My bed stopped heating.

I have a printer of my own design that uses a Rambo controller. It has worked flawlessly for well over a year...until this morning. I tried to do a print and discovered that the bed was not heating. The red light associated with the bed was not lighting. The hot end and all the motors worked normally. I checked the power connections to the bed and found no issues. I went ahead and replaced the bed female power plug just because it had been in service a long tome. Nothing worked. I replaced the Rambo card with another I took out of an older reprap that I knew to be good and the problem persisted.

I think I remember that the thirmister had to be in place for the heater to kick on. It was. I checked the resistance across the thermistor leads and got well over a mega ohm. Is this what I should expect from a cold thermistor? Am I right to assume that resistance comes down as the thermistor heats up? I don't mind replacing it, but that means ordering one and I'm not crazy about waiting a week only to find that's not the problem. There were no changes to the printer prior to this problem. It has been as dependable as sunrise.

Anybody got any suggestions of things for me to look at?

most thermistors reprap uses are normally 100k at 25c and decrease resistance with temperature increasing (ntc negative temperature coefficient)

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Dust
most thermistors reprap uses are normally 100k at 25c and decrease resistance with temperature increasing (ntc negative temperature coefficient)

I checked again and I guess I was moving the decimal in my head, because this time it reads ~99k. So I am back at squire one.

I am using a 24v power supply and I checked all three output channels and they are right on. I also did a continuity check on all three fuses and everthing there looks good. What the hell am I missing?

Edited 1 time(s). Last edit at 11/10/2016 08:08PM by charlieRC.

Does the controller read room temperature on the bed when its not heating? if so its not thermistor related

If the thermistor is reading room temperature correctly

Possibilities are
Dead io pin (replaced controller board should have fixed that)
Power wires not connected to heated bed plugs, broken wire somewhere?
Blown bed fuse (you say you checked this)
bad cables from heated bed plug to heated bed
Blown heated bed (just check continuity)

Basically stuff a multi meter into the heated bed socket, set the meter to dc voltage and turn on the heated bed (eg M140 S55)
are you seeing voltage?
yes: issue is in the bed or the bed to controller cables
no: then either a dead mosfet or io pin or fuse or just not getting power.

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Dust
Does the controller read room temperature on the bed when its not heating? if so its not thermistor related
If the thermistor is reading room temperature correctly

Yes, it shows 26c which is ~ room temperature

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Dust
Dead io pin (replaced controller board should have fixed that)

Well now that's become complicated. I took the controller out to the other machine that was working last time I used it, and now neither work. I now know that I shot myself in the foot because I just assumed that the controller from the other machine was working because it had worked last time I ran the machine. I did not fire it up before putting it into the problem machine. Now I don't know if there was some unknown issue with the board from the "working" machine that caused it to act the same way, or putting testing that controller on the non-working machine (and more to the point, the bed of that machine) might have caused the same problem in that controller as well.

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Dust
Power wires not connected to heated bed plugs, broken wire somewhere?

I get a steady 2.8 ohms of resistence across the leads from the bed heater. I am not sure what it should be, but I assume that rules out a continuity problem. I do show continuity across the leads but I assumed that is because of the low resistence. Do we know what the resistence on the bed leads should be?

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Dust
Blown bed fuse (you say you checked this)

Yes...I even took them out of the controller and checked a second time and they are good.

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Dust
bad cables from heated bed plug to heated bed

Blown heated bed (just check continuity)

Basically stuff a multi meter into the heated bed socket, set the meter to dc voltage and turn on the heated bed (eg M140 S55)
are you seeing voltage?

Little or none. Once of the boards gives me 1.9v and the other nothing.

I don't know what else to do except throw money at it. I guess I will order another Rambo board and because the bed heater might have blown both boards out, I will need to replace that as well.

Still not sure what I should expect for resistance across the bed heater leads. Does 2.8 ohms sound reasonable? Not being a electonics genious, I am thinking that the bed heater would be a very low impedance circuit, so I wasn't real surprised by that reading.

One thing I noticed on both boards: There is a red LED right next to the mosfet on both the hotend and bed that are very bright when the respective circuit is on. Doing the test with turning the bed on using M140 S55, the red light for the bed DID come on, but very dimly. When this is working correctly, these LEDS are VERY bright. I guess that supports the blown mosfet cause. But if that's the case, then I have to think that trying the other (assumed) good board, would have instantly blown it. Why else would they both act the same?

I would have a look at the soldering and wires for the heatbed. The resistance test might come out OK on a multimeter but may struggle when moving current.

The solder joints on heatbeds are often just blobs of solder held on with dried flux as the aluminium cools the solder too fast. It will work OK for a while but will break down.

Edited 1 time(s). Last edit at 11/12/2016 06:48PM by ohfurryone.

The good folks at ultimachine are helping me with diagnostics. They are a great company for supporting there products. I think we have proven that both controllers now have bad heat bed mosfets. Just trying to pin down how they got that way. It very well could be poor connections. I did find discoloration on the output pins. I cleaned that off and replaced the female end on the bead leads. I also improved my stress relief on both ends of the bed cable. Ultimachine sent me firmware code that would turn the bed full on and it still won't come on. I attached the bed leads directly to the p/s 24v output and it heats up as it should.

So, I know the boards are broken and have a hunch why. I guess the only worry is blowing the new board. I did order a new 12 X 12 PCB heat bed and thermistor.

Well, I was wrong. The boards are fine. I found that the problem was in the power supply connector. I tested the voltage early on and it was ok and a visual inspection seemed to support that assessment. But yesterday I noticed a slight discolorization on one of the pins and looked again at the female p/s connector on the bed heater channel with strong magnification and found it was heat damaged. Changed it and like magic, the bed came on and heated.

I can not say enough how helpful all your comments were and how supportive Ultimachine was. This was a terrible experience for someone as tied to there printer as I am, but at the same time a wonderful experience in people helping people. Thank you all very much!