Faulty Stepper or Driver?

Discussion in 'General Electronics Chat' started by solissydney, Oct 13, 2010.

  1. solissydney

    Thread Starter New Member

    Sep 21, 2010
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    HI all.
    I have a stepper (from old floppy drive) 12 V on my telescope. The fast forward no longer responds. The stepper turns at 100 revs a min. with good torque. fast forward should be 200 revs. I have spend hours looking and probing at the connections, Every thing looks OK. Should I replace the relevant IC's or could the problem be with the transistors. I have no one to ask for assistance. Another stepper behaves in a similar manner. When pressing the fast forward button on the hand-controller the stepper sounds as though it tries to speed up.
    I know it is a difficult question for me to ask of you. Any ideas, or guidance please.
    I have attached an image of the controller box. It has a lot of (funny) things inside it.
    I use a volt meter to test resistance and voltage. Just basic stuff.

    Yours sincerely
    Ken
     
    Last edited: Oct 13, 2010
  2. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    A schema wud help a lot
     
  3. solissydney

    Thread Starter New Member

    Sep 21, 2010
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    Sorry.
    No such luck
    Does it look like being the stepper or the driver?
    Ken
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    If the stepper runs in one direction but not the other, it's a problem with the driver circuit; it could be as simple as a bad switch or bad wiring between the non-working direction's switch and the board; this is very likely because there are no strain reliefs on the cabling where it enters/exits the az/el switch box, and it would get the most flexing of any of the cabling.

    I can't read the part numbers of any of the IC's.

    If you don't have a schematic, that's where you need to start.
     
  5. solissydney

    Thread Starter New Member

    Sep 21, 2010
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    The only 2 chips, relevant to this particular RA drive, I have not yet replaced, are
    RCA845 - CD 40103BE and H9623 - CD74HCT40103E.
    Is this helpful?
    Ken
     
  6. solissydney

    Thread Starter New Member

    Sep 21, 2010
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    Further information.
    The stepper is able to rotate in both directions
    Ken
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    So if it's able to rotate in both directions, what is the problem?
     
  8. awright

    Well-Known Member

    Jul 5, 2006
    84
    7
    If it is functioning normally at 100 RPM the motor is OK and the problem is probably in the driver circuit. You indicate that it once worked at high speed so apparently the basic design is OK (but probably far from optimum). You say that "...the stepper sounds as though it tries to speed up," when you press the fast button then possibly the driver is going to too high a pulse rate that the motor can't respond to.

    It would help a lot to determine the pulse rate going to the motor. In the absence of an oscilloscope or a DVM with a frequency counter, you might try listening to the pitch of pulses to the motor or at some other point downstream of the IC that the "fast" switch goes to. See what you hear with a crystal "speaker" connected across a motor coil. You may not hear a tone because the pulse rate is going to be below 200 Hz and the crystal transducer can't reproduce that low a frequency but you should hear a string of clicks like a buzz. The crystal will click at each transition of the coil drive waveform. When you press the "Fast" switch you should hear the rate of the pulses double. Might be hard to detect that it is doubling, but try.

    If the pulses disappear that tells us that the frequency divider is not putting out double the standard frequency. If the frequency shifts but to a very high frequency, that tells us the frequency shift circuit is malfunctioning. If the frequency seems to double and the AC voltage stays the same - well, that's a tough one (but we're learning).

    You can salvage a crystal "speaker" from an old computer board or buy one at Radio Shack for a buck or two. It is usually in a round black plastic housing with a small hole in the center and two leads. Some are about the size of a quarter and some are about 1/2" diameter and 1/2" tall. I'd advise you to reduce the voltage to the speaker but that gets a little complicated and may not be necessary for a true crystal speaker. Be sure you are using a crystal speaker, not an electrodynamic speaker (that looks like a miniature version of a regular loudspeaker). An electrodynamic speaker would probably burn out or might overload the motor driver.

