Headbed stuck on and incorrect sensor values on brand new kit
I finished putting together my mk4 kit today and much to my dismay, I immediately started encountering issues with the heatbed. The first power on, I ran the self-test wizard, and everything passed with green checkmarks up until it got to the nozzle/heatbed test. The nozzle passed just fine, but the heatbed test appeared to be stuck on "preparing" for 4+ minutes while the bed temp slowly climbed to 125C, when the machine reset and displayed a heatbed overheat error. I let it cool down unplugged and tried again, and the same thing happened.
Next, I tried a full factory reset, wiped the firmware and reinstalled from a flash drive. The entire time the firmware was flashing, the heatbed remained on and heating up. By the time the flash was done, the bed temp was around 120C and I powered the printer down to avoid damage. No matter what I try, the heatbed is always turned on. I triple checked all of the cables, all possible pinch points, the thermistor leads, and all other parts of the wiring and didn't find anything wrong. From the moment I turn on the PSU to when I switch it off, the heatbed is always receiving power, giving the printer about 8mins of time spent turned on before throwing an overheat error and resetting.
Here's some points of all of the data I've gathered and tests I've done.
- The thermistor measures ~75kOhms when the bed temp is about 30C. I can watch the resistance slowly climb as the bed cools, and wiggling the cable doesn't change the reading. Other measurements confirmed that the readings were as expected, and the temperature readouts on the display screen matched my measurements with an IR thermometer. Most likely my thermistor is fine.
- The heatbed measures ~3.5 Ohms across the terminals. The heatbed seems fine as well.
- The moment the PSU is switched on, the red LED on the heatbed lights up and the heatbed immediately sees ~23.06V applied to it. This stays on at all times when the PSU is switched on, even while flashing firmware or holding down the reset button. Is this roughly the expected voltage drop for the heatbed when it's powered on and heating?
- The sensor info screen on the printer displays the correct bed temperature at all times, verified by an IR thermometer. The same temp is displayed on the bottom of the screen on the main menu, and it flashes blue as the temp rises.
- The sensor info screen shows 0.0V and 0.0A are being applied to the heatbed, despite it being on and heating all the time. I measured 23.06V across the heatbed contacts while the screen says 0V. Something seem wrong with the power sensing circuit for the heatbed here.
- The sensor info screen says that the printer in consuming (-7.0A). 7A sounds correct given Ohm's law and the bed's resistance and voltage it sees. Is this reading supposed to be a negative value though?
- The only other account I could find online of a heatbed stuck on was for a mk3 where someone's heatbed power MOSFET had visibly blown on their buddy board. I'm starting to suspect that I'm experiencing the same issue, though I don't see any obvious signs of damage on my board. Then again, I'm not sure where the heatbed power MOSFET is and it's of course possible for it to fail closed without burning up.
Naturally I contacted support, but they were quite insistent that it was a thermistor failure despite my testing it a half dozen times at different temps with my multimeter, and all of the readings coming back nominal. After going over this a few times they agreed that it was a strange issue that is hard to pinpoint and they weren't sure what to do. I asked if the xBuddy could be warrantied and they said they did not think that would help. I'm a bit lost because now I have a printer that doesn't work, and the xBuddy board is not in stock (and I need one with a 2-wire panic connector, not the 1-wire one that is in the parts shop). Any ideas? This is a very frustrating failure mode and even more frustrating that support isn't sure what to do. I'll contact them again tomorrow of course, but I'm not sure what else I can try. I'll update this post with any other info I find.
RE: Headbed stuck on and incorrect sensor values on brand new kit
I took a few more measurements this morning with everything at 18C room temperature. Both the heatbed thermistor and the nozzle thermistor read 133kOhms, so I am fairly sure this is not a thermistor issue.
RE: Headbed stuck on and incorrect sensor values on brand new kit
Some more data I've collected:
- This failure doesn't appear to be Haribo-related. I metered the bears out according to the directions and I still have the final ten bears to be consumed once the self-test passes. They are currently sealed for freshness as it seems this won't be resolved quickly.
