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Carl A
(@carl-a)
Estimable Member
Bed Thermal Runaway

Maybe I should start a new post on this as it has been a while since this problem happened. But here goes for now.....

Well it is back again. LCD screen shows the correct bed temperature (same as before). Same printer as before (my MK2.5S).

Here is what happened: I was printing a 4 color print with a filament change in between each color (do not have an MMU). I had printed this 2 or three times with no problem. Well this time I left the printer and it stopped while I was away from it (for a color change). When I came back after an hour or so, the printer was beeping (normal) BUT the screen gave me the dreaded Bed Thermal Runaway. The bed temp and the estruder temp were back down to room temperature. I tried to get the print to restart but to no avail. Shut down printer and restarted but same error. So I raised the extruder temperature and removed the filament. Shut down printer and have not gotten back to troubleshoot but fully expect to see the same results as I had last time. After that time, the problem was resolved by changing out the control board. Hope that is not the resolution this time as the board is somewhat expensive. 

I hope this is not a problem with the Rambo Mini. Any suggestions as to what to do before I buy another one to solve the problem. Note that the bed thermsistor does show a temperature change when heated but no power lite. Same indicators I had before.  I will be back to rechecking all bed connections at the psu this week as well as voltage at the bed heater connection with the bed connected and unconnected. 
It just occurred to me that since my PSU is not physically mounted on the PRUSA frame but separately outside the plexiglass enclosure that the printer is contained in, a PSU box to Printer frame ground strap may be needed. It does not have one presently and over time may be causing the failure of the Rambo Mini. The PSU has again been mounted directly on the frame but was not when I got the Bed Thermal Runaway error.
Mounting it back on the frame did not clear the error but the damage may already have been done. Any thoughts?

Best Answer by Carl A:

Tim, Measurement was done out of circuit.

Karl, you were right. I reflashed the FirmWare and all is working well. Now the question is, why did it happen? May be something that PRUSA needs to look at as well as Ultimachine.

I guess that my old board may be OK and only needs to be reflashed. That is a good thing I guess.

How do I get all this tracking to PRUSA for review?

Posted : 05/11/2020 10:07 pm
Carl A
(@carl-a)
Estimable Member
Topic starter answered:
RE: Bed Thermal Runaway

Ok, here are some readings:

Bed has no light on when set to heat on LCD; Thermsister does react to heat;

With bed wires disconnected: wires read 3.39 M Ohms on Ohm meter while connected to Rambo; with meter leads reversed the wires read 20 K Ohms

The bed with wires disconnected reads .8 Ohms

Bed wires disconnected from bed and PSU OFF, I get .01 VDC and .015 VDC with meter wires reversed

Bed wires disconnected from bed and PSU ON, I get .063 VDC and .043 VDC

With all wires connected and printer set to heat on PLA, extruder heats up but no light on bed and I get 2.88 VDC and 2.90 VDC 

Heat-up ceases due to bed not heating.

All this is similar to what I had last time this happened and could not fix it until I replaced the Rambo with a spare I had.

So, any suggestions before I buy another board (not too keen on that as I have had 2 failures and the boards are $100+ each not counting S/H).

Note: Since I am a pack rat, I still have the first failed board.

Posted : 06/11/2020 11:19 pm
Carl A
(@carl-a)
Estimable Member
Topic starter answered:
RE: Bed Thermal Runaway

I have emailed Ultimachine technical but no response to date. So, just for kicks, I took out the first board that had failed and took the same Ohm readings at the Bed 12 volt DC output connections on the Mini-Rambo 1.3a board. I fully expected to get the same or very similar readings.

Well, with the common multimeter lead connected to the positive pin, I did get the same 20 K Ohm reading that I got on the installed (failed board #2). Then I reversed the leads and put the common on the negative 12 Volt DC pin and the reading was 10 time what it is on the installed board, 34.6 Meg Ohms. Double checked both boards and got the same results.

