Assembled printers - cannot get a good print, turns out to be bad hotend thermistors on arrival
 
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Assembled printers - cannot get a good print, turns out to be bad hotend thermistors on arrival  

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Terence - AMakerDad.com
(@terence-amakerdad-com)
Active Member
Assembled printers - cannot get a good print, turns out to be bad hotend thermistors on arrival

Long time Prusa owner here - just purchased two CoreOnes assembled for work, one directly from Prusa Research, the second one from MatterHackers.  Both of them out of the shipping box had no firmware installed, I had to install the firmware before I can get it to work.  Both of them had bad thermistors for the hot end, which lead to a lot of head banging and troubleshooting before I could figure out why I couldn't get a successful print off the new machines.  

(BTW, this post isn't meant to shame / cast blame - but I think Prusa got a bad batch of thermistors in and there are other people may be affected and maybe I can save someone a lot of grief).

On my first printer, I could print ABS, but prints in PLA and PETG would fail.  The print won't stick, or the first few layers will stick and then it would come off the bed.  I thought at first it was bed cleaning and bed adhesion, but after a few weeks of trying, I wasn't getting ANY successful prints in PETG and about 50-50 on PLA.

Then one day as I was troubleshooting the printer I noticed that after a reboot and cooldown (tried a cold pull to see if the nozzle was stuck) the hotend still read 292 deg C.  At first I thought it was a firmware bug of some sort, and I started filming for a bug report... only to find that the hot end was at room temperature.  The hot end thermistor had failed in such a way that it was reading 292 deg C at room temperature.  I replaced the thermistor and suddenly, viola - prints were coming out great.

I took delivery of my second printer - and could not get a single good print off of it.  Knowing the issue I ran into (and since it too was missing firmware), I grabbed a thermocouple and data logger from my lab and recorded the hot end temperature running the same G-Code file.

In the plot, the blue line is the bad printer from Matterhackers, the orange line is the Prusa Research printer that I fixed by replacing the hotend thermistor.  The first plateau is 170 deg C during the probing cycle, then the part is printed at 230 deg C for PLA.  On both printers, a K-type thermocouple was wedged into the front facet of the hot end under the silicone sock.

The bad hotend thermistor was consistently 20-30 degrees under the setpoint.  This meant:

1)  I had a hard time with bed levelling; a lot of times when it's cleaning it would report a dirty nozzle because the nozzle was only 140, 150 deg C.
2)  The print would print for a few layers, and then detach off the bed because I was barely printing at 170 deg C.

To Prusa's credit, they are shipping me new replacement thermistors.  But it surely looks like a bad batch of thermistor made it past QC and into printers into the field, and if someone else has the same issues... well, this is one thing to check.

-=- Terence

Posted : 02/07/2025 5:41 am
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