Metcal MX500P-11 acting strangely

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My good old Metcal MX500P-11 station is starting to misbehave

When I turn it on I get the green light
If I remove the tip it turns amber
After tip reinstall and a power cycle it turns green again
So far so good

But it only heats the tip for a few minutes (still green light)
First I suspected the tip, and changed to another one (which was brand new) and it started heating up
But all my old tips are dead, it seems that new tips are working

Any suggestions?

Need to find a suitable screwdriver to take it apart

[thm_w]

Did you measure the resistance of the tips, to see if they've gone open circuit?

So the new tip works for a long time now?