Not having any circuit information on the board I could not determine if this was to be expected (since it was NOT installed) or if it provided some insight into the failure.

I am operating with just one printer now and do not intend to spent more money Easter egging the problem (old Navy term) without knowing the cause of two boards failing in the same manner. The only difference is that the second board failed with much less operating hours on it than the first.

If anyone has an idea or can point me to a good troubleshooting/technical source, please advise.

Posted : 16/11/2020 8:38 pm
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(@)
Illustrious Member
RE: Bed Thermal Runaway

Fuse?

Posted : 16/11/2020 9:59 pm
Carl A
(@carl-a)
Estimable Member
Topic starter answered:
RE: Bed Thermal Runaway

All fuses are OK and so are all connections. PSU is working fine as the extruder has no problem warming up (until the software stops it due to no heating of the bed). Thermister checks out OK also.

Posted : 16/11/2020 11:13 pm
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(@)
Illustrious Member
RE: Bed Thermal Runaway

Blindly ohming out drive pins without understanding what is behind them is not helpful. 

You say the bed begins to heat then the printer shuts off:

Heat-up ceases due to bed not heating.

Have you monitored the temperatures during this shut off event? Have you monitored the bed LED during this event?

Is there any bed movement near the time of this event?

If the bed is not moving, the temperature on the display when the heating stops is important. Report that temperature. I'd also recommend you test it two or three times to ensure the shutoff is consistent. Let us know what you find.

 

 

Posted : 17/11/2020 12:55 am
Carl A
(@carl-a)
Estimable Member
Topic starter answered:
RE: Bed Thermal Runaway

Tim-M30,

I have sent all the readings to Ultimachine as I am sure that they are very familiar with their boards, what the reading should be, and the readings could help defining the problem. I added it to my post here just in case they made sense to anyone monitoring the Forum.

I will go back and actually keep track of the temperature at shutdown and error message.

The bed DOES NOT begin to heat and there is no light indication on the bed. Sorry if that was not clear.

The EXTRUDER begins to heat up but at some point (I will get for you) the EXTRUDER heating stops due to no bed heating.

If your highlighted "Heat-up ceases due to bed not heating" area is supposed to link, it does not work. I will type in as a search and read.

Bed set to 60 C and extruder set to PLA 215 C. (will get actual shutoff temp of extruder)

The bed does move right after power on on initial system check. I will check if it does move after PLA heat-up selected on LCD. Don't think so but will check.

Posted : 17/11/2020 3:21 pm
Carl A
(@carl-a)
Estimable Member
Topic starter answered:
RE: Bed Thermal Runaway

Here goes:

The bed LED is NOT on at any time which would indicate no power to the Bed or a burnt out light led. Multimeter measurement of DC power at bed connection point is zero with multimeter leads both normal and reversed. 

Preheat PLA 215/60 C on LCD selected. Z axis moved up a few mm, Bed temp 27C Extruder temperature went up to 99C with the following message on LCD "Err: MINTEMP BED" Z Axis moved up a few mm again when the ERR Message appeared, extruder had about a 5C drift up after Error message; #2 Bed 28C, Extruder at 79C to start. Z Axis moved up a few mm when preheat PLA selected then when extruder reached 139C received ERR message and z axis rise by a few mm and an extruder temperature drift up to 144C ; #3 Bed at 29C extruder at 34C, Z axis moves up, Err at extruder temperature of 106C, z axis moves up and temperature drifts to 110C; #4 Bed 28C, Extruder 50, z axis moves up at start, Err at extruder 117C, z axis moves up, extruder drifts to 122C; #5 Bed at 29C Extruder at 34C, z axis moves up at start, Err at extruder temperature of 105C, z axis moves up, extruder temperature drift to 111C

Then I did same PLA Preheat but at the start went to settings and set bed temperature to 27C, Extruder acted normally and went up to 215C.

