MakerGear Forum

Was printing ABS tonight with bed temp of 100c when I went to check it the part had broke loose from the bed. Looking at octoprint temp graph showed the temperature had dropped to 29C heading for ambient. Set temp was still 100c. The heater led6 on the Rambo was not lit. Great i think, MOSFET is blown. But, then I found the heater 12v was pulled down to 0v. Fuse ok, then I unplugged the board and the led came on. A resistance check of the heated bed showed 0.00 ohms, or a dead short. That's what is pulling down the heater 12v.

So, anybody had this problem and know how to fix it. Really can't afford $80 bed.

tj

9alfred99 wrote:anybody had this problem and know how to fix it

Order a new thermistor, install it, and your problems will Magically Go Away. Until then, DO NOT flash new firmware, mess around with slicer settings, or change anything else.

While you're waiting, you can verify the heater isn't causing the problem …

The 12 V version of the heater has cold resistance around 1 Ω, so an ohmmeter set to a scale suited for "normal" resistors will read 0 Ω; autoranging meters often misread low values. Set the meter to the lowest scale (maybe 200 Ω or so) and short the probes together: if it doesn't read about 0.2 Ω, due to the resistance of the test lead wires, then it's probably not sensitive enough to measure the heater. If you see a shorted-probe reading around 0.2 Ω, then the heater should show a resistance around 1.2 Ω when measured directly at the pins on the red connector, not at the end of the cable on the RAMBo board).

So maybe the heater is OK.

The heater draws about 10 A in normal use, which can destroy the connector at the RAMBo board. If it looks charred or melted, you've found the problem. If not, make sure those little screws are nice & tight, because that problem definitely lies in your future.

Also check the connector & screws on the heater cable where it plugs into the RAMBo, because those can fail, too.

A broken wire in the cable between the heater and the RAMBo can mimic a dead heater. Measure the end-to-end resistance of each wire (unplugged at each end) while wiggling & jiggling the braid: the resistance should be near 0 Ω (equal to the shorted-probe reading) and stable while you flex the wires.

If it's the heater cable, you replace the wires with flexy 12 AWG silicone sheathed wires made popular by RC drone builders. Not cheap, but much better than the common (and stiff) PVC insulation. You want the "soft & flexible" wire with hundreds of fine copper conductors, not the common 19-ish strands in ordinary 12 AWG wire.

Most likely, the heater & cable will be fine.

The thermistor's cold resistance should be around 100 kΩ, measured at the end of the cable where it plugs into the RAMBo. If that resistance varies as you flex the braid, then there's a broken wire that needs replacement.

If the resistance is constant, but not within maybe 50 to 200 kΩ, then the thermistor has failed and the one you just ordered will save the day …

"Order a new thermistor, install it, and your problems will Magically Go Away. Until then, DO NOT flash new firmware, mess around with slicer settings, or change anything else."

Do you just have that bound to a keyboard macro?

insta wrote:Do you just have that bound to a keyboard macro?

Rated: **** 7/10, would type again

I kinda wish it weren't the right advice so often, though.

The thermistor seems to be the most failure-prone part of the whole printer. Perhaps if the RepRap project had sprung for thermocouples, back in the day, we'd all be better off.