Heat bed suddenly stopped working  

Heat bed suddenly stopped working

In the middle of a print, the heat bed suddenly stopped working. Everything works fine with the exception of the heat bed

Troubleshooting:

• LED on heat bed connector - No
• LED6 on mini-Rambo - No
• Power input (X10 AB, and X10 CD) – 12V, Molex connectors ok
• F3 Fuse (15A) – Ok
• Terminals on heat bed - Ok
• Heat bed cable – Ok
• Heat bed resistance – 0.9 Ohm
• Heat bed connector (X3 AB) – OK
• 12V3 – 10.55V when idle, 11.85V when heat bed and hot end is set active

I have gone through the wire diagram and verified all component, where everything seems to be ok. The mostfet is difficult to test without de-soldering it, but there are no short circuits which would be natural of a mosfet failure.

What I did found was that the PWM output (port 1) on the Atmel I only gave 0,85V when heat bed was supposed to be on. When the target setting is set well above the thermistor readback, I would expect something in the range of 4.5-5V?! The PWM output (port 2) for the hot end shows 4.99V when activated.

I have done a factory reset and upgraded to the latest firmware – heat bed still not working

This is actually the second time this happens – last time I replaced the mini-rambo with a second hand piece I got my hands on.

Any thought? Please help.