Then I did a filament autoload but went into settings and set bed temperature to 27C with PLA at 215C. Filament Autoload worked normally as did extruder temperature at 215C (Actually autoload started about 200C)

Then I sliced a short print with PLA extruder set to 215C and bed set to 27C in Prusa slicer. Extruder went up to 215C and began print normally (bed started at 28C but with heat applied by printing drifted up 3C - no light on bed); second layer at 210C/27C and print finished normally.

Hair Dryer test of thermister showed a temperature rise in a normal manner. Tested from 27C to 40C.

NO BED Movement at time of event or anytime except when Bed Temperature lowered under Settings or in the slicer as described above.

Note that I did a reset of printer between some of the tests but saw no difference. Also, the difference of the final extruder temperature, at ERR point, appears to be about 70C so when I started with an extruder at 34C the final ERR temp was lower than when I started at 76C. The Bed temp was about 27C in all cases. This indicated to me that the ERR point was more of a temp difference from extruder start temperature  than a temperature difference between the extruder temperature and the bed temperature.

 

 

Posted : 17/11/2020 6:56 pm
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(@)
Illustrious Member
RE: Bed Thermal Runaway

Sounds like a simple bed heating issue, no voltage getting to the bed heater.  That would be a blown fuse or mosfet, or a driver or related circuit failure.

Have you checked the BED connection voltages at the Rambo end?  

This is a dangerous measurement to make in that you can easily damage the Rambo if you mess up: Unplug the BED connector, very carefully attach the meter to BED- to BED+ then set bed temp to 50c.  It would be a good idea to use the connector housing to avoid damaging the board with a probe slip, mark and pull the bed wires out and insert the meter probes into the sockets and screw them down, then replace the wires when done. Do you see voltage at the Rambo BED connector when the temp is set to 50c?

 

Posted : 17/11/2020 7:16 pm
Carl A
(@carl-a)
Estimable Member
Topic starter answered:
RE: Bed Thermal Runaway

This is the second board with same problem. I would hate to replace the Rambo before I have made sure I do not have another problem. Seems unlikely since the bed is a simple circuit and reads point 8 ohms with Rambo disconnected.

I did check the voltage but not with the bed set to 50C. Will do and report. thanks for nthe heads up on connecting the multimeter probes.

Not a blown fuse. All 3 checked OK.

 

Posted : 18/11/2020 12:28 am
Carl A
(@carl-a)
Estimable Member
Topic starter answered:
RE: Bed Thermal Runaway

Here it is:

I had and used a spare connector and attached meter as suggested - with power off.

with bed set to 50C  and power on pins read 2.76 VDC

with psu power off I get a reading of .160 VDC

with PSU off and power cable to PSU disconnected I get (after some maybe capacitor discharge) .056 VDC

Posted : 18/11/2020 12:52 am
Carl A
(@carl-a)
Estimable Member
Topic starter answered:
RE: Bed Thermal Runaway

Here it is:

I had and used a spare connector and attached meter as suggested - with power off.

with bed set to 50C  and power on pins read 2.76 VDC

with psu power off I get a reading of .160 VDC

with PSU off and power cable to PSU disconnected I get (after some maybe capacitor discharge) .056 VDC

I hate to think I have an intermittant problem with the bed (don't know how) and I may have to replace the bed and the mini-Rambo.

If I am printing a filament that does not need bed heat, I can slice the object with the bed temp set to 0 or 27C as that works.

This post was modified 11 months ago by Carl A
Posted : 18/11/2020 12:58 am
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(@)
Illustrious Member
RE: Bed Thermal Runaway

And you have verified the 15a fuse is good?

Posted : 18/11/2020 1:06 am
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(@)
Illustrious Member
RE: Bed Thermal Runaway

ps: You might examine the heat bed - look for any areas where the epoxy coating has worn off exposing the copper traces ... a spring steel sheet can scrape off the epoxy and later when you are putting the sheet on while the bed is heating, short things out and blow the Rambo.  

