Trouble shooting damaged Melzi Ardentissimo

Hey folks! About a month and a half ago I got a damaged Melzi from a friend building a mendle Prusa. From what I know he was testing the printer and making adjustments with the pots on the stepper driver ICs and he managed to blow the 5v reg and whenever it connects over USB it draws to much current (on my mac the port gets disabled).

He told me that he kept powering it on and off again and poking at it with a screw driver, he enlisted the help of someone else to break off the 5v reg and replace with with a through hold LM7805 crudly soldered on there but that didn't fix it. When I say break off the 5v reg I mean it, its was like someone tore it off with pliers, the reason being is that these guys are somewhat inexperience and could not desolder the board (RoHs compliant solder I presume) becaue the solder was too high temp. I managed to remove the remains of the 5v reg with a hot air reflow workstation. I also replaced the FTDI chip but that did not fix it. Somewhere on the board its still shorting but I don't know where to check. My next step is to replace all the ICs one at a time until it works again. In the meantime I will be removing all through hole components so I can flux down the PCB and relow it all to clean up any excess solder or shorts.

Any ideas? What IC should I try to replace next? I got this board for free so I'm trying to get it working for my first RepRap (Rostock). I have a lot of electrical engineering experience as well as a few years doing PCB design but I am not seeing any obvious shorts from examining the PCB. All I can think of is badly damaged ICs.

Is the jumper set to VREG or VUSB?

The most likely chip shorting the 5V would be the stepper driver he was adjusting.

One way to find a problem like that is to inject 5V into the board with a current limited bench supply and see which chip gets hot.

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[www.hydraraptor.blogspot.com]

I'll have to give it a try next thanks! If I needed to replace the pots on the melzi where can I even find those? Whats the aprt number? I tried searching for it but I couldn't find anything.

here is the wiki for the schematic [reprap.org]

the pots are standard surface mount 10k trimmer pot. It does not say what they use, but I've found an equivalent part that people can reference.

like found here at digikey. here is a link. cost $1.38 each.
[www.digikey.com]

the part # I'm referencing is

Manufacturer Part Number
ST32ETA103
Description TRIMMER 10K OHM 0.125W SMD
Lead Free Status / RoHS Status Lead free / RoHS Compliant

this should help you find it about anywhere. i would imagine that 1/8 watt would handle the current at 10k resistance. it is used to reference feedback voltage of output to steppers.

for size and detailed specs look here [copal-electronics.info]

Edited 1 time(s). Last edit at 06/17/2013 06:17AM by jamesdanielv.