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Need help debugging possible short on Melzi

Posted by larsrc 
Need help debugging possible short on Melzi
January 24, 2019 05:17PM
Hi Reprappers!

I have a Mendel90 with a Melzi 2.0 board on it. It has run fine for a long time, but recently I decided to finally fix the rather messy cabling around the extruder (see [repstrapdk.blogspot.com]). Something untoward must have happened during this, as I now have a connection between the hotend connector and the thermistor connector - even with the wires disconnected. Other functionality seems fine, including X motor and X-stop.

Before I go buy a(nother) replacement Melzi board, I'd like to find out if it's something that can be fixed, but I have little experience debugging hardware. Is there a guide to what resistance/operating voltage can be expected where on a working board, or other way I can use a multimeter to diagnose the issue?

Re: Need help debugging possible short on Melzi
January 24, 2019 08:35PM
everything leads back to the mpu (atmel 644p or 1284p) this what is most likely damaged

Can you replace it, yes, if you can do SMD rework (that's soldering with hot air)
If your eyes are failing and you cant hold your hands still any more... then no you can't.

you haven't said what pins are connected... just which plugs

the hotend connector has two pins
+12v and a gnd that is switched via a mosfet

the thermistor connector has two pins
signal going to the mpu and gnd

so if the hotend is on there is normally a connection from the switched gnd and thermistor gnd.

Edited 1 time(s). Last edit at 01/24/2019 08:36PM by Dust.
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