MakerGear Forum

Thanks Ed. I certainly hope what's happened doesn't remain a mystery. New heatbed and RAMBO show up on Friday so by end of weekend I ought to be back printing and have had a closer look at whichever turns out to be the faulty component.

For anyone who stumbles on this thread in the future, replacing the MOSFET was cheap and easy and worked great, $0.92 vs $170 new RAMBo.

Mouser part: https://www.mouser.com/ProductDetail/ne ... cycode=USD

So, my question is: Any idea what cooked it?

Kalborg,
you were lucky. I thought trying my heat bed on the extruder heater socket looked attractive, but guess what? I tried it an fired my Rambo board. The fault which failed the heatbed was a shorted heat bed (reason so far unknown) which when connected to the extruder heater socket fired the board.

fortunately I received a new board and a new RAMBO today and will be able to get up and running sometime tomorrow.

my Rambo heat-bed mosfet was probably already fried because of this short so nothing really lost.

nothing wrong with the wires from the heatbed. I'll try it on a separate 24 volt supply to see if it really is shorted.

very strange.

New bed, New Rambo, and back in business. I did have a hiccup though. After getting everything reassembled and connections checked 4 times in different sequences I fired it up without the lid on the control box and got everything except hot bed. Unbelievable!?!

So I rechecked connections again wiggling connectors etc. everything looked good. Repowered and everything worked except the bed heater. I was using the LCD controller to summon the bed heat. the little red light didn't come on either. turned it off, checked output of power supply and had 24 volts on all three connections. plugged it back in and checked bottom pins on the jack and they all showed 24 volts. turned it off, did some otheer things, turned it on did the same routine and this time it worked.

Does anyone have any idea what might have been troubling this thing?

john

jferguson wrote: ↑

Sat Jan 11, 2020 12:59 am

replacing the MOSFET was cheap and easy and worked great

The force is strong with this one …

what might have been troubling this thing?

That uneasy feeling generally happens when I've done a great job fixing the symptom, but haven't found the real cause.

It could be "one of those things", but, more likely, you'll only figure it out after the same thing happens again.

Hope I'm wrong … again! [grin]

Hi Ed,
the worst thing is when there are two intermittent problems either of which can produce same symptom. It looks like present situation is that there is something which sometimes prevents power going to heat bed. If the little light doesn't light then the bed won't get hot.

I would have thought that it might have been a sensor problem, but nothing intermittent there. since RAMBO is new, firmware is also new, and if firmware works sometimes, it would seem to me to have to work all the time. so buried in there somewhere is something the firmware expects to see if it's going to heat the board and for some reason isn't seeing.

I'd considered possibility that although power supply is putting out measurable 24 v on all three channels, something might be tricky with the one which feeds the heaters. ??

I hate stuff like this.

It's working fine. bed heat comes up when asked for. I still wonder what the earlier intermittence was. ???

jferguson wrote: ↑

Sat Jan 11, 2020 2:31 pm

If the little light doesn't light then the bed won't get hot.

Well, it's worse than that. The LED indicates the voltage difference between the (platform) power supply and the MOSFET drain terminal. When the LED is dark, the MOSFET is supposed to be OFF and conducting no current. When it's lit, then the drain terminal should be near circuit common (aka "ground") and the MOSFET should be ON = conducting current.

If the MOSFET is ON and the LED is dark, then something horrible has happened to hold the drain terminal high against the MOSFET's intense desire to conduct current. The most common cause would be a dead short from the drain to the power supply: no matter how much current the MOSFET tries to draw, the power supply wins, the drain stays high, the LED stays off, whereupon the MOSFET eventually emits magic smoke and burns up (or, ideally, the fuse blows, which seems not to have happened). That seems to describe the the original situation you reported, complete with incinerated MOSFET.

I'm certain there's something else wrong in there, but you've covered all the obvious debugging hints, so I'm at a loss for what else to test.

Double ugh!

HI Ed,
I think your take on this is what I looked for. I'm going to try running the "bad" bed on another power supply at 24v and see what happens. I got no magic smoke on the RAMBO and the mosfets look fine.

Since I'm back online and printing like crazy, most of this is a bit academic. but ....