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[Fixed] Thermistor short

Posted by rdomunky 
[Fixed] Thermistor short
October 20, 2015 09:42PM
Hi all,

Finally getting around to posting a summary of my fix for a problem I had with my M90, in which the thermistor sporadically gave wildly varying readings. This is what my hotend temp looked like after heating it up and then turning it off to let it cool:

Nophead suspected a short at the thermistor. When I disassembled the hotend, I found that black ABS had seeped between the red thermal tape and the hotend metal. Much of the cement holding the thermistor in place crumbled away immediately. I suspect that the plastic somehow contaminated the cement and partially bridged the thermistor leads. I drilled out the old thermistor and completely cleaned out the thermistor cavity in the hot end.

The biggest challenge was in sourcing replacement parts stateside. Nophead provided the following information on the original parts:

This is the thermistor: [uk.farnell.com]
This is the red tape: [www.ebay.co.uk]
This is the cement we use: [www.ebay.co.uk]

Turns out that the thermistor is currently out of production, although there is some remaining stock available from UK sellers. Digikey in the US indicated that the direct replacement to the thermistor part is model B57540G0104F000; however Nophead warned that the specs are indeed different, including a lower max temp. I was eventually able to find the exact replacement part through a third party seller on Amazon.

I believe I was able to source the same E-Z Fuse tape in the US, although mine is black. I got it on Amazon.

The cement was problematic, as the product that Nophead used is not available in the US and can't even be shipped here. At first I tried Blue Magic 18003 QuikSteel High Temperature Metal Repair, which seemed promising due to its high temperature tolerance. However, it also turns out that this stuff is partially conductive, so it did not work at all. Fortunately I'd ordered a pair of replacement thermistors, and had one left after drilling out the replacement from this failed attempt.

Neil then suggested Permatex 80335 Muffler and Tailpipe Sealer, which I was able to get through Amazon. This cement appears to have worked, and has held strong for a few dozen hours of machine use at this point. I made extra sure that the thermistor cavity was packed with cement before inserting the thermistor, so that there would be no trapped air. Follow the instructions for both the cement and the tape, as they both require a curing period before being heated up.

Hope this helps if anyone comes across the same problem. Thanks to Nophead and Neil for their guidance.

- rdo
Re: [Fixed] Thermistor short
October 21, 2015 10:50AM

as you already suggested in the thread I started, I might have a similar problem. Is there any way to make sure that the problem lies with the thermistor?

I have already tried to heat the nozzle with a hair dryer and compared the values given in Octoprint with the values measured with a infrared thermometer. They are almost identical but I only managed to heat the nozzle to about 45°C. I could use a heat gun but I am not sure if that would prove anything and I am afraid of accidentally damaging some other parts of the printer.

I guess I should check the wiring of the hotend next, maybe there is something wrong there.

Ok sry for bothering you, I am more than a little ashamed now, I guess I should´ve checked the wiring first, in fact I discovered that a cable has become loose, so I simply reinserted and screwed it back in place again and everything is back to normal now!

Regards, Jens

Edited 1 time(s). Last edit at 10/21/2015 11:12AM by D4RK1.
Re: [Fixed] Thermistor short
October 21, 2015 12:51PM
Glad you were able to find a solution simpler than rebuilding the hot end!

- rdo
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