    Your circuit is gonna have an oscillator (at the upper left corner of your photo where the oblong crystal can is located), a frequency divider to bring the frequency down to the pulse rate required for the motor (apparently with dip switches for fine-frequency adjustment), a decoder to generate the proper sequence of on/off voltages to the motor leads to drive the motor in the right direction, and probably a power darlington transistor array to provide enough current to the motor coils. In the absence of a schematic, it would help a lot if you would give us a listing of all ICs, transistors, and other devices on the board and where they are located on your board photo. Tell us what component and pins the "Fast" button is connected to.

    This is obviously an amateur-built device which explains the absence of a schematic or any place to request one.

    One characteristic of stepper motors is loss of torque with increasing step rate, particularly in the absence of clever current drivers or series resistors that make the supply look more like a current source than a voltage source. If the frequency is shifting too much it may go above the frequency range that the motor can respond to.

    Let us know what you find and we can gnaw the bone further.

    awright
     
  9. awright

    Well-Known Member

    Jul 5, 2006
    84
    7
    Continuing; The CD40103 is a presettable 8-bit binary counter. I'd guess that the frequency divider is loading a preset number (possibly selected by the dip switches?) into the counter which then counts down at a rate controlled by the crystal oscillator. When it reaches zero it reloads the preset and counts down again. Thus, the preset number loaded and the crystal frequency determines the frequency out of the frequency divider. The frequency could be doubled by changing one bit in the preset number with the "Fast" switch.

    Just a guess late at night.

    awright
     
  10. solissydney

    Thread Starter New Member

    Sep 21, 2010
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    This is great information for me to continue working to solve the problem with my scope.
    This is greatly appreciated

    Many thanks
    Ken
     
  11. solissydney

    Thread Starter New Member

    Sep 21, 2010
    9
    0
    Chip numbers, should it be useful.
    In the image chip one will start at the top left chip 2 to the right, going downwards.

    Chip 1
    P9742AP
    CD4060BCN
    2
    905
    CD4040CBN
    MM5640BN
    3
    HD74HCO2P
    6E46
    4
    P9730SD
    MM74HCO2N
    MC7402N
    5
    CD40103BE
    RCA845
    6
    CD74HCT40103E
    H9623
    7
    L297
    995A9635
    8
    L297
    R99089828


    Hoping this info is useful

    Ken
     
  12. awright

    Well-Known Member

    Jul 5, 2006
    84
    7
    What? You have a scope? What did you mean by, "I use a volt meter to test resistance and voltage. Just basic stuff"?

    OK. That makes it easy to observe the frequency shift when pressing the fast button. Just be careful not to short one of the motor drive outputs to ground via the shield connection on the probe. Probably not a problem unless the power to the motor drive circuit is grounded.

    Soooo, how much do you really know about troubleshooting?
     
  13. campeck

    Active Member

    Sep 5, 2009
    194
    3
    He means his TELEscope.
     
  14. solissydney

    Thread Starter New Member

    Sep 21, 2010
    9
    0
    I mentioned the word telescope in my starter Post. OK, Multimeter then, Digitor Q1467
    I am not just sitting here, letting you guys doing all the work for me. I am educating myself about electronics as mush as I can by trawling the Internet learning about steppers and electronics in general. I know how to find the 'common' wire on a stepper, and also how the measure voltage with the stepper turned on. I have found someone who I believe have a oscilloscope. So give me a bit of time to see what he can do to help me follow up on your kind suggestions
    Thanks Ken
     
  15. solissydney

    Thread Starter New Member

    Sep 21, 2010
    9
    0
    Having just read the suggestions by Awright again regarding pulses to the stepper when pressing the fast forward button. I can clearly hear the pulses increase I am sure at double the rate. That was what I meant in an earlier post when I said that I could hear the sound from stepper trying to go faster.
    I have listed all the IC's. I will try to locate transistors and other devices as you suggested.

    Thanks
    Ken
     
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