Omerod 1 Nozzle Temperature slow to rise

On my Ormerod 1, when I set the nozzle temperature to 205 degrees say, the temperature rises fairly quickly up to within 5 degrees but rises little or no further. As a result a timeout is exceeded and I get the message that heater 1 has not reached temperature in 150 seconds. I need to switch the machine off and on again in order to do anything else with the nozzle temperature.
Looking at the temperature/time graph the temperature rises linearly to within 10 degrees of the target then continues at a much slower rate.
I replaced the whole of the nozzle mount including heater and thermistor but still have the same problem.
Interestingly, if I set the target temperature to 250 degrees the temperature rises linearly to about 240 degrees (passing the previous 205 degree target temperature) then slowing down and never reaching 250 degrees. A similar effect is seen setting the target temperature to 170 degrees, although in this case it does actually reach 170 degrees.
If I heat the nozzle to 170 degrees, then after a minute or so set the target to 205 degrees the temperature of the nozzle increases from 170 to 205 degrees and reaches that temperature well within the time limit.

All the above shows to me that the heater is perfectly capable of heating the nozzle to 205 degrees but something in the system is slowing the rate of rise as it nears the target temperature.
Is the power supplied to the heater reduced as it nears the target? If so, is there any way to control when and how this low power mode is initiated?

Or is there another problem?

Hello Peter

I can't advice you about PID settings but look in this string or do a search

dc42 firmware - nozzle heater PID
[forums.reprap.org]

Erik

The heating control code changed at firmware version 1.15. Now we use auto tuning. See [duet3d.com].

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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].

Thank you very much.
I am still running firmware version 1.09 so Auto-tuning is unavailable to me, so I used M301 in a 'suck it and see mode' and found that M301 H1 P10 S1.5 appeared to give optimum results for me.
I might look into installing the new 1.17 firmware when I have the chance.