Bad bed thermistor?

A couple of days ago I started having a weird problem with getting the bed up to temperature. Up to about 65 degrees the temperature increases in a normal fashion, but then the web interface reports that it drops by 5-7 degrees suddenly, and fluctuates by +/- 2 degrees, before jumping back up to a higher temperature. As I normally print at 70C this often means it jumps up to around 75 degrees, then when it cools to the target 70 (and overshoots a little) it will sometimes drop by a few degrees again, and it gets stuck in a cycle. Eventually it does settle at 70 and will remain there for a whole print, but it means the warm up time can be up to 10 mins longer than normal. I've found that if I catch it around 75C and then command it to 74C, followed by 73C and so on I can get it to stabilise down to 70C.

This happens with the hot end heater on or off, and I'm running the Duet on 13.8V. I've checked the underside of the bed and there is no short on the thermistor and the PCB coating is still intact. Is my thermistor on it's way out?

Cheers,
static65

Which version of the Duet are you using, and which firmware version are you running? Some older firmware versions suffered from crosstalk between ADC channels when run in the Duet 0.8.5.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

I'm running 1.09m-dc42 on a Duet 0.6.

1.09m is quite old. Try a more recent firmware version, in case you are seeing a similar crosstalk problem to the one that particularly affected the Duet 0.8.5.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Righto, I'll give it a go when I get some downtime.