I just measured on the Taz 6 I am building that is on the bench front of me, and when I turn on the heat bed the right most heat bed pin reads 24 volts on the connector and ~2.5-3.3 volts on the one right beside it.
Thanks, kmanley57. Good idea. I just tried this and got roughly the same as you (but ~3.8-4.5V on the neighboring pin). I tried both beds, including bypassing the wiring harness, and everything is good until I connect the bed, then both lines falls down to ~3.8-4.5V (both at the RAMBo and at the bed. I checked the thermistor, and it is working and responsive.
Anyway, I think this just confirms that there is some other issue on the RAMBo that is hard to diagnose, so I’ll just replace the whole bard and see how it goes. I’ll update when I receive and install the board. I really appreciate everyone’s help on this.
The one line(right most) is your 24 volt supply to the RAMBo, so check to make sure you have good connection(AKA - low ohms) to your 15 AMP fuse and the power supply to that right most pin. Sounds like not.
OOH. This is definitely it. 1MOhm from the fuse the the right pin on the mosfet and 1.3kOhms from the fuse to the terminal block. What the heck could that be?
Rereading your message, I think you might have meant the main 24V power supply box to the fuse. I get pretty much no resistance from the power supply terminal block (coming from the huge 24V power supply box) to the fuse. Does that seem right?
[EDIT: I originally said 12V - I meant 24V]
If you do have a 12 volt instead of the 24 volt supply it comes with then you would have problems.
But read between the right most pin of the heat bed connector on the RAMBo to the second from the top of the power connector. Should read around 0 ohms.
Shoot, I meant 24V, sorry.
Whelp. There’s the issue. 1.6kOhms. Not 0 Ohms. Not even close… Hmmmmmm… So I guess I just chase the trace to see where the resistance is coming from… 
Thanks kmanley57! For the pro-tips!
Thank you so much for your help kmanley57 and piercet! I finally figured it out!
I replaced the RAMBo and after examining the old one, I found that the right pin for the heater wire was melted off of the RAMBo. Pretty scary/surprising stuff. It’s possible to fix, but just wanted to add this info for those of you looking for solutions at home.
Thank you for all of your help. Pictures below for the curious.
Note the long pin on the right of the long black connector - it is marginally melted off the board. 
From the back you can see the solder has been melted on the leftmost solder pad in the line of through-hole connectors for the black connector. Crazy that those pins get that hot. Perhard the connection was just not quite good enough and heating up with all of that current passing through?
Edit you can’t see the edit icons on android tablets anymore…
That would do it! Should be a pretty easy fix to make yourself a usable backup board too. Given that’s s 6, even if it’s out of warranty I might call support on that one, they may be willing to swap the board so they can examine it in house.
Looking at your pictures I think it was like that when it was built. If the pin had got hot enough to melt the solder in thew board, the plastic on the connector wire would have turned dark along with the pin in the connector.
Hi, interesting story you have. I have a TAZ5 and like I posted yesterday 28 of May 2018 on Forum, I am unable to have more than 76C on heat bed Modular or original one. Exchange with Luzbot support, photos during this 29 and nothing. I came back to old CURA. Tested all cable, I have a heat problem. CURA shows 110C bed and 240C head, when I test with IR Gun I got 66 to 70 on bed and 110 on square top of the nozzle. IWhen I try to print a ABS, showing 240 and 110, the print on a brand new modular bed, temperature at 66C, after 2mm, the print head move the part I print because not stick. The support told me to put blue stick or sand. Sand a old bed, OK, glue stick T-Glase ok. New bed why? The issue is on the heat and control made with the heat sensor. I am lost after tested all wired and fuses. May be the RAMBo is bad too?
JP
TL;DR: If your bed heater fails check your board connector along with the wires for crispiness.
Resurrecting an older thread as it helped me out with troubleshooting what seems to be a problem with the Rambo boards on this and other printers.
I had my bed heater die on my Taz 6 towards the end of a print, with the firmware (Klipper) giving me a warning about the heater not responding. Traced all the wires checking for continuity, checked that the mosfet seemed to be functional, etc. Prior to pulling the whole board out I did a search on the forum and came across this thread. Pulled the bed connector off and found this:
Pulled the board out and found this:
One interesting note is that the fuse is fine. In reading up a little on these standard spade-style fuses, I learned that they don’t deal well with low-overload conditions (~110%-135% of rated current).Many will continue to conduct current @110% indefinitely, and can take hours to fail for greater currents. Looking into heated bed failures, I see a lot of burned pins and wiring harnesses, so the chosen fuse setup seems suboptimal relative to a significant cause of potential failure.
Also note that I found sputtering of material (likely solder) on the case directly behind this pin:
I’m not sure if this was because of vaporized solder condensing after sublimating, or if it represents high-current arcing to the grounded case. I did find it a little odd that a bare board with high-current pins is mounted over bare metal. I threw some kapton tape behind mine before reinstalling, just for my own peace of mind.
At first, I thought I would just replace the board. But I decided to just see if I could reuse safely as-is. I ran a 16awg jumper from the fuse holder, and spliced it into the 24V bed heater wire. Reassembled, and everything is working with no heating issues. I’ll keep an eye on things for now, but I couldn’t justify buying a new board at full price with the same design flaw.
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Glad to hear it! Just FYI, I’ve been running constantly since Last year with the jumpered connection with zero problems.
I just started having the same issue after it completed a print. Can you post a picture of your jumper wire so I can see how you did it?
@intelinc Can you post a picture of your jumper connection? The same exact thing happened to me and I also don’t want to have to order another board 
Your help is much appreciated!
Images below. In the first image you can see the board header for the heated bed (the two screw terminals to the left of the large capacitor). The position on the right is the V+ trace that burned out on my board. The large white wire in the foreground is the wire that should be plugged in to this terminal, but is now jumpered to the fuse. You can see that the GND wire returning from the bed heater is still in its original position in the header
The white wire in the foreground of the first image is not just big and blurry because camera, but also because it is two wires spliced and then covered with shrink tube that you can see in image 2. Basically, I cut off the crimped contact at the end of the V+ wire and spliced in some additional length of heavy (I think I used 16awg) wire to add some length for a jumper. In the second image you can see where that new wire end is seated in the left-hand blade of the fuse. This is the negative side. Current flows positive => negative, so from the negative contact of the fuse to the positive contact of the bed heater, through the bed heater itself and then back out to ground. Basically, the burned-out V+ pin is directly connected to the left-hand blade of the fuse, so the pixies just taking a slight shortcut around the burned-out contact.
To secure the new end of the wire I removed the fuse, took the end of the stranded jumper wire and seated it in the slot, and then reinserted the fuse, sandwiching-in the jumper wire. I’ve put hundreds of hours on the machine since then with zero bed-heater issues. HTH!
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Thanks a lot!
It looks like it fully fried the connector, I had to scrape out the burnt ends of the connector from the motherboard. After I did that and plugged it back in, I had a successful bed heat! I don’t think mine is safe since half of the connector was burnt out, which is pretty scary stuff!
Luckily I can order a new one for cheap, I might try to print a few things to see if can work for the time being though.
Also, I didn’t even need to cut off the crimped in, I just wired mine directly to it in case I decide to replace the motherboard. I also don’t have a crimping tool so I wouldn’t be able to crimp it again later on.
If you send me your payment info, I’d gladly send you $20 for your help!
I’m honestly a little shocked that something as nice as a Lulzbot machine would allow this kind of defect. Has anyone had any luck reporting this to them?