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finding a compatible thermistor

Discussion in 'General Electronics' started by Mike, Jan 7, 2005.

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  1. Mike

    Mike Guest


    I am looking for a compatible thermistor to replace a damaged thermistor
    in the fuser assembly of my laser printer. The laser printer is black
    and white and it was last made by Apple in 1998. It is the Apple
    Laserwriter 8500 - an excellent laserprinter except that after 5 years
    of use the thermistor in the fuser assembly is worn out - it was
    actually damaged by becoming coated with blackened iron oxide from the
    toner. Could someone recommend where I could find a replacement
    thermistor that would have all the correct specifications? Apple no
    longer supports this obsolete printer and no longer shares any
    information about it and therefore unfortunately, I don't have the
    specifications for this thermistor but I would be willing to mail this
    thermistor to someone so that they could use it as a model to create an
    identical one (if that was possible or practical) or to find a
    compatible one. I don't know how many volts it used but I have the
    wiring diagram of the printer in pdf format if that would be of any
    help. I was told when the temperature rises, this thermistor's ohm value

    Here is a photo of the old damaged thermistor after I cleaned it off:

    It is situated on a small bar magnet in the fuser and it also touches a
    heater roller (used to fuse toner onto the sheet of paper) and regulates
    whether this heater roller is turned on or off to keep it to the correct
    temperature. The heater roller heats up to between 150 and 200 degrees

    How can I replace this thermistor if I have the old thermistor but not
    its specifications? Is it possible to find a compatible thermistor in
    this situation? I could just buy a rebuilt laserwriter 8500 fuser
    assembly from ebay but I would rather just fix my broken fuser assembly
    by finding a way to fix or replace just this damaged thermistor.

    Thank you for any help or suggestions as to who I could contact for
    duplicating this thermistor.
  2. Robert Baer

    Robert Baer Guest

    If you have no make and part number info, and no specifications, you
    are SOL.
    Since it is damaged, then measurements would be misleading at best.
  3. Mike

    Mike Guest

    Thank you Robert,

    Good point but there may still be a solution...

    Apple doesn't indicate a part number for the thermistor - they only
    indicate the part number of the entire fuser assembly (which is part
    number 661-1418). This is unfortunate because they expect you to replace
    the entire fuser assembly if just the thermistor become faulty. Apple
    most likely contracted a company to build a custom thermistor
    specifically for this Apple Laserwriter 8500 fuser.

    You are right - I have no specifications to reference unfortunately. But
    I do have one other possible solution: I could find a working thermistor
    from another properly functioning second-hand fuser unit from the exact
    same printer model - probably from ebay. Then I would have a properly
    functional thermistor from which measurements could be taken in the aim
    to make a duplicate or to find something compatible. (I would end up
    with a good thermistor from ebay plus a future solution for when it
    breaks down in the future.) But this goes back to my original question:
    who might I contact to create a duplicate or to help me find an exact
    compatible match?

  4. Karl Uppiano

    Karl Uppiano Guest

    You might be able to find a replacement fuser assembly. I recently got an
    old HP LaserJet IIP working again by purchasing reconditioned parts off the
    Internet. You might try typing "laserwriter 8500 fuser" in Google and see
    what comes up.
  5. Robert Baer

    Robert Baer Guest

    No company will custom make a thermistor, unless they see a goodly
    sized market; you canot afford it - just like you cannot afford the
    Queen Mary.
    As far as a match, one must first characterize the good one by
    plotting tresistance as a function of temperature.
    With that data, one could calculate the multipliers used in typical
    thermistor specs (alpha and beata at minimum).
    Then a lot of looking at specs for thousands of thermistors made.
    Picking one maker, get an idea of the range of alpha and beta that a
    given thermistor series/type covers (some makers do that, making it
    Failing that, look at the specs (usually in a chart, and sorted) of a
    given series/type can give one the same idea - to see if that series
    might contain a possible "match".
    Each perusal of a given table should take no more than 10 seconds of
    look-see of possibly hundreds of part numbers.
    Not as bad as the "looking at thousands" sounds...
  6. MG

    MG Guest

    You are dealing with an NTC type thermistor (Negative Temperature
    These devices are pretty standard, I doubt that the OEM went to the trouble
    to specify a custom unit.
    They are characterized by two parameters, The resistance at 25C (called Ro)
    nad the exponent Beta to be used in aformula.

    R = Ro Exp[B(1/T - 1/To)]

    T=actual temperature in Kelvin
    To= reference temp in Kelvin, same temperature that give R=Ro

    Once you get your hands on a good part, you place it in a plastic baggie,
    place the baggie in a glass of ice and water and measure the resistance and
    record the R reading at 273K.
    Then you do the same into a glass of boiling waater and record the R at

    Then you play with the equation to extract the value of Ro and B.

    Any catalog that shows a part physically suitable that duplicate the same Ro
    and Beta is OK.
    Within limits Beta is not that critical.

    Look for catalogs of NTC manufacturer they explain this stuff.

  7. Mike

    Mike Guest

    Thanks Robert and MG,

    I actually found a few used fuser units on ebay for this particular
    printer model - there are even some broken ones for sale for parts. I
    will probably buy one of the broken ones and take the thermister out in
    the hopes of finding a functional thermister. I will also take some
    measurements off it as MG and Robert suggested - just for reference for
    the day that there are no longer available thermisters on ebay for this
    old printer (which Apple stopped producing in 1998 or 1999). Thanks for
    all these interesting and helpful suggestions.
  8. eco17

    eco17 Guest


    I have the same printer with the same problem - I think. A malfunction
    appears after the printer idles for 30 min. or so and the fuser lamp
    runs too hot - the thermistor does not cut power.

    Let me know how your search is going will you? I would surely like to
    save this printer from the dumpster and with a part spec it would be
    an easy enough repair.

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