Hot-end fluctuation (Solved)

Hi Gang,
I'm a bit puzzled here...

If I heat the print bed up to say, 90 degrees, then check the temp (according to Pronterface) it remains very close to target i.e. it stays at 90c +/- 1c.
The hot-end however, if I set to say, 230c will heat to around 245+ then drop to around 190c before it heats again. The same fluctuation occurs at lower temps, e.g 185c (goes to maybe 220 then back to maybe 165 before heating again. This can't be right and would lead to severe extrusion problems I would have thought. I have changed the thermistor but the behavior is the same. Anyone have any suggestions?

Thanks

Edited 1 time(s). Last edit at 02/14/2013 07:04PM by waitaki.

I would have thought they would use the same circuitry for both, they use the same output transistors in any case. Yes it sounds odd to me too. maybe check that they thermistor is tight in the head?

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Richmond, New Zealand
Thingiverse ~ YouTube

Well, I removed the heated bed wires and bed thermistor input and put the hotend connections there. i.e put the hotend wires in D8 and the thermo in T1.
Then switched on the "bed" so to speak (100c),and the hotend temp rose to just past 100 but stayed within acceptable parameters. This leads me to suspect that the ramps circuit for the hotend bit has issues - maybe a dodgy resistor? Shame because I'm almost printing...

Well, would you believe, it was a resistor on the Ramps board.
R13 which links to leg1 on the Q1 mosfet (gate?) had a dry joint so just touched with the iron and all good now - rock steady temp!

Cool, makes you feel 10 feet tall when you find a real problem and fix it.

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Richmond, New Zealand
Thingiverse ~ YouTube

Yeah Simon, bit of a relief there!

By the way, have I got this right...
If I set the print bed/ nozzle height to one sheet (0.1mm) of paper, in the home position,and then send a job to the printer with a 0.2mm layer height, does that mean I should be able to measure a 0.3 mm gap when it starts? (assuming no plastic is being fed of course)