK the one that varries voltage?

    Best Regards
  2. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    The small box shaped capacitor sounds more like a possible RELAY to me. Relays should have zero ohms across some pins and open across others. There should be a set of pins with 100-1000 ohms as well. That would be the coil of the relay. Recheck that component to be sure about this

    Pictures of your board and the motor and wires would be extremely helpful to us.
  3. franzschluter

    Thread Starter Active Member

    Jun 1, 2009
    Some guys in the forum figured what it apparently was. They believe it to be a Pulse transformer. I will try soldering it back and see what comes out of it. Regarding pictures. I've posted a link of the album.. machine and board pics are located in there. I don't like to post it here since some of the resolution might not fit nicely on the forum.

    Relay was also my first doubt. But showing zero ohms and no coil somehow erased the benefit of the doubt. If it still fails.. I will try to reverse engineer it and draw its schematic.

    Best Regards

    ALBUM for the component in doubt:
    http://s747.photobucket.com/albums/xx117/olftdm/weird capacitor/
    Last edited: May 30, 2011
  4. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    what horsepower is your motor. KB drives offer inexpensive drivers.

    AAC Fanatic!

    Jul 1, 2008
    With the limited search that I did it appears to be a pulse transformer, but I confess, at first glance, I would have guessed a relay.
  6. franzschluter

    Thread Starter Active Member

    Jun 1, 2009
    Well I tried soldering it back. All active and burned passive burned components I have replaced. Still no luck. I tested almost each component and have found nothing wrong with its components. I did some conductivity tests on the pulse transformer and it seemed fine. For some reason reason the SCRs aren't triggering. I tried turning the pots for some luck but no avail. I did put them back to its original position.

    I'd love to power it up and probe the copper lanes for voltage. But 220V is a bit high. My hands are shaky and would love to avoid sparks >.<.

    There are pictures of the board and machine in the album for viewing. For now I'm going to try to draw it's schematic. I read about SCRs in a DC supply. The SCR seems to be a function of firing at a certain angle in order to produce a certain level of DC supply.

    I'm currently looking at http://www.kbelectronics.com/Variable_Speed_DC_Drives.html
    For a retrofit on alternative. I'm still unsure which is best suited.

    Best Regards

    AAC Fanatic!

    Jul 1, 2008
    Have you searched to see if anyone has ever posted a schematic of your controller?
  8. franzschluter

    Thread Starter Active Member

    Jun 1, 2009
    Yes I have searched for schematics. Even schematics of the machine itself seems to be very very hard to find. I had to rip out the wires to see where it leads them.

    The machine is a Maho MH600 from the 70s. Unfortunately DMG hasn't made any replies since 3-weeks despite the follow ups.

    A schematic for the board driver will be even more unlikely since there is no brand or board number on its board. I figure this board to be the confidential type in those days..

    The control seems pretty simple.

    Control panel has the following: Rapid traverse, Traverse motor on/off, pot for speed control. Reverse/forward,limit being mechanically makes life easier for me.

    The board has 16 pins.
    I can only do a rough guess on some pins...
    PIN 1: 220VAC
    PIN 2: GND
    PIN 3: 220VAC
    PIN 6: FIELD +
    PIN 7: FIELD -
    PIN 8: POT?
    PIN 9: POT?
    PIN 10: TACH +
    PIN 11: TACH -
    PIN 12: LOW AC VOLTAGE (leads to small rectifier)
    PIN 13: NOT USED
    PIN 14: LOW AC VOLTAGE (leads to small rectifier)
    PIN 15: LOW AC VOLTAGE (leads to small rectifier)
    PIN 16: BOARD GND (2 wires connected, gnd-shield from tach is also wired here)

    Note: It does rapid traverse by switching an adjustable resistor in parallel to the pot via a contactor.

    Motor Specs on dusty Name plate:
    Motor# GN15110
    Armature 180V 5,3A (confirmed)
    Field 190V 0,17A (confirmed)
    RPM 5000
    Class: B
    Rating: 100%
    6 wires for tach,armature,field.

    I had a post that someone kb electronics. I've been looking at them lately...
    I am not sure of this one is the one for me..

    KBMM-225D perhaps? It seems to match somewhat. I sent an e-mail for inquiry.

    AAC Fanatic!

    Jul 1, 2008
    Odds are, the control board was not made by the mill manufacturer but instead farmed out to an electronics manufacturer. Schematics for it were probably never available to the end user. BTW, I hope you took notes or marked where each wire went. Judging by how thorough you are when supplying information, I'm sure you did though! ;)
  10. franzschluter

    Thread Starter Active Member

    Jun 1, 2009
    Thanks everyone for the excellent wisdom and advice. I got my motor up and running. I just bought the KBMM-225 model from KBelectronics. I'm still trying the tach feedback to work properly. But this will only be a matter of calibration and time. The case is solved. Thanks a lot