finding a compatible thermistor

[Mike]

Hello,

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
decreases.

Here is a photo of the old damaged thermistor after I cleaned it off:
http://www.ryznardesign.com/thermistor.jpg

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
celcius.

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.

 

[Robert Baer]

Hello,

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
decreases.

Here is a photo of the old damaged thermistor after I cleaned it off:
http://www.ryznardesign.com/thermistor.jpg

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
celcius.

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.

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.

 

[Mike]

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.

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?

Thanks,

 

[Karl Uppiano]

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.

 

[Robert Baer]

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?

Thanks,

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
easy).
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...

 

[MG]

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.

You are dealing with an NTC type thermistor (Negative Temperature
Coefficient).
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
373K.

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.

MG

 

[Mike]

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.

 

[eco17]

Mike,

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.

Simon