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BED PREHEAT ERROR was a toasted fuse  

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spark
(@spark)
Reputable Member
BED PREHEAT ERROR was a toasted fuse

My heated bed stopped working after an attempt to calibrate the temperature.  Returned to find BED PREHEAT ERROR.  Talked to chat and pointed me to check fuses again.  Yes it was a charred bed fuse but also the LED was melted like a fuse too.  Replaced the fuse from my emergency kit in my car but the LED is another matter.  But perhaps I should start to find out what causes the melt-down.

What is the LED and can I replace it?

What tips other than found on https://help.prusa3d.com/en/article/multimeter-usage_2117 should I do to find the source of the over-voltage/current at the fuse?

Thanks guys!

This topic was modified 12 months ago 3 times by spark
MK2S kit owner since 8/15/2017...
Posted : 28/12/2020 12:33 am
spark
(@spark)
Reputable Member
Topic starter answered:
RE: BED PREHEAT ERROR was a toasted fuse

Sorry, here is the picture using forums own server.

blown MK2.5S bed fuse

MK2S kit owner since 8/15/2017...
Posted : 28/12/2020 12:40 am
spark
(@spark)
Reputable Member
Topic starter answered:
RE: BED PREHEAT ERROR was a toasted fuse

Test print worked well.  Second PLA print failed due to Bed thermal runaway.  Out of ideas.

MK2S kit owner since 8/15/2017...
Posted : 28/12/2020 8:44 am
spark
(@spark)
Reputable Member
Topic starter answered:
RE: BED PREHEAT ERROR was a toasted fuse

Went though https://help.prusa3d.com/en/article/multimeter-usage_2117.

Just for academic purposes...

  • Hotend heater resistance, 3.4 kΩ, within spec.
  • Bed heater resistance, 0.8 kΩ, within spec.
  • Hotend thermistor resistance (not relevent)
  • Bed thermistor resistance at 19°C ambient, 121 kΩ.
  • Rambo shows 11.25V at both heater headers with no load.
  • Rambo shows 11.55V at both heater headers with extruder heater connected only.
  • Rambo shows 11.55V at extruder but 3.1 mV at the bed heater when both heaters are connected.  Also, when bed heater is connected a red LED on the Rambo turns off.  If the bed heater is unplugged, the bed's receptacle returns to 11.55V with the LED turning back on.

I resoldered the LED which was working for the one PLA good print I got from replacing the 15 Amp blade fuse for the bed heater.  The LED was working perfectly for that one so I don't think it is a resoldering issue.

Any help?   I need to print MK3 parts.

MK2S kit owner since 8/15/2017...
Posted : 28/12/2020 7:45 pm
spark
(@spark)
Reputable Member
Topic starter answered:
RE: BED PREHEAT ERROR was a toasted fuse

Conclusion for LiveChat is a failing power regulation on the RAMBo 1.3.  It is discontinued so this may be the end of the road for my MK2.5S unless I can repair myself but what is wrong and what can I reflow/solder?  Which MOSFET which capacitor?  I'd prefer not to go on a wild goose chase when I need a working printer by February.  It would seem this board is a bit of a ghost town as well so this note in a bottle will be left to float but I'm moving forward with an upgrade or new printer...might not be a Prusa.

MK2S kit owner since 8/15/2017...
Posted : 30/12/2020 1:53 am
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