Repairing Heindenhain VRZ 753B + LS803 scales - Page 7
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  1. #121
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    queston on the bulb replacement: doesnt it get a sinusoidal signal (i read that somewhere)? in that case, how does the diode work?

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    Dian,

    the bulb is normally fed about 5V DC and provides a steady light source. The light is formed into a parallel beam with a collimating lens and then passed through the scale. This has a single grating with a 40 micron period (20 for LS803D). Below the scale, the read head has two channels A and B.

    Each channel consists of pair of grating lines, each with a 40 micron period. The pair are offset by 20 microns (180 degrees) so that when light shines through the first pair, it is blocked from the second, and vice versa. Below each of these offset gratings sits a photodiode. The two diodes are connected "in series" with cathode to anode and vice versa. The channel output is taken across the diodes, so when one diode is illuminated and the other is dark, current flows in one direction in the external circuit, and when the diode illumination flips, so does the current direction in the external circuit. If both diodes are illuminated equally, the current flows through both diodes and no current flows in the external circuit.

    The two channels A and B are as described above, and are at a 10 micron offset from each other, so generate quadrature signals A and B.

    The adjustment screws control how much light falls on each of the members of the pair. There is also an additional reference channel which I have not described.

    Cheers,
    Bruce


    PS: just realised that you are asking about replacing the bulb with an LED. Normally the scales are fed DC for the bulbs. If they are fed AC and an LED is present, then there better be a full-wave rectifier and capacitor somewhere downstream in the read head!

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    thank you bruce. would you know if the 501 is similar to the 803? mine gave up the ghost and i wonder if i can fix it myself. there is virtually nothing to be found on the 501 online.

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    Quote Originally Posted by dian View Post
    thank you bruce. would you know if the 501 is similar to the 803? mine gave up the ghost and i wonder if i can fix it myself. there is virtually nothing to be found on the 501 online.
    I don't know, I have only worked with LS803 scales. If you post some closeup pictures of the read head and I might be able to tell you more. Is the 501 an 11uA sinusoidal signal, a 1Vpp sinusoidal signal, or a TTL digital signal?

    Is the problem a burned out light bulb in the read head?

    PS: see this old post by our very own Peter Veltman (Peter from Holland):
    Led for Heidenhain LS501
    Last edited by ballen; 11-15-2021 at 03:57 PM.

  5. #125
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    i finally found the ls 501 instructions on h.h.s site (they are under "ls 501d"). its a 150 ma ttl signal. i was able to check the power feed pins for the lamp and they are o.k. so im pretty sure its the lamp itself, although h.h. sais they usually degrade and not just quit.

    https://product.heidenhain.de/JPBC/i...ontentUid%5D=0

    so i wonder what diode to look for. "near ir"? will i be able to use the existing collimating lens? peter never followed up on the thread.

    this is the "situation", im not even sure yet, how to get the reading head out without damaging it. does it just slide out?
    Attached Thumbnails Attached Thumbnails 0-350.jpg   0-351.jpg   0-352.jpg  

  6. #126
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    surprise, the lamp seems to be working. has it deteriorated to the point where its not funktional? but all of a sudden? the yellowish light is exactly what i would expect if feeding 5v to a 12v lamp. (i saw somewhere they did this to extend its life.)

    so what else might cause the scale to stop working? if its dirdy, would it stop like that in any position?

    i also wonder about the cable. thats probably not a connector. what happens if i unscrew that fitting (or whatever it is)?
    Attached Thumbnails Attached Thumbnails 0-357.jpg   0-361.jpg  

  7. #127
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    Clearly its dirty....Will it stop the scale from reading...depends where the crud has collected and how much....looks pretty gorse from here...

    Can't you test the cable using a multi meter to run a continuity test to the connections at the read head to the 9 pin connector.....
    Should be a rectangular cover plate on the back of the read head held on by small screws.
    Removal of that plate (think you need to remove the read head from its mounting bracket to get access) will give access to the connections of all the wires in the cable...and from there you can test for continuity
    with everything disconnected of course....

    Cheers Ross

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    connectivity was o.k., i soaked the scale in hot dish soap water and rinsed the head and electronics with propanol. now it works again. was probably a piece of dirt blocking the optics.

    i have been using the scale without any seals for 15 years and p.o. probably for a few years too. i found remnants of the seals, a brownish, crumbly substance soaked in oil. it looked like some candy and i was tempted to try how it tastes. what could it have been? cork?

    btw, why is propanol better than ethanol? is a bit more polar, but does that make a difference? its very hard to get and the cost is several times higher.

    oh, and i was not able to remove the reading head from the aluminum block. never found out how its attached. maybe glued, but what a strange idea, as there are nice mounting holes.

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    Hi Dian,

    Quote Originally Posted by dian View Post
    connectivity was o.k., i soaked the scale in hot dish soap water and rinsed the head and electronics with propanol. now it works again. was probably a piece of dirt blocking the optics.
    It's amazing how much abuse these scales can take, and keep on working correctly. Just like a Deckel mill...

    I have been using the scale without any seals for 15 years and p.o. probably for a few years too. i found remnants of the seals, a brownish, crumbly substance soaked in oil. it looked like some candy and i was tempted to try how it tastes. what could it have been? cork?
    I won't comment on your tastes in food, personally I prefer swiss chocolate. But buy some new seals from Franz Singer, they are not expensive, under 20 Euro/meter, and put them in place.

    btw, why is propanol better than ethanol? is a bit more polar, but does that make a difference? its very hard to get and the cost is several times higher.
    In the electronics industry, isopropyl alcohol (IUPAC name propan-2-ol, also called isopropanol or 2-propanol) is the standard washing agent for components, circuit boards, and assemblies. So all of the plastic, sealants, glues, inks used in electronic components are rated/safe for that. IMO that's the main reason to use it, but it also seems to be more effective than ethanol at removing various types of adhesive.

    I buy 2-propanol in 5 liter plastic cannisters, online in Germany for about 20 Euro, delivered. That amount lasts a few years.

    oh, and i was not able to remove the reading head from the aluminum block. never found out how its attached. maybe glued, but what a strange idea, as there are nice mounting holes.
    Oh my, imagine that someone glued the read head onto the carrier. Crazy!

    Cheers,
    Bruce

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    If someone is looking for a replacement cable for a read head, I suggest using ethernet cable (after cutting off the RJ45 connectors from both ends). Cat 5, 6 and 7 ethernet cables contain four twisted pairs. Use one pair for pins 1/2 (zero phase), one pair for pins 5/6 (quadrature phase), one pair for pins 7/8 (reference), and one pair for pins 3/4 (lamp).

    Most ethernet cables have a braided shield, and that's what you should use. Don't use a cable that has only a foil shield.

    If your existing cable is smaller than the ethernet cable, then you may need to enlarge the entry hole slightly. If that's the case, keep in mind that Cat-5 cables tend to be smaller diameter than Cat-6/7, because the individual conductors are slightly smaller diameter.
    Last edited by ballen; 12-22-2021 at 11:32 AM.

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