I've been using a Melzi (v1 I believe, with an Atmega644A) on my Mendel90 for several years now with Jumper 15 set to USB. It has suddenly dawned on me that I should be powering the Melzi from the PSU so I set the Jumper to VREG (the pins closest to the screw terminals) and tried heating the bed. It seemed to take a while so I checked the voltage at the terminal and it read 6.53V. It was at this point that I touched the USB cable and found it was very hot. Why is the heated bed being powered via the USB cable?

The extruder heats up using the PSU power regardless of whether JP15 is set to USB or VREG.

Could it be that my Melzi is a clone? I'd be pretty disappointed if it is because it came with a Prusa i2 kit from a reputable seller (RepRapKitStore) many years ago.

The USB cable getting hot is a sign that you have a poor connection between the PSU negative terminal and the electronics, or a ground loop that is carrying a lot of current. See [duet3d.com]. It's written for Duet electronics but it applies to all electronics except Rambo and Archim.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks for that useful information.

I narrowed the problem down to the power terminal on the Melzi and while desoldering the terminal to replace with a new one, the solder pad came off. It must have been making a poor connection due to the pad being damaged. The terminal twisted a bit when I tightened the screws so that could have caused it.

Thanks to nophead for suggesting I scrape the solder resist off next to the hole and solder a wire between the new pad and the ground pin. Everything is working again.