Bed not heating after years of sterling service
My Original Prusa MK2 has been working brilliantly for a long time now and I suppose I was about due for an issue. This one has me stumped.
The bed stopped heating mid print but then worked on the next one. It now does not heat anymore.
It's a miniRambo board and I replaced the troublesome heater plugs a year or so ago when they were melting. Since then I have printed literally miles of material.
Here's what I know for sure:
1) Everything else appears to work fine, including nozzle temp.
2) The led on the heat plate does not light when it is supposed to be heating.
3) When I apply 12 volts from another source to the heat bed, it lights, heats and the thermister registers the temp increase as displayed on the LCD..
4) All fuses are good.
5) The connectivity from the board to the bed has been checked.
6) I measure 2.7 volts across the heater output
I sometimes get the red LED6 to illuminate. No idea what that means or how I get it to come on.
Any ideas where to look? Blown output driver?
Solution, a replacement board?
Any suggestions appreciated.
Re: Bed not heating after years of sterling service
3) When I apply 12 volts from another source to the heat bed, it lights, heats and the ....
How exactly are you doing that?
Do you apply the power directly at the bed where the wires are soldered on?
Or do you do that at the cables/conector?
That one by the way often fails and melts which could exactly be your problem.
I would guess it is either this or broken cables.
Re: Bed not heating after years of sterling service
Hi Robert,
if you move the heatbed to the front and try to heat it, does the BedLED illuminate?
if so, if you slowly push the heatbed backwards, during the early stage of heating, does the LED go out?
I suspect that one of your heatbed wires has failed through flexing. this has happened on both of my prusa's
replacing the wires with Flexible Silicone insulated radio control model wires has cured the problem for me
I believe the bed wiring is 16awg for the heater and 26AWG for the thermistor
typical domestic flex has 7 strands of wire in each conductor,
Prusa heatbed wire if the same gauge has 21 strands of wire in each conductor, which makes it more capable of resisting fatigue from flexing,
Flexible silicone wire has 252 strands of wire in each conductor, making it much more resistant to fracturing due to flexing
replacing the wires may well resolve your issue.
regards Joan
I try to make safe suggestions,You should understand the context and ensure you are happy that they are safe before attempting to apply my suggestions, what you do, is YOUR responsibility. Location Halifax UK
Re: Bed not heating after years of sterling service
To clarify 3)
I applied 12v from a Liion battery directly to the solder connections of the plate to see if the temp rises and is measured but the thermister and displayed on the LCD. It worked as it should.
Cable issues.
I cannot get the plate LED to light at all.
Cabling was certainly my first thought (after the fuse) but I have measured the resistance from the board pads to the plate connections and they are about 100mOhm (0.1 ohm) on both wires. When I replaced the the bed connectors a while back I used silicon wire AWG16 that I use in building drones. I only replaced this at the board end with bullet connectors to the existing wire. What I will try, despite the resistance measurement, is replacing the whole wire from board to plate - I have nothing else to try at the moment.
Has anyone ever had a power stage (I assume a power mosfet drives the heater) go out?
Thank you for your inputs.
Re: Bed not heating after years of sterling service
I just looked at the miniRambo schematics at https://github.com/ultimachine/Mini-Rambo/blob/1.3a/board/Project%20Outputs%20for%20Mini-Rambo/Mini-Rambo.PDF and see that the output stage has the aforementioned LED6 in series. That needs to illuminate when the bed is heating (on the board). I'll keep looking.
Fixed: Bed not heating after years of sterling service
After re-soldering the heat bed connections and doing all I could think of I bought a new mini rambo board. It's all up and working fine now. I had either messed up the tracks on the board soldering directly to it for the heat bed (although it looked OK and measurements of resistance were OK) or the power mosfet had given up the ghost.
I am back in business. Thanks for you suggestions.
Re: Bed not heating after years of sterling service
shame the fix was so expensive!
regards Joan
I try to make safe suggestions,You should understand the context and ensure you are happy that they are safe before attempting to apply my suggestions, what you do, is YOUR responsibility. Location Halifax UK