Printer Bed Continuously Heating with new V4.2.7 board

[Steven F. Le Brun]

At last Tuesday New River Maker meeting, I told the group about the problem I was having with upgrading the motherboard on my Ender 3 Pro to the Creatlity V4.2.7 board.  The problem was that as soon as the printer was turned on, the bed started heating and continued heating until the printer was powered down.  If I didn't manually turn the printer off, the thermal runaway protection would.

All indications was that the MOSFET that controlled power to the bed heater was fried.  A voltmeter showed that the bed heater was always receiving full power, 24V.  I contacted the company, a third party seller.  They verified that my wiring was correct by photos that I sent them and sent me a replacement board.

The new board had the exact same problem.  I felt that the odds of two boards having the same problem was rather unlikely and started looking more closely at what I could have done wrong.  My primary suspect was that I had screwed up something in the Marlin V2.0.9 firmware that I configured and compiled.  The seller had sent me a pre-compiled version of the firmware.  Both boards failed with the same problem.

 At this point, I felt that the firmware was not the source of the problem.  My next thought was that maybe something on my printer was frying the MOSFET when the board was first powered up.  With a third board, I decided not to complete wire it up, with only the power leads and the display contented.  The board did not power up.  My printer uses a relay controlled by OctoPrint on a Raspberry Pi 4 to turn on the printer.  First, the relay was not providing power.  That turned out to be the dupont cable.  It had broken and had came off the GPIO 5V pin.  I was able to replace the connector without replacing the whole cable.

Now the relay was delivering the full 24V.  The motherboard was still not receiving power.  I traced the cable running from the relay to the motherboard.  The first connector showed that the cable was receiving 24V while the extension was still at 0V.  

It turns out that the power wires and the bed heater wires are identical.  I had misread the labels I had put on the wires a year ago when I tried to move the motherboard to the rear of the printer.  The power cables were labeled with a B/H.  The problem was a serious case of pilot error.  I had connected the power wires to the bed heating terminals and the bed heating wires to the power input terminals.  This explains why the voltmeter showed that the the bed heating terminals were always at 24V.  The motherboard was receiving power thought the bed heating terminal and supplying power to the bed via the power input terminals.  After closer examination of the label, the B/H was a smudged B/R which shows the order in which the wires should be connected.

Now that the printer is working again, I have started printing.  It still needs to be fully calibrated and I have only run a flow control test.  That test showed that the flow control was perfect at 100%, a value I have never seen on my printer before.  I am guessing that the silent stepper motor drivers on the V4.2.7 board not only make the printer run much quieter, it is also having better accuracy with the nozzle movement.  Time and a full calibration suite will tell.