- I tried swapping the power cable pairs going form the PSU to the xBuddy board. This had no effect.
- I tried pulling fuses and the heatbed remains stuck on if I pull either or both of the 5A fuses. Pulling the 10A fuse disables the heatbed as expected.
To me, this points to a failure of some component or components that are in the flow of power between the 24VDC input to the xBuddy board and the output to the heatbed. Given that there is no publicly available schematic, I have to assume that a MOSFET or some sort of current switch being used to modulate heatbed power and that is where the problem lies.
Unfortunately I am continuing to strike out with support. Two separate chats led to nothing and no offers to replace the faulty hardware. Apparently this has been escalated to a mk4 specialist and I was told to expect an email with next steps today, but that email never came. Hopefully next week brings a solution. Until then, I have a very nice paperweight.
RE: Headbed stuck on and incorrect sensor values on brand new kit
I was bored at home on a Friday night so I did some more sleuthing while I wait to hear back from support next week.
It looks like the power path is fairly simple for the hotbed. The left PSU(+) input terminal is connected through the 10A fuse to the Hotbed(+) output terminal. The Hotbed(-) terminal connects to an IC, Q4, which is connected to the left PSU(-) return terminal. I was able to make out the part number on Q4, CSD18503, an N-Channel power MOSFET. The part's datasheet indicates that the on resistance (Rds) is under an ohm when the gate is pulled sufficiently high, around 4.5V. With the PSU disconnected from the xBuddy and all cables unplugged, I measured the resistance across Q4 with my multimeter and found that it was conducting - with zero volts on the gate that seems top point to the MOSFET being fried. There might be other factors that I'm not considering, but I'm fairly sure this xBuddy board just needs replacement.
RE: Headbed stuck on and incorrect sensor values on brand new kit
Prusa support followed up with me and is sending a new xBuddy board kit, which I should have in the new few days. Just for fun I may try doing some rework on the broken board on my own since Pruse doesn't want me to send it back. It shouldn't be hard to get this FET off, and if I can find a new FET with similar characteristics, I may even be able to resurrect the board. Of course I don't plan on connecting any modified hardware to my mk4 because I value my warranty, but it would be a fun experiment to hook up the (reworked) defective board to my bench supply by itself to see if my suspicions of the blown FET were correct. In any event, kudos to Prusa for the ongoing support and righting the issue before too long. I'll follow up again with the results of the replacement board and my own tests with the defective one.
RE: Headbed stuck on and incorrect sensor values on brand new kit
Great sleuthing! Is this a TO-220 package (easy to replace) or a surface mount version (less easy to replace)?
The part seems appropriately specified and in-expensive. Plenty of stock at Digikey.
https://www.digikey.com/en/products/detail/texas-instruments/CSD18503Q5A/3478067
https://www.digikey.com/en/products/detail/texas-instruments/CSD18503KCS/3673425
RE: Headbed stuck on and incorrect sensor values on brand new kit
Thank you! This is the tiny SON package but there's just enough exposed pad and solder that I could remove it with my smallest iron tip. I could definitely just order a new one from DigiKey but I was also hoping I could find something close enough in my spare parts bins. The current ratings for the CSD1850 are surprisingly high for such a small package, but I don't think it ever sees more then 6A or so across the drain and source pins. The real trick is finding one with a similar Vgs threshold. I'll probably just grab a few from Digikey and try my hand at the rework if I don't turn anything up in my spare parts bins.
RE: Headbed stuck on and incorrect sensor values on brand new kit
Hi,
I have exactly the same problem as you described.
The support chat suggested replacing the board. But unlike you, I have to send my board in first and only then can I get a replacement. I'm now waiting for an email with instructions on how to return the board
RE: Headbed stuck on and incorrect sensor values on brand new kit
I received my new xBuddy board, replaced it, and everything is working great now. I haven't yet had time to try replacing the FET on the original one, if I do get around to it I will update this thread with the results.