Rambo Heat Bed Terminal Melted

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gchristopher
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Rambo Heat Bed Terminal Melted

Post by gchristopher » Thu Aug 13, 2015 3:16 am

While adding a copper heat spreader, the heat bed stopped heating. While probing the RAMBo to check the bed voltage, I noticed that the insulation melted on the wire leading to Pin 9 (one of the Heat Bed leads.) There was also less than 12V (between 11.3 and 11.5) between the two terminals, though the voltage difference at the PSU supply never dropped below 12V.

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.

Image

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.
Image
Image

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.)

Initial Thoughts:

  • 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.

Next Steps:

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?
Last edited by gchristopher on Fri Sep 04, 2015 6:41 pm, edited 1 time in total.

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teoman
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Re: Rambo Heat Bed Terminal Melted

Post by teoman » Thu Aug 13, 2015 4:17 am

10 awg may be a little overkill. That is what i had on hand.

12 awg would be a little more convenient.


I also put 10 awg wires "feeding" the rambo.



OFFICIALLY THE RAMBO ACCEPTS 14AWG WIRE
You can use bullet connectors but i was lazy.



I also had the idea of removing the connector assembly and putting a new one. But did not trust the pcb (mine may have been a little more fried than yours).


MY INITIAL SYMPTOMS WERE HEATED BED SLOWING DOWN AND NOT BEING ABLE TO EXCEED 80 DEGS.
Last edited by teoman on Mon Feb 29, 2016 4:26 pm, edited 1 time in total.
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Re: Rambo Heat Bed Terminal Melted

Post by teoman » Thu Aug 13, 2015 4:24 am

http://m.banggood.com/12-AWG-10-Feet-3M ... 83012.html

I his is where i got my wire. (This is the 12 awg).
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Re: Rambo Heat Bed Terminal Melted

Post by KAS » Thu Aug 13, 2015 9:29 am

I'm using Turnigy 10 awg silicone wire myself. Super flexible and easy to work with. Might be an overkill for the 12v power supply but since I had everything apart I soldered it on for a future 24v conversion.

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Re: Rambo Heat Bed Terminal Melted

Post by bvandiepenbos » Thu Aug 13, 2015 11:58 am

sorry to see your trouble.
but, that happens pretty easy with screw terminals. I have even had them loosen over time. probably good to check them every once in a while.
wish there was something better on the board.
spring cage connections maybe?
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Re: Rambo Heat Bed Terminal Melted

Post by Eaglezsoar » Thu Aug 13, 2015 1:13 pm

Hey Geneb, he used a dremel tool on his Rambo let's see your explosive response! :D

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Re: Rambo Heat Bed Terminal Melted

Post by geneb » Thu Aug 13, 2015 1:44 pm

Not my circus, not my monkey. :D

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Re: Rambo Heat Bed Terminal Melted

Post by Eaglezsoar » Thu Aug 13, 2015 3:20 pm

geneb wrote:Not my circus, not my monkey. :D

g.

Excellent response!

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Re: Rambo Heat Bed Terminal Melted

Post by gchristopher » Fri Aug 14, 2015 3:49 am

Thanks for the link, teoman!

I looked closer at the failed connection and it looks like the wire heated up and the wire insulation melted first. This was extra bad because the wires exit the RAMBo upward in the Rostock. So the melted insulation dripped down into the screw terminal, probably making the connection even worse and even hotter.


I found the reference to geneb's violent outburst: yet another thread with a terrifyingly fried hot bed connector.

geneb wrote:Image

NO. No, no no, no no no no no, no no - no. No!

A Dremel tool is NOT THE RIGHT TOOL FOR THIS TASK. NEVER WAS, NEVER WILL BE.

If you have a bad connector problem and you pick up a Dremel tool to fix it, you now have TWO problems.

Bad Eaglezsoar, no donut! :D

g.


Bwahahaha, heeeheee. Geneb is correct, a dremel is not generally a wise choice for PCB rework. I'll beg out that I was only clearing melted plastic off the screw?

However, while I have a high-wattage iron and hot air, but I do not have a heat bar soldering iron to desolder all 10 pins of the terminal block. So I reserve the right to do something silly like take a hot knife or jewelers saw to it.


geneb wrote:Not my circus, not my monkey. :D


Plus the monkeys in this case are dinosaurs!


So, let's talk about how to actually fix a RAMBo!

I found the BOM, and the screw terminal is a MULTICOMP MB612-50810. 5.08mm pin pitch. (Which is weird, because caliper measurements had be thinking things were 5mm.) It's rated for 16A, 300V and a maximum 14AWG wire.

The housing is Polyamide 66, which is a nylon, but that's not specific enough to get a definite data sheet. Too bad it's a solid 10pin piece. That's a little harder to rework.

5.08 mm pin pitch is pretty exclusively the domain of screw terminals. So unless I want to solder wire directly to the board, that's what I have to work with. I have a Mouser order coming with a few different plug-in terminal options, like the ones used to connect the RAMBo to the PSU. The screw terminal plugs are rated 18A up to 12AWG wire, so that gives me a little more to work with. They have tin contacts, so I might just break one open and try soldering wire to it, depending on how much surface area there is to work with.

The local hobby store has 12ga wet noodle wire and Dean's connectors, so I'll grab some when the rest of the parts show up, do the board rework and hopefully get back to testing out the heat spreader and Volcano hot end.

