Bed thermistor failure. Negative temperature readout (please help)

Hi all, first time poster here and very inexperienced with software and configuring it.

I have a very old Mendel with a Melzi 1.0 board (original from 2013)

The other day the bed thermistor failed and the bed did not heat. That's understandable.
I replaced the thermistor with another of the same type. The temperature is reading about -19C. The thermistor checks out at about 100k ohms.
I swapped the bed and hot end thermistors but the problem is still there.
The bed thermistor does detect changes in temperature but because its reading 30 lower than ambient temp it's preventing the bed from heating up.
Any ideas what's going on? What can be done to fix it?
Any help is appreciated as it's stopping me from completing a very important project

a very low temperature like that is indicative of a break in the thermistor wire somewhere

Check the resistance through all cables.. not just at the thermistor

If that is reading correctly (which it sound like it is) , its the controller ADC port that is damaged...

Hi Dust,

Yea I checked the resistance at the cable ends and all my stocked thermistors and the one fitted to the hot end all read about the same, 100k ohms at room temp.

If the ADC port is damaged, is there a way of fixing it?

It seems odd that once the thermistor is plugged in, it starts off reading about -19C but the temp elevates then I hold it in my hand. So it does seem to react correctly, it's just the start value is low.
Is there a way to fudge the start temp in the controller configuration?

Thanks!

Quote
Dust
a very low temperature like that is indicative of a break in the thermistor wire somewhere

Check the resistance through all cables.. not just at the thermistor

If that is reading correctly (which it sound like it is) , its the controller ADC port that is damaged...

the ADC is part of the processor on the controller....

You can replace the processor, If you can do surface mount electronics, but realistically a 1284p or 644p is no longer worth the time or effort.

Re flash your firmware in case its somehow lost its programming (very unlikely)

But most likely it is time to consider a new controller. I would strongly recommend a 32 bit controller.

Edited 1 time(s). Last edit at 12/24/2019 06:34AM by Dust.

Thanks for your help Dust,

I had a feeling that would be the case. I've been looking into new controllers.
What would you recommend as a replacement? I've been looking at maybe a Duet2 or a smoothieboard

Thanks!

Quote
Dust
the ADC is part of the processor on the controller....

You can replace the processor, If you can do surface mount electronics, but realistically a 1284p or 644p is no longer worth the time or effort.

Re flash your firmware in case its somehow lost its programming (very unlikely)

But most likely it is time to consider a new controller. I would strongly recommend a 32 bit controller.

Can't comment on duet, its way outside my price bracket.

I wouldn't go for a real smoothieware board anymore...

But I do like the bigtreetech SKR 1.4 (uses lpc1768 32 bit cpu) Cheap, socketed stepper drivers supporting modern drivers, has all the basic features... well supported by Marlin2

Thanks Dust,

I'll look into it!

Quote
Dust
Can't comment on duet, its way outside my price bracket.

I wouldn't go for a real smoothieware board anymore...

But I do like the bigtreetech SKR 1.4 (uses lpc1768 32 bit cpu) Cheap, socketed stepper drivers supporting modern drivers, has all the basic features... well supported by Marlin2

I'm trying to remember... I had this same exact issue on my skr 1.3 but I don't remember how I fixed it. I'm on my mobile right now, but I can try to find it back later on my laptop.

Also, the skr is a very nice upgrade and dirt cheap so it wouldn't hurt to get one.

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