Heatbed temperature keeps increasing.

I have bought a second hand printer about a month ago (was the best option for me when cost is concerned). The vendor owns a hobby shop and had this printer he built for sale.

The printer is a coreXY configuration, running on a RAMPS 1.4 kit. It is also running on repitier firmware. I use Cura to do the slicing.

The printer printed fine for a wile with no (observable) problems. A few days ago when I started a new print I could smell electronics burning and I observed the heatbed temperature was increasing past the set temperature of 45C. I turned the printer off at the pain power supply and switched it back on. When I turned on the heated bed it did not heat up at all. Reading other forums I understood that the MOSFET was most likely the culprit (I have measured the fuses and they were functioning correctly). I replaced the MOSFET on the RAMPS board and carefully measured everything around it to ensure that I had not accidentally shorted something in the process. I replaced everything and turned the printer back on. Everything worked fine, the heatbed LED on the RAMPS was off. I turned on the heatbed and it heated up. Soon, though, the heatbed temperature increased again passed the set temperature and I immediately turned the main power off. I disconnected the heatbed and measured it and it had a resistance of less than 2 ohm. With the heatbed disconnected I turned the printer on and immediately the heatbed LED on the RAMPS was on.

I have printed a part, with the heatbed disconnected, and everything works fine (except the heatbed).

Will I need to replace the RAMPS board? Is there any thing else I can try and do before replacing the RAMPS?

Its still probably the MOSFET..

1) the mosfet listed on RAMPS BOM is a P55NF06L, many cheap ramps come with a P55NF06 instead, this is bad.
2) most "replacements" being sold are also P55NF06 not P55NF06L they don't turn on fully at logic levels and over heat and burn out.
3) the P55NF06L isnt that good to start with and should be replaced with a IRLB8743 (be weary of counterfeits)
4) you can also use a external mosfets eg here This takes all the load off of the RAMPS and moves it to the external MOSFET

I tend to replace the MOSFET with a IRLB8743 and run an External MOSFET.

Edited 1 time(s). Last edit at 04/26/2019 10:13AM by Dust.

Also, make sure that the tabs on the three MOSFETs are not touching each other. I've seen several instances where they were touching, and (for example) turning on the part cooling fan would also end up turning on the bed!