Did I fry my mini RAMBo?

Today, while doing some maintenance on the nozzle of my Mini the USB connection with Octoprint went down so I restarted the printer to make it reconnect. This happened before so I didn’t think anything of it. But now, my printer seems broken. G29 makes some effort, but never actually probes the washers. It turns out something is wrong with my Y-min and Z-min endstop sensors. When I move the extruder so that none of the switches should trigger, M119 gives me:

> M119
< Reporting endstop status
< x_min: open
< x_max: open
< y_min: TRIGGERED
< y_max: open
< z_min: TRIGGERED
< z_max: open

What I did so far:

  • Tried reflashing firmware
  • Opened electrical chassis to check end-stop connectors, all are tightly connected.
  • Disconnected Y-min end-stop to see if I could force an open on Y-min
  • Disconnected Z-min and Y-min connectors on RAMBo to see if I could trigger an open status

I can move Y only forward untill endstop hits Y-max and then won’t budge anymore. Y will move back and forth while homing Y (it moves a bit back on G28 Y0 in order to re-approach Y-max and then backs off again).

Have I fried my Z-min and Y-min RAMBo end-stop sensor logic?

:cry: :cry:

Looks like it.

If so, I’ll take my loss and replace the board, but can anyone explain how the board can so easily be fried by holding the hot-end with one adjustable spanner and turning the nozzle with another while heating the hot-end? I understand there’s a lot of current going to the heater, but that should be insulated and I don’t touch the heater wires (I approach the hot-end from the left, nozzle from below, fanduct still in place). The only wire on the left is the Z probe lead I guess?

So, according to this post I shortened the heater to the thermistor, but like con-f-use I find that hard to believe when approaching the hot-end from the left?

Anyway, if that’s true and if the hot-end needs to be heated for proper nozzle tightening, how to prevent this failure in the future then?
I tried to inspect the translucent insulation on the thermistor but it wouldn’t budge, is it thread-locked inside?

btw: I’ve ordered a refurbished mini RAMBo from UltiMachine using U.S.P.S. First Class International shipping to Europe. Hope it arrives in time. Found out that mini RAMBo’s are a tough replacement when living outside the USA :frowning:

The nozzle and heat block need to be hot, but it doesn’t need to be on. Heat it up a bit higher than the last filament that you used usually starts oozing at, turn the whole machine off then unscrew the nozzle. Then turn it back on and get it heating up, hand screw in the new nozzle while you are re-heating, turn the machine off and tighten with wrenches once back up at temp.

Yeah, I guess I should turn off the printer next time, but that wouldn’t prevent the thermistor from making contact to the heatblock if one of the leads had shifted enough during maintenance?

Thinking out loud I can’t help but think that a glob of high-temp silicone around the thermistor before it goes in would be extremely helpful in this regard? If so, I’d still need to get the thermistor out in one piece first which seems quite difficult in my assembly?

I’m not the only one thinking this: https://www.youtube.com/watch?v=Y3CffV7VkOQ
Although he’s doing it for mechanical and thermal reasons I think it wouldn’t hurt.

It is not shorting the thermistor to the block, it is having one of the heat resistor leads hitting the block. That puts 24 volts into a 5 volt input on the Mega chip. BOOM!!! Shorting the thermistor would cause temperature errors.

Check to make sure the red insulator is fully down against the heat block on both leads and that the heater leads are parallel to the screw-on retention plate that holds the thermistor in. Some times they are VERY close to the plate.

Ok, this sounds more plausible in a failure explaining kind of way, but looking at the hot-end I still find it hard to believe I made the heat resistor leads touch the block (wires were nice parallel upright) :frowning:

Anyway, while I’m at it and have the mini on the bench I might as well drip some exhaust sealant in the thermistor hole for better thermal conductivity and electrical insulation and put a glob of high-temp silicone on the leads end of the heat resistor to make it harder to short them on the block?

Looking at, at least, two weeks downtime for the RAMBo to arrive I’m quite eager to solve this for good!

Mini RAMBo arrived today, printer is back in business!

This is how I hope to solve heater shorts in the future.

It’s Permatex Exhaust Sealant. Good for 1000C, perfect thermal conductor and electrical isolator. Took the opportunity to center the heat resistor for better heat tranfer.

It sounds like you got this sorted out, but just to clarify:

You did check the fuses, right?

I have inadvertently caused the same short several times. I use quick-disconnects for the heater and thermistor wires and if I unplug all of the wires at the same time, and then get distracted, the heater can easily short to the therimistor–even if the NZL set point is 0.

BUT, each time I just had to replace one of the 5V automotive-style fuses on the board. Then it kept on chuggin. It is, however, possible that the fuse failed to react fast enough in your case…

Anyway, if it makes you feel any better, I just had to replace my MiniRambo also because the X-motor driver burned out.