Ramps 1.6 problem

Two of my RAMPS 1.6 boards have suddenly broke
(one broke at first run and other broke after printing several weeks)
- bed is 200*200
- turns on when connected with usb
- two printers in similar setups/same settings works fine
- replacing mega 2560 did not help
- 12V setup

I found that F2 (i assume that is fuse) was not conducting on those broken ones and soldered it close.
I also redused bed heater cycle down to 50/255.

When heating bed it start to broke 15A fuse between board and psu.

???

Edited 2 time(s). Last edit at 06/13/2021 06:51AM by Hexentric.

Since the heated bed is only a pcb, and the tracks are only covered in a thin layer of lacquer. make sure what ever you mounted the bed on hasn't rubbed through this layer, on top or bottom, causing a short

Edited 1 time(s). Last edit at 06/13/2021 07:28AM by Dust.

Element is under glass and measured R is 1,3 ohm when wires connected or disconnected (same as my few identical/unused heating elements)

Edited 1 time(s). Last edit at 06/13/2021 07:58AM by Hexentric.

well that circuit only goes through the fuse, the mosfet then the bed.
There has to he a short in there some where...

I assume it was heating cartridge.

I changed 20A fuse on power in and find that both beds heated up fine but hot ends had problems.

printer a
When heating hot end one mosfet (furthest from power in) got very hot.
Cartridge seems shorted and mosfet is constantly in conductive state.

printer b
Cartridge gives random R values and printer shows "heating failed" when heating hot end.
Mosfet seems to work because another cartridge heats up.

Cartridge in picture is similar as broken ones.

Edited 1 time(s). Last edit at 06/14/2021 06:51PM by Hexentric.

Fuse 2 is the heated bed only.

well, after changing also hot end thermistors both printers work