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MKS GEN L V1.0 heated bed not working

Posted by qrp-gaijin 
MKS GEN L V1.0 heated bed not working
May 02, 2021 09:57AM
I am setting up a new MKS GEN L V1.0 board with Marlin 2.0, and have everything working now except the heated bed. A multimeter shows 0 volts across the bed heater terminals. I defined TEMP_SENSOR_BED to be 1 in Configuration.h. Pronterface seems to show correct data from the bed thermistor. But the bed heater never seems to turn on, even after I set the bed temperature in Pronterface. My power supply is 12V, 10A. Previously, with the same heated bed, the same power supply, but using a gen6.d controller board, the heated bed (120mm x 120mm) worked.

Any ideas what might be wrong? I read one Amazon review that said the FET for the bed heater on this board is prone to burning out.
Re: MKS GEN L V1.0 heated bed not working
May 02, 2021 12:12PM
Check for continuity in fuse F1

A tiny bed like that shouldn't be an issue
Re: MKS GEN L V1.0 heated bed not working
May 02, 2021 11:12PM
Check for continuity in fuse F1
Fuse F1 and F2 (located at the lower-left corner of the connection diagram) have continuity as measured by a multimeter.

Connection diagram:

Just to confirm I'm not being stupid here: is it correct that the J11 connector (labeled with "H-BED") is a voltage output node intended to power the heated bed? Again, I am always getting zero volts measured across the J11 connector.

Is it also correct that the heated bed is non-polarized, so the polarity of the connections does not matter? I inspected the bed and could not observe any polarized elements.

Measured resistance across the heated bed terminals is 10.2 ohms. This seems rather high. Is this a reasonable value? Details of the heated bed are here: [github.com] (though I don't have the required software to open the .brd and .sch files).
Re: MKS GEN L V1.0 heated bed not working
May 03, 2021 12:04AM
Enable pins_debugging (in Confuration_adv.h

send the machine a M43

post the results

check the heated bed is on D8 as it should be in the the M43 output
Re: MKS GEN L V1.0 heated bed not working
May 03, 2021 01:06AM
Woo hoo, the heated bed seems to be working now! The little LED (D4, right next to the connector for the heated bed) on the board lights up when I turn on the bed heater, and the bed temperature graph in Pronterface slowly but surely increases.

During my failed test yesterday, I was playing around with the PIDTEMPBED define, so maybe I messed something up with my configuration. Also, yesterday's test was done on an outdoor, cold balcony, so maybe the heater just didn't have enough power to heat the bed in the cold environment. With 10 ohms resistance and 12 volts, that's only about 14.4 watts to heat the bed.

The other thing that fooled me is that Marlin 2.0 (and maybe earlier versions?) seems to completely halt the software as a safety measure, if the bed fails to reach the target temperature in a certain amount of time. With only 14.4 watts into my bed, and my corresponding slow heat-up times, I triggered this software halt. However, even after the board enters this halted state, the Pronterface UI still allows you to attempt to adjust the bed temperature with the buttons and comboboxes, and Pronterface still misleadingly updates the animated temperature graphs with the target temperature, giving the misleading impression that the board is still alive and is still responding to temperature-related commands. But in this halted state, the board is actually not alive, the temperature graphs are showing old and stale data, and no user operations carried out in Pronterface can cause the heater to come on, because the board is halted and requires a reset.

Edited 2 time(s). Last edit at 05/03/2021 05:48AM by qrp-gaijin.
Re: MKS GEN L V1.0 heated bed not working
May 03, 2021 01:15AM
[duplicate post]

Edited 2 time(s). Last edit at 05/03/2021 02:17AM by qrp-gaijin.
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