Bed Not Heating M2 Rev. E

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KaiborgStudios
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Bed Not Heating M2 Rev. E

Post by KaiborgStudios » Sat Jan 04, 2020 10:18 pm

Hello!

Some background info and what I've already tested:
-Heatbed and thermistor wires have been completely removed from sheath, no sign of breakage or wear.
-Heat bed ready a consistent 3 ohms even while wiggling up and down wires.
-Thermistor reads 100 kOhms and adjusts to temperature from my hand
-Extruder heats up fine.

-with both thermistor and headbed wires plugged into rambo, when heatbed is turned on via software, red heat LED does not turn on, and low voltage is read at heatbed connector on rambo. As soon as I unplug the heatbed connector, leaving the thermistor connected, heat LED turns on and connector outputs 24v.

Thanks for your help!

Cameron

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ednisley
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Re: Bed Not Heating M2 Rev. E

Post by ednisley » Sun Jan 05, 2020 3:42 pm

KaiborgStudios wrote:
Sat Jan 04, 2020 10:18 pm
heat LED turns on and connector outputs 24v
Excellent troubleshooting!

The heater cable connector on the RAMBo has pins for the +24 V supply and the MOSFET switch; when the MOSFET turns on, it connects its pin to "ground" = 0 V. As a result, a multimeter across the connector will read 0 V with the switch OFF (both terminals at 24 V) and 24 V with the switch ON (MOSTFET at 0 V), exactly backwards from what everybody expects.

In your M2, the LED stays off when the heater is plugged in and (supposedly) turned ON, which means the MOSFET can't pull its pin low enough to light the LED. That suggests a moderate short circuit between the cable wires or (possibly) inside the heater pad, enough to force the MOSFET pin high (and keep the LED off), but not quite enough to blow the power supply fuse. Because the short keeps the current out of the heater, there's no heat where you expect it, but the cable may be warming up.

Because you say the wiring looks good, I'd inspect / measure the connectors and the heater mat for obvious problems. Although it's not recommended, you can probe along the length of a wire with an ordinary pin: poke it though the insulation to make a connection at that point. If the wire-to-wire resistance is the same at both ends, then the problem is in the heater.

In that case, you should contact MG support to find out what the heater resistance should be before ordering a replacement. I think 3 Ω at room temperature is too low (which is why I think there's a short), but I don't have one of those heaters for comparison.

Good hunting …

KaiborgStudios
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Re: Bed Not Heating M2 Rev. E

Post by KaiborgStudios » Sun Jan 05, 2020 10:56 pm

Is this the mosfet in question, and is it supposed to be covered in dark brown stuff? Already wiped a bunch off the top of the chip.
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20200105_135532.jpg

KaiborgStudios
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Re: Bed Not Heating M2 Rev. E

Post by KaiborgStudios » Sun Jan 05, 2020 10:59 pm

Additional testing :
Tried swapping the heat bed connector and the extruder connector themselves, same issue, so the connector hasn't failed.

Tried the pin method all up and down both wires and the resistance stayed around 3ohms.

Tried tugging up and down both wires, if there's a weak point silicone wires should break super easily in my experience.

KaiborgStudios
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Re: Bed Not Heating M2 Rev. E

Post by KaiborgStudios » Mon Jan 06, 2020 12:32 am

Just thought to swap the bed and extruded connection on the Rambo, and the bed heats right up. Rambo must be the problem, I'm assuming that mosfet. Can I replace just the mosfet or is that a symptom of a bigger problem?

jferguson
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Re: Bed Not Heating M2 Rev. E

Post by jferguson » Tue Jan 07, 2020 2:26 am

Hi,
I have same problem. I'm not at all certain whether bed or RAMBO is bad. I hadn't thought of trying the bed on the extruder heater connection, but have ordered both a new RAMBO and a new bed. I will try the new bed on my existing RAMBO and if it works, which will confirm there is nothing wrong with the RAMBO, I'll keep the new one as a spare. If it doesn't work, then I'll know problem is with RAMBO and having an extra bed isn't the end of the world either.

There was some Ed NIsely advice a bit back about how problem could be that the wiring between the connector and the bed itself could be bad and you wouldn't see it because it's buried in the loom. I took it out and it appears there is nothing wrong with it, physically.

So I'm guess I've got a bad RAMBO.

I may try to troubleshoot the bad one if that is what is wrong. Replacing MOSFET doesn't sound too difficult. I have the equipment.

It should be interesting.
john

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ednisley
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Re: Bed Not Heating M2 Rev. E

Post by ednisley » Tue Jan 07, 2020 8:10 pm

KaiborgStudios wrote:
Mon Jan 06, 2020 12:32 am
Can I replace just the mosfet or is that a symptom of a bigger problem?
Given that picture, yeah, things look grim in there. The fuzz is normal, but the MOSFET looks like it's been way too sweaty.

The MOSFET is firmly soldered to big copper pours on both sides of the PCB specifically designed to suck heat out of the solid tab on the chip, so, unless you're set up with PCB rework equipment, you're not gonna get that sucker outta there.

In principle, you could re-flash the microcontroller firmware with the Extruder Heat 1 output pin assigned to the Heated Bed function and move the cable over. That might be with considering, although it'll probably take a while to get all the firmware configuration options sorted out if you've never done such a thing before.

The least awful outcome: a new RAMBo is in your future.

KaiborgStudios
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Re: Bed Not Heating M2 Rev. E

Post by KaiborgStudios » Tue Jan 07, 2020 9:09 pm

Thanks for all your help Ed!

Talking to support over the phone, unfortunately I can't swap the connector via firmware, as the heat bed uses that 15A blade fuse, which none of the other connectors are wired to.

I've got some replacement mosfets arriving tomorrow and I'll give that a shot before condemning myself to a new board.

Cameron

jferguson
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Re: Bed Not Heating M2 Rev. E

Post by jferguson » Wed Jan 08, 2020 1:09 am

Hi gentlemen,
My M2 lives in the shop which is remote from home. I often turn it loose printing something which will take 18 hours, and then am not able to get back to it for a day, which means the RAMBO is powered up but the heaters have cooled down. So my question is whether there is any internal load in the MOSFET which would suggest that extended idle may not unload it, so that it could overheat even though it wouldn't be doing anything.

Is this possible?

John

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ednisley
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Re: Bed Not Heating M2 Rev. E

Post by ednisley » Wed Jan 08, 2020 11:20 pm

jferguson wrote:
Wed Jan 08, 2020 1:09 am
Is this possible?
Nope, barring unfortunate events like power glitches and component failures.

When a MOSFET is OFF, it's not conducting any current and dissipates no power.

Conversely, when it's ON, there's (essentially) no voltage across its terminals, so it's (ideally) dissipating no power, either.

Trouble happens when it's supposed to be ON, but an external dead short to the power supply forces both high current and nonzero voltage, whereupon it gets all sweaty (for a while) and fails.

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