Intermittent heating of hotend and bed
My Mk3 worked perfect for about a year but all of a sudden I am getting errors and having issues getting both the hotend and extruder to heat. Sometimes the red lights on the that indicate power is going to the hotend/bed come on and sometimes they won't. Sometimes one will turn on sometimes the other, sometimes neither. If the extruder does heat up it will turn off just after the fan turns on during the second layer (fan speed 50%)
-I have gotten both thermal runaway and preheat errors.
-After I started having issues I upgraded to firmware 3.8.1 - no change
-measured hot end heater resistance = 14.1 ohms
-measured hot end thermistor resistance = 122k ohms at just above room temp.
-at one point when I could get the hotend to come on it measures 24.1v at the connector
-all three fuses on the control board are fine
-both thermistors seem to work fine when tested with a hair drier.
-I unplugged the black/white power panic cord and tried to print - no change
-During the most recent attempt the bed heated up to 90C. The hotend made it to the target temp of 245c while the bed was heating but by the time the bed finished the bed was struggling to stay at 225, then the red light went out and the extruder temp dropped (while still showing a target of 245c)
I am leaning towards a control board or power supply issue but I am pretty much stumped. Any help would be greatly appreciated.
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RE: Intermittent heating of hotend and bed
What's the power supply doing while this is happening? Staying at 24v or moving around?
Also - if you metered the bed when it seemed to be working and saw 24v, what was the voltage when it wasn't heating?
The bed fuse to the PS or EINSY drivers could be failing; but more likely the PS itself. But something to check.
RE: Intermittent heating of hotend and bed
1. Replace the wires.
2. Check all connectors.
3. Check PSU.
I'm always wandering why people assuming firmware and board first. This are always last items on my list. My bet is on wires, but surprises never end.
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RE: Intermittent heating of hotend and bed
I would be more suspicious of the wiring if it was only the heat bed or only the hot end.
For two wires, in different bundles, to fail simultaneously strains credulity.
Hang a voltmeter on your PSU and watch it during some failure cycles.
If that is not it, contact Support and get them to help you with board debugging.
RE: Intermittent heating of hotend and bed
Thanks for the suggestions! I just measured voltage across across each of the two pairs of PS wires that go to the board. Each pair remained at 24v while two failure cycles occurred. Both times the hotend decreased in temperature and the red light by the hotend heater output stopped flashing followed a little while later by a thermal runaway error.
The first time (also the first time i had turned the printer on today) it printed for a while after the fan kicked on before the error happened. The second time happened much quicker and didn't even make it to the fan turning on.
RE: Intermittent heating of hotend and bed
The bed shorting (a wiring problem) crowbars the power supply. The bed circuit is fused at 15 amps, the drivers can do 50 amps. The power supply shuts down at 10 amps.
So replacing the wiring is not unreasonable. But I'd want to know where the problem is before shot-gunning it. Otherwise, you don't really know it was fixed.
pps: and it might be a thermistor wire that is intermittent; that can cause all sorts of oddities.
RE: Intermittent heating of hotend and bed
Update on this issue: I replaced the hotend thermistor and heater cartridge (I ordered the parts from Prusa so I wanted to just do both while I was paying the shipping). I have a lot of print time since the repair and the problem is solved. I have believe it was the hotend thermistor that was the problem.
Thanks for the help!