In a couple subsequent tests, the heat bed terminals intermittently registered 0V across them! (And the FET indicator LED on the RAMBo did not light up.) That was scary! Maybe a resettable fuse was at work, because after turning it off, the FET started working again.
When trying to remove the RAMBo from the printer, it looks like the screw terminal had partially melted, locking the screw in place. I dremeled away the extra plastic above the nut head, then heated the nut with a soldering iron until it could be turned and removed. The exposed copper wire still stuck in the terminal was visibly corroded. Heating the wire let me remove it.
This looks like the screw terminal didn't form a great connection, heated up and partially melted the screw terminal. The hot wire probably corroded over time as well, worsening the connection and the problem.
Here are pictures of the damaged terminal, after dremeling away plastic to expose the screw head, but before heating the screw and wire to remove them.
I measured the resistance of the heat bed, and it came out to 1.3 ohms. To help eliminate the copper heat spreader as a cause for the meltdown, I put it back on the bed and moved it around while measuring the resistance, and didn't see any change. It seems clear that this was a problem with the wire connection at the screw terminal, as many others have encountered.
It looks like this is a pretty common failure mode for RAMBo boards, judging by a past thread on this forum. An upgrade to the head bed wiring was already high on the list, thanks to teoman's thread on switching to 10AWG noodle wires. KAS's thermal imagery of the wire heat was also pretty compelling.
So, I'm pretty certain that adding the heat spreader plate was coincidental, and that the wire connection had been corroding and worsening over time until it heated enough to noticeably fail. Luckily, the damage to my RAMBo is MUCH less worse that others have encountered. (teoman's capacitor melted by the proximity heat was terrifying.)
- I got lucky, this could easily have been a lot worse, frying my RAMBo and forcing me to buy a Smoothieboard.
- Screw terminals are a bad idea for this purpose.
- I don't have a RAMBo BOM, but representative terminals on Mouser are rated 12A. That's sufficient for the expected 10A-ish draw of the heated bed, but isn't that cutting it a little close?
- I don't see how I could possibly have the wire any cleaner and the connection more secure than when the printer was first assembled.
There's no way that terminal pin can be repaired and ever provide a reliable connection. Replacing the terminal block is an option.
Others have soldered larger wire directly to the board. That's okay, but I'm not averse to doing a little rework on the board to have a nice connector.
So a few options are:
- Solder good wire to the back of the board, then run that to a Dean's or other good high-power connector before it heads to the bed.
- Saw/melt/cut the last two terminal pins off the RAMBo, and solder wires in their place. (Again, soldering to the pads.)
- Remove the terminal pins and solder in a better 0.2" pitch connector in their place. (Which might be a tight fit if the other 8 screw terminal pins are left in place.
Does anyone have any thoughts or advice?