Please let me know if anyone has any better ideas. I'll post more pics when progress happens.

Also, I think I've seen enough evidence for this failure mode to at least justify a strongly worded recommendation to SeeMeCNC to consider beefier heated bed wiring

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Re: Rambo Heat Bed Terminal Melted

Post by teoman » Fri Aug 14, 2015 4:40 am

If you remove enough plastic, the "tongue" of the screw terminal will be exposed. You can solder to that.

Bullet connectors may be better than deans here. Depending on your wire routing, you can pull the wires out individually if you have 1 hole per wire leading to the bed.
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Re: Rambo Heat Bed Terminal Melted

Post by geneb » Fri Aug 14, 2015 8:25 am

The version of the connector that Ultimachine ships with their "generic" RAMBo is much better - it's essentially the same as the style used for the power connections, but is broken up into a series of pluggable 2 pin terminal blocks. I really wish they'd use those in the SeeMeCNC boards.

A vacuum desoldering pump might do the job.

I'd completely forgotten about Eagle's Dremel adventure. Thanks for the laugh. :)

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Re: Rambo Heat Bed Terminal Melted

Post by Eric » Fri Aug 14, 2015 1:46 pm

5mm and 5.08mm pitch differences are negligible when you're only talking about two pins. There's enough slop in through-holes to handle the 3 mil difference. That may open up your replacement choices a bit.

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Re: Rambo Heat Bed Terminal Melted

Post by Eaglezsoar » Sat Aug 15, 2015 11:25 am

Thanks for posts about the Dremel tool!
That was a real hoot at the time and still is.
Geneb is a great guy but avoid his wrath :lol:

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Re: Rambo Heat Bed Terminal Melted

Post by rpress » Sun Aug 16, 2015 9:12 am

There are lots of 0.2" connectors out there.

http://www.digikey.com/product-detail/e ... ND/3189889

Larger gauge wire acts like a heat sink, pulling heat out of the connector. It's also cooler itself of course, because of its lower resistance.

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Re: Rambo Heat Bed Terminal Melted

Post by gchristopher » Mon Aug 17, 2015 8:34 pm

Desoldering the terminals was easier to do one pin at a time, so they were first cut from the rest of the header using a skill saw, .... er.... dremel tool ... wait, no, jeweler's saw and hot xacto knife. Pictures of dinosaurs with dremel tools and circuit boards will probably have to happen some time, I guess.

Anyway, the RAMBo board both uses a very high temperature solder and has very good heat dissipation, so a high iron temperature was needed. that part seemed to go okay.

Before the edge of the remaining header is filed down, check out the bubbling melted plastic from the hot wire!

Image

Also, I failed to notice that the capacitor appears to be damaged as well.

Image

Looking at my earlier pictures, that damage wasn't present before. Go me! Looks like I should grab a replacement before plugging it in.

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Re: Rambo Heat Bed Terminal Melted

Post by gchristopher » Thu Aug 20, 2015 2:29 pm

The repairs seem to be working! :)

Thanks to everyone for the advice. I took teoman's advice and went with 4mm bullet connectors for the 12ga Wet Noodle wire. Stega is double-checking the gigantic wire-to-pad solder job.

Image

I still had to enlarge the center hole of the bottom plate slightly to get the wires through without too much friction.

This Molex terminal plug (Molex PN 39533-2002), was an incredibly pleasant surprise; it has a clip-on case that opens easily to access the terminal leads!

Image

Since I'm now very wary of hot copper oxidizing inside screw terminals, I soldered a section of 12ga wire between the terminal leads and the bullet connectors. Again, the result was surprisingly clean and easy to achieve.

Image

The board rework wasn't too bad, either, using a jeweler's saw and hot xacto knife blade to first separate the terminal pins, then removing the two problematic ones, filing down the remaining terminal and replacing them with a mating through-hole connector (Molex PN 39531-0002)

Image

I'll need to do some longer-term testing, but the bed works again for a brief test print! The major remaining question is whether the RAMBo will fit back into the stock mounting location with the much larger terminal plugged into the top of the board and much larger wires. I'm guessing it probably won't, so maybe the board will have to be moved around a bit.

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Re: Rambo Heat Bed Terminal Melted

Post by teoman » Fri Aug 21, 2015 6:16 am

I placed copper heatsinks behind the rambo especially the motor driver and the bed driver.

I am also running 15v to the bed.
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Re: Rambo Heat Bed Terminal Melted

Post by THLycanthrope » Sun Feb 28, 2016 4:23 pm

Just had this problem happen. Can someone explain why I can't remove the melted terminal and solder the bed wires directly to the board?

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Re: Rambo Heat Bed Terminal Melted

Post by IMBoring25 » Sun Feb 28, 2016 5:33 pm

Others have. You just won't be able to disassemble for future upgrades or maintenance without desoldering something.

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Re: Rambo Heat Bed Terminal Melted

Post by gchristopher » Mon Feb 29, 2016 2:03 am

THLycanthrope wrote:Just had this problem happen. Can someone explain why I can't remove the melted terminal and solder the bed wires directly to the board?

I think it was teoman's advice to put bullet connectors on the bed cables, so you can still remove either the bed or the RAMBo? That worked out well for me.

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