Page 1 of 1

No Heated Bed = Power Connector Fail

Posted: Sat Jul 11, 2020 10:15 pm
by Mach
M2 Rev D purchased November 2015. Its been in climate controlled workshop or an air conditioned house since that time. The last year I've been printing 20 hrs a day 5 days a week. Primarily PETG with heated bed at 90c. The power connector positive socket has failed in a surprising way. I am looking for advice on what could be the cause? Has anyone seen this before? Should I scrap the original Rambo board? Would a short in the board or heated bed cause this?

Diagnostic details below.

Comparison to a new power supply (bottom), the top connector second socket (heated bed positive power input) from the left looks darker and wider.

The Rambo board positive power input pin shows discoloration not present on any other pin.

Dismantling the power connector, shows corrosion that I'm guessing is from arcing? The one next to it is the negative power input socket.


Recently the bed stopped heating mid print. I searched the forum but did not see this problem before.
Steps to diagnose
1) Power connector checked for 24V at the terminal screws on the power connector - Verified to be receiving 24V
2) Replaced bed thermistor with known good thermistor. Both new and old read similar 80-85 and responded to hand temperature change - No change
3) Replaced heated bed with known good heated bed. - No change
4) Replaced Rambo board with known good board - No change
5) Replaced PSU with known good psu - Heated bed successfully heating
6) Replaced old PSU power connector with new power connector - Heated bed successfully heating

I've swapped back in the old heated bed and confirmed that its now heating as expected. My next step is to swap back in the old Rambo board but wanted to check first if that's wise.

Re: No Heated Bed = Power Connector Fail

Posted: Mon Jul 13, 2020 1:35 pm
by ednisley
Mach wrote:
Sat Jul 11, 2020 10:15 pm
printing 20 hrs a day 5 days a week
Yup, industrial usage on a consumer-grade machine will do that.

Bonus: The wiring to the extruder and platform has surely worn out, too. While the machine is dead, take the looms apart and see what's going on inside. If (when!) you find any chafed wires, it's time to replace them, too.

AFAICT, the only permanent solution is runing the platform heater current through an external DC-DC solid-state relay, with control from the original MOSFET on the RAMBo. This was back when my M2 had a 12 V supply: ... hermistor/


I'm now using a 30 V hotrod platform, but the same wiring applies.

You must connect the platform heater power supply pin on the RAMBo to one of the other supplies:


Given the wrecked pins on your board, you may need to solder a jumper on the underside of the board; anything will suffice because it's only carrying a few mA to let the MOSFET control the SSR.

Essentially all eBay-grade SSRs are counterfeit Fotek units, so don't believe their max amp ratings.

Re: No Heated Bed = Power Connector Fail

Posted: Mon Jul 13, 2020 2:49 pm
by Mach
Thanks Ed, I appreciate the advice. I inspected the wiring on the extruder earlier this year when the connector failed at the stepper motor. It looks ok but I may take the opportunity to replace it with new wire. The HBP wiring is ok but I've got a new HBP unit that I can swap in just to be safe. I swapped back the old Rambo. So far no problems.

Thanks for the link. I'll take a look at the SSR option.