Mendel90 dead - Heatbed or Melzi?

Hey RepRapper,

I need your help with my Mendel90.

in March this year, i bought a Mendel90 Dibond from nophead. I really love that machine and it printed well all the time untill this evening.
I was printing two small parts and then the printer suddenly stopped working and octoprint says that serial connection is lost.

I use octoprint with an raspberry pi and from time to time i had the ground-issue if someone was switching on the light or some other electronic decvices. Then, the usb connection was disturbed.
But this time i cant get my Mendel90 back on printing.

I restarted the Mendel90 and also rebooted the Pi manny times. No chance to get i printing again. It always looses the usb connection. Even if i use Pronterface, it crashes after a short time after connection via usb.


Then i found out that deisocnnection comes from the Heated Bed. I grabbed my multimeter and measured the voltage of the + and - Terminals for the extruder heater. Everything works fine. The extruder heats up. Voltage rises.
After that i measured the Heated Bed Terminals on the melzi. 0V on idle. Then i set the temp. to 30° and heated it the "set" button in pronterface.
Instantly i lost the connection, but the heated bed gets voltage and heats up until it reaches the 30°.

Video 1: [www.youtube.com]

Video 2: [www.youtube.com]

I also tested the thermisistor on the heated bed with an hot air gun. It works.

So whats wrong with my Mendel90. Is the melzi dead or the Heatbed or .....? You guys have any idea?

Here you can see how i wiried it up when i build it: [blog.seidel-philipp.de]

Edited 5 time(s). Last edit at 11/07/2015 06:12PM by ps915.

Check that the screw terminal for the ground connection on the Melzi is tight. It looks like the ground has a high resistance so the bed is only getting part of the voltage and the rest is across the USB cable.

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[www.hydraraptor.blogspot.com]

Yeha, the problem was the main ground lead. The big one wich i soldered together. Maybe i have to add a short wire that holds better in the screwterminal. That was the second time a wire got out of the terminal. Is that, why they changed from terminals to pins? [www.geeetech.com]

I definitly will add the screw terminal to my maintenance plan.

Thank you Chris for the fast support!

Screw terminals are more reliable than pins as long as you tighten them properly and don't use too much solder. In fact the best way is probably to solder the bunch to a short piece of thick cable and leave that without solder where it goes into the terminal.

I have read it is a bad idea to tin wires in terminals as the solder creeps but I have never had a problem with it on any of my machines that run for years. I tighten them once and forget about it. On the other hand all my machines that use Molex connectors fail eventually and then get replaced with screw terminals.

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[www.hydraraptor.blogspot.com]

Quote
nophead
Screw terminals are more reliable than pins as long as you tighten them properly and don't use too much solder.

I think this is here the problem. I will add some 18 AWG wire. Is 18 AWG good or do i have to go bigger?

Quote
nophead
On the other hand all my machines that use Molex connectors fail eventually and then get replaced with screw terminals.

Okay, good to hear. Then i stick with terminals.

It should really be 16 AWG for the 16A that the machine takes but I think you will get away with it if you keep it very short as the heat will be dissipated by the thick bundle soldered to it.

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[www.hydraraptor.blogspot.com]