Bed won't heat  

RE: Bed won't heat

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.

RE: Bed won't heat

@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.

RE: Bed won't heat

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!

RE: Bed won't heat

@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.

RE: Bed won't heat

@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.

RE: Bed won't heat

@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.

RE: Bed won't heat

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?

RE: Bed won't heat

@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.

RE: Bed won't heat

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.

RE: Bed won't heat

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!

RE: Bed won't heat

Did you solve this problem? I am having the exact same problem.

RE: Bed won't heat

Follow up. This thread was very helpful. I figured that last post might have solved the problem. It did for me. The connector at the bed in + had gotten hot over time and was no longer making a good connection when trying to heat. Soldiered that wire straight to the board and bed is heating again. Thanks for that last tip.

RE: Bed won't heat

@dougd

Be very, very careful.  I had this happen and almost had a fire.  

RE: Bed won't heat

I had this problem of the bed not heating up, it was intermittent at first but I also found that the connections from the PSU were bad. In fact it was not even tightened up, I'm surprised it ever worked

RE: Bed won't heat

   Ah, sorry, I thought I'd updated.  It WAS in fact the headbed connector to the RAMBO board.  Looked good, tested good...but was intermittently not good.  Since this was a high-amperage connector to the heatbed, intermittent connection had caused some arcing, melting of the plastic shell, and shifting of the connector.  What finally turned the clue light on for me was removing the heat bed leads from the RAMBO, looking at all 4 holes end-on, and noticing that 3 holes were shiny...and one was not.

   So it was very subtle, and very vexing.

RE: Bed won't heat

@dougd

I also had some problems with the bed heater, because of bad contacts and therefore I assembled new heat resistant and flexible AWG16 silicone wires with suitable plug and screw connections.

https://www.amazon.com/Electrical-Silicone-Flexible-temperature-resistance/dp/B0746HRVZP?th=1

RE: Bed won't heat

sorry to revive this thread, but I am having the exact same issue that you had.

What did you wind up doing to correct this issue on your mk2s please?  I'm about to start swapping components on the rambo board...  I suspecting the mosfet, but after reading your issue and what you did, and nohting fixing it, I'm hesitant to pull the mosfets off 🙁

   I have 2.5V when trying to heat the bed at th epins on the rambo, without the heat bed wires plugged in and he LED lights on the rambo.  I plug the heatbed in, and no voltage and led goes out.  I have replaced the thermistor (was working fine before too), same issue occurs if it's plugged in or not as well.  I've replaced the heatbed wires too.  no damage on the heatbed that I can see... this issue happened once, I took the print sheet off and put it back on and it worked again for 10 mins... then poof no more work 🙁

what did you do to correct your issue please?  

thank you!

Mike B

Posted by: @erik-a-2

@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.

 

RE: Bed won't heat

ahhh, gotcha!   2 boards with the same issue 🙁   I'll reflow the rambo side connector in the morning.  

I was on chat with support, and they didnt' mention this being a common problem, but I guess it is.  i mentioned to her I was going to replace the mosfet.  the bed and heatblock both have the same mosfets so I was just gunna swap them across.  but doesn't look like I'll have to do that now.

 thanks!

  Mike B

Posted by: @erik-a-2

 

   Ah, sorry, I thought I'd updated.  It WAS in fact the headbed connector to the RAMBO board.  Looked good, tested good...but was intermittently not good.  Since this was a high-amperage connector to the heatbed, intermittent connection had caused some arcing, melting of the plastic shell, and shifting of the connector.  What finally turned the clue light on for me was removing the heat bed leads from the RAMBO, looking at all 4 holes end-on, and noticing that 3 holes were shiny...and one was not.

   So it was very subtle, and very vexing.