Heatbed Thermistor Issues

My printer started going into dry mode because of a heatbed thermistor error. I removed the thermistor and found that it had split. The bad thermistor was replaced with what I believe is the same type of thermistor (both purchased at one time from same vendor, both in same bag, but no data sheets).

With no FW changes, the new thermistor shows -50deg temp - causing default.

New thermistor appears to be working ok (115K at 23deg and goes down when heated).

Using the form provided on the [reprap.org] page, I was able to calculate the Beta value and then used the createTemperatueLookup python script to create a new table.

Using the new table, my heatbed temp is correct at startup but it never seems to change.

I can turn on the heatbed and it does heat, but the temperature reading never changes and the heatbed won't turn off without a reset.

Also, when viewing the Temperature Curve in Repetier, the heatbed output graph does not change when turning on/off the heatbed (like it does when I do the same thing with the extruder).

I have more thermistors on the way but right now I'm not sure if it's a thermistor, RAMPS or firmware issue.

I am testing using version .83 of the firmware because the remarks in the configuration.h file make it easier calculating and setting the thermistor tables.

Thx

So I decided to try walking through the .91 config, using a generic thermistor chart and my calculated Beta value.

Temp now reads -20deg and still no display change if I turn on the heatbed. But I can now turn it on/off manually and the heatbed output graph shows power output and a blue line that goes on/off with the average temp checkbox. The top graph will show the heatbed target temperature (when heatbed is on) but no actual temperature readings are plotted (temperature display in manual mode, or on LCD, never changes from -20deg).

I have to manually maintain the heatbed temp, but at least I can print again.

Hopefully the new thermistor(s) will resolve the problem completely.

There is a chance that the user generated table is created in the wrong order letting it stick at the lowest temperature. I have to check that.

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Well, I had a feeling this wasn't a thermistor problem.

Received my new thermistors today, installed one in the heatbed, changed the type (now ATC Semitec 104GT-2 [type 8]) and fired up the printer: -20deg reading on the heatbed - WTF!!!

Time to debug. Switched the pins/thermistor types with the extruder (which was reading correctly) and readings switched - not a problem with the board.

Got out the multi-meter and started checking wires. Soon found that I had no continuity on one of the heatbed thermistor wires leading to the RAMPS board. When the original thermistor shorted it must have taken the wire with it.

Ran a new wire, and I now have the correct heatbed temperatures being detected/displayed with either thermistor installed (1 or 8).