Bed won't heat  

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erik.a4
(@erik-a4)
Active Member

Not a new printer, my Mk2S started intermittently losing heat to the heat bed.  Prints would end due to bed thermal runaway, which wasn't runaway at all, but the bed ceasing to heat and reverting to ambient temperature.

Gradually, this got worse.  Now it won't run at all.

When I turn on the heated bed, it doesn't get warm and the red LED doesn't light up.  The red LED >DOES< flash briefly at power-up, so I know it works.

I tested the RAMBO board output to the heated bed, and came up with 2.5v.  Obviously, that's the problem...so I replaced the RAMBO board.  And that DIDN'T fix it.

So I also replaced the heated bed.  And the wiring between that and the board.  This also has not fixed the problem.  The new RAMBO board output to the heated bed is 2.9V.

 

Okay, what's left?  What can possibly be going on here?  Did I really get a replacement RAMBO with the same MOSFET bad in the same way, or do I have a conceptual problem here?

 

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Posted : 20/07/2020 5:33 am
Karl Herbert
(@karl-herbert)
Famed Member
Posted by: @erik-a4

Not a new printer, my Mk2S started intermittently losing heat to the heat bed.  Prints would end due to bed thermal runaway, which wasn't runaway at all, but the bed ceasing to heat and reverting to ambient temperature.

Gradually, this got worse.  Now it won't run at all.

When I turn on the heated bed, it doesn't get warm and the red LED doesn't light up.  The red LED >DOES< flash briefly at power-up, so I know it works.

I tested the RAMBO board output to the heated bed, and came up with 2.5v.  Obviously, that's the problem...so I replaced the RAMBO board.  And that DIDN'T fix it.

So I also replaced the heated bed.  And the wiring between that and the board.  This also has not fixed the problem.  The new RAMBO board output to the heated bed is 2.9V.

 

Okay, what's left?  What can possibly be going on here?  Did I really get a replacement RAMBO with the same MOSFET bad in the same way, or do I have a conceptual problem here?

 

Have you ever checked the thermistor. It should have an internal resistance of 100k ohms at an ambient temperature of 25 degrees C.

In addition, I would check all plug connections and wires for firm seating and possible loose contacts.

Statt zu klagen, dass wir nicht alles haben, was wir wollen, sollten wir lieber dankbar sein, dass wir nicht alles bekommen, was wir verdienen....
Posted : 20/07/2020 6:55 am
erik.a4
(@erik-a4)
Active Member

@karl-herbert

Thanks, but thermistors (both of them) are good.  Can't find a loose connection, and of course I've re-seated every one in the process of switching RAMBO boards.

 

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Posted : 20/07/2020 8:49 am
grahame.b
(@grahame-b)
New Member

Hi,

I have aMK3 which has exactly the same fault with it not producing 24V for the bed..intermittent now permanently faulty. The led on the Rambo that normally flashes red doesn't. I assume the thermistor must be working as the display whilst doing a preheat shows the correct ambient temperature. I was going to order a new board but after reading your post I'm hesitant.

Let me know if you get it going!

Posted : 20/07/2020 9:26 am
Karl Herbert
(@karl-herbert)
Famed Member

@erik-a4

Another thing that comes to mind is that I would check the output of the PSU if the voltage remains stable when the nozzle or bed is heated.
Have you checked the PowerPanic connection?
In any case of problems, simply disconnect the black/white cable and then test the bed and nozzle heating again.

Statt zu klagen, dass wir nicht alles haben, was wir wollen, sollten wir lieber dankbar sein, dass wir nicht alles bekommen, was wir verdienen....
Posted : 20/07/2020 10:14 am
erik.a4
(@erik-a4)
Active Member

@karl-herbert

Power supply!  That's good thinking; I really hadn't checked it.  Unfortunately, I don't think that's it.  Both power supply connectors are 12V, with heaters on or off.

Mk2.5S, so I don't think I have a PowerPanic connection.

The bed heater, BTW, is reading 3.0V only if one of the wires is disconnected.  Hooked up, it reads zero.

 

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Posted : 20/07/2020 5:11 pm
Karl Herbert
(@karl-herbert)
Famed Member

@erik-a4

Have you flashed the latest FW for the MK2S, done a factory reset?
If that doesn't help either, I would contact the Prusa Chat.

Statt zu klagen, dass wir nicht alles haben, was wir wollen, sollten wir lieber dankbar sein, dass wir nicht alles bekommen, was wir verdienen....
Posted : 20/07/2020 10:04 pm
erik.a4
(@erik-a4)
Active Member

Firmware was the latest for the first board, but updated the new one...no change, of course.

Then I got an idea, I swapped the heater connections for the extruder and the bed.  They should be swappable, right?  For diagnostic purposes?  I thought they were...but it didn't work; just blew the 5A fuse on the RAMBO board.  

Or was I supposed to swap the 5A with the 15A for that test?

 

...
Posted : 21/07/2020 3:19 am
Karl Herbert
(@karl-herbert)
Famed Member

@erik-a4

I am not an electronics specialist and therefore I would never do such experiments, because the bed needs much more power than the nozzle. Maybe you can destroy the board with it. I hope it didn't just blow the fuse. Have you installed the MiniRambo V1.3A?

As I said, in your case I would turn to the Prusa Chat. You only have to log in with your account, choose the eshop. After a few seconds a chat window will open in the lower right corner.

Statt zu klagen, dass wir nicht alles haben, was wir wollen, sollten wir lieber dankbar sein, dass wir nicht alles bekommen, was wir verdienen....
Posted : 21/07/2020 7:47 am
erik.a4
(@erik-a4)
Active Member

Yes, that was foolish.  I'm pretty sure it would work if I swapped the 5A and 15A fuses, but I'm not anxious to try.

What it DOES suggest is that the power supply has more than enough juice to instantly fry a 5A fuse, so it's probably not at fault here.

Which suggests that I did, in fact, get a second RAMBO board which is bad in exactly the same way as my first one.

 

...
Posted : 21/07/2020 3:10 pm
keri7240
(@keri7240)
Active Member

Suggestion:

You say the terminals at the Power Supply check OK at 12v.

Have you checked all four power supply connections at the MiniRAMBO? My suspicion is that one of the four wires from the Power Supply to the MiniRambo has high resistance, likely at the crimp connector at the PSU end or the Molex connector at the MiniRAMBO end.

Without a load, the voltage is likely to read normally at 12v.  This test is best done (carefully!) with the bed trying to heat up.

 

Good Luck!

 

 

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Nein! Nicht Versteh!
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Posted : 26/07/2020 6:47 pm
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