 

Posted : 18/11/2020 1:17 am
Carl A
(@carl-a)
Estimable Member
Topic starter answered:
RE: Bed Thermal Runaway

yes, verified all fuses - all read zero ohms and are visually intact.

will look at the bed as suggested. top and bottom.

If that is the problem then I would expect the Rambo to be bad. Will advise after I check it.

This post was modified 11 months ago by Carl A
Posted : 18/11/2020 1:55 am
Carl A
(@carl-a)
Estimable Member
Topic starter answered:
RE: Bed Thermal Runaway

Top and bottom of bed look as good as new.

Posted : 18/11/2020 2:16 am
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(@)
Illustrious Member
RE: Bed Thermal Runaway

Last place to check to verify the Rambo driver is probably shot is to measure the BED IN voltage. If there is 12v there, then something on the board is keeping th heater from working. Could be a bad fet, a driver gone bad, could be one of the error signals like AC fail ... There really is no way to troubleshoot something like this remotely any more than "Yep, looks like the board is bad."  Just too difficult asking for this voltage that voltage, what happens when ... add that I'm having trouble just finding a schematic and board outline that match and that are supposed to be for the Mk2.5.

Maybe someone else has an idea. 

 

ps: a bad fuse can look 100% perfect. The only way to know is to ohm it out when out of circuit or measure voltage across it when active. 

This post was modified 11 months ago by --
Posted : 18/11/2020 5:23 am
karl-herbert
(@karl-herbert)
Illustrious Member
RE: Bed Thermal Runaway

@merrill-a

Check the "power panic" functionality. Simply unplug the 2 wires and test how it reacts when trying to heat up the bed and nozzle.
Also make sure that the polarity of this connector is correct. Factory reset and fw reflash also often helps.

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 : 18/11/2020 11:04 am
Carl A
(@carl-a)
Estimable Member
Topic starter answered:
Bed Thermal Runaway

Tim - The voltage at the input to board for bed is 12.6 VDC. Should have mentioned that I checked it earlier. The fact that the fuse measures 0 (zero) ohms says it is OK. I can remove the 15 Amp fuse and measure voltage across pins but think the Ohm readings say it all, also can replace it with a new fuse.

karl - I assume you mean to disconnect the two wires going to the bed. That has been done and I get same results. Extruder heats up to about 70C above ambient temperature and then stops and gives ERR message as previously stated. Polarity of connection is correct. Not sure how to do a Factory Reset but can reflash printer with no problem (advise on factory reset - I would assume more than just pressing the reset button the board). 

Note that the board has worked for about 6 months before failing. Again, the board failed (got Bed Thermal Runaway error) after the printer stopped for a filament change (printing with 2 colors) and I was not there to make the filament change. Before, in such a case, after a few minutes the temperatures of bed and extruder decreased and had to preheat before I could unload old and autoload new color.

Also, this is the second board that has failed. The first lasted for over 2 years and thousands of hours of printing. The second board failed after about 6 months of use (maybe less). My fear is that I have an unrecognized problem and a new board will ultimately fail in the same manner.

I have a spare LCD, but would need to buy and change out the mini-Rambo 1.3a and the 12VDC Mk52 bed if I were to eliminate all possible problems (not to mention doing the same with the wiring). Not an option at this point as I really want to isolate and fix the problem.

Again: Right now I can set the bed not to heat (in the slicer or through overriding under settings) and can use the printer but that is a temporary workaround.

I have not received any response from Ultimachine technical yet. I have sent them most of the testing I have posted here. They may have a schematic for the board.

I appreciate all the help.

Posted : 18/11/2020 2:57 pm
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(@)
Illustrious Member
RE: Bed Thermal Runaway

@merrill-a

The fact that the fuse measures 0 (zero) ohms says it is OK. 

No - not if the measurement was done in circuit. TLDR;  That's why I was asking for a different check of the circuit.

Here's to hoping the board designer is able to help.

Posted : 18/11/2020 4:32 pm
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