Rostock Max V2 heated bed short

[peterk]

After successfully installing & starting to use octoprint on my V2 (and printing a good number of stuff) today in mid-print I see the heated bed temp dropping. This was midway through a 4.5 hour job. At that time everything else was working fine except heated bed (although I didn't check in my haste what temp the hot end was)

I took off the onyx heated bed and after optical check everything looked fine.

Then I looked at the RAMBO and where the heated bed is connected one of the leads was partially fused into the connector. It must have gotten pretty hot there. Checking resistance between heated bed wires shows they are shorted. Ok. That's fixable. But how can I verify that the RAMBO is working fully? Should I measure 12V at the red/black heated bed terminals on the board? I want to make sure that the board is ok before I put it back in.

Thanks

[Eric]

If it was a true short, the 15A blade fuse would be blown. Is it? Remember the expected resistance of the bed is around 1 ohm, which isn't very much. Some of the cheapest multimeters might not even be able to measure 1-ohm directly.

Do a search for "melted connector" if you want to find other cases of this in this forum...you aren't the only one. There's a lot of current through the bed connector, and poor connections will generate enough heat to do damage.

The positive (+) terminals on the bed/heater/fan outputs are all connected directly to the red input line, via the blade fuse. You can do a continuity check on those.
The negative (-) terminals are connected to ground via the mosfets. You'll need a live board with a way to operate the mosfets to test those.
But yes, when the associated mosfet is 100% enabled, you should be able to measure the input voltage between the two pins on each output.

[peterk]

Thanks for the input.

15A fuse is fine

Looking at the trouble spot it looks to me as if the red and black wires which feed the bed heat are getting pretty hot during operation and over time weakened the insulation enough that the wires got exposed and shortened out.

[joe]

Were you using the right gauge of wire for the heated bed? It looks like the hot end wires are the thicker gauge as indicated by your photo.

[Jimustanguitar]

A loose connection will cause arcing, and that will heat up and melt a connector like this as well. Anytime you're in there, it's worth making sure that those things are snug.

[peterk]

I built this over a year ago and I *think* I followed the instructions - this means the hot bed should be 18ga? But, yes, the hot end wires are thicker.
So, replacing the hot bed wires with a heavier gauge (15?) would be a good thing.

I measured the resistance on the bed and it is 1.1ohms.

Also, the hot end terminal does provide 12V when requested. The Heated bed terminal provides about 9.8V. This seems to be a bit low.

Also, the way the melted wires crossed over each other just before entering the terminal.

[Xenocrates]

I'm wondering if you're not losing some of the voltage to the melted connector. I personally would use 14 AWG for this, as it will handle 15A at 90C, which is enough higher that the fuse is actually protecting the wiring, rather than being useless (The fuse will blow under sustained 15A loads, and should only handle ~12A to be at the normal 80% threshold, lest it blow at any random little spike, while being rated for power transfer at 90C means that it will survive that long term with minimal chance of failure). I would also crimp ferrules onto the ends of the wire, to ensure that it is properly connected, and that no stray strands short.

[Eric]

Also, the hot end terminal does provide 12V when requested. The Heated bed terminal provides about 9.8V. This seems to be a bit low.

I misspoke above...the 15A blade fuse is ONLY for the heated bed. The other mosfets run through one of the 5A nano fuses. Should be the same input voltages either way, but they're different rails on the black power connector. So a poor connection on the power input connector could contribute to low voltage just on the bed. Worth checking.

I agree that the wires for the bed look too small for the job. The bed draws way more current than the hot end.

[peterk]

Ok. So I am in the process of rebuilding my Rostock after the connector melt down.

So far I cut off the part of the screw connector with the two melted terminals. I have rewired the heated bed with 14ga wire and also soldered 14ga wire directly to the Rambo board and secured it with kapton tape. I am still looking for a good reliable connector to put these 14ga wires together so the Rambo stays removable. Tested the heated bed with a temporary connector and it works fine.

In the meantime I am still only measuring 9.8V on the heated bed terminals at the Rambo. Would it make sense to use this to control a relais and get the actual power for the heated bed from... where? Could that be connected to the power connector feeding the Rambo?

[Xenocrates]

You could use that to control a relay. If you're using the original ATX power supply that earlier V2's shipped with, you may need to find any and all remaining yellow wires and run them through the relay. If you've got a different ATX supply, I suggest using a 6 or 8 pin PCIE connector's wiring. If it's a rail power supply, run a new wire from the contact on it to the relay.

[peterk]

Thanks. In another thread on the forum I did see a recommendation of a relay. I think I'll go that direction.

(Sorry for spelling relais - that's how we spell it in German)

[Eric]

If you're still looking for a high-current connector, look at Ebay, Amazon, or RC hobby stores/sites. Any connector used for serious battery packs will have the current rating for the job. XT60 and Deans Ultra are popular choices with a 60A rating. XT30 would work too, 30A is still plenty in this case.

[peterk]

Thanks, will do. I'll post back when the machine is back up running.

[HComet]

Arrrgggg.... Just had the same bed heater connector meltdown. I haven't had my Max V2 opened up since I installed an HE280 upgrade in September. Lots of printing since then with no issues. Suddenly today a few minutes into a print theres an ugly burnt electronics and plastics smell. I went over to the printer and it was still going and everything looked fine. No smoke, just a really bad smell. Canceled the print and opened it up to sniff around. Found the melted connector.

Ugly burnt heated bed connector

Well, I just happen to have some screw terminal connectors that were the right size so after a couple of hours of swearing and desoldering, prying and desoldering, more swearing, prying and desoldering its back up and running with a new connector block:

New connector block

Downside is that the heater bed wiring is just a bit too short after triming off the damaged ends. Guess I'll be replacing that wiring now...

[peterk]

Finished rebuilding my Rostock V2 last week after some parts arrived.

I ended up cutting off the molten section of the screw terminal and soldered two 12ga wires directly to the RAMBO. Since I only measured about 9.8V there I used that to control a relay which switches power to the bed on and off. That power comes directly from an unused 12V bus on the power supply and now a large heat sink is gracing the outside on the side of the printer. As an added benefit I used a few Wago connectors to split the 12V feeding the power supply cooling fan and run one line outside of the machine to power a small LED. I replaced all the wiring feeding the bed with 12ga wires and kept them as short as possible.

So far had one afternoon of printing and it works well. The little red LED on the bed that comes on when the heater kicks in is much brighter than before and the bed gets from 20C to 80C in 12 minutes. Before it could take twice as long (although I took the advice I also read here sometime back: putting a towel over the bed while it is heating speeds it up a lot)

Unfortunately I didn't have a camera or phone in my workshop and didn't document.