A20 LIME SMBJ6.0A Detached - power supply overheated

[julesS]

Hello,

I have just got A20 LIME without NAND, and am having difficulty powering it.

Yesterday I tried USB-OTG powered either from my Laptop, then from a Wallwart that I have used to power an RPi.
The power LED glowed faintly.
In both cases I measure 5V on the VBUS pin of the USB connector, but do only see very low voltages on power pins on the GPIO connectors.

Today I tried powering the board using PWR_JACK. I first measured the supply voltage which is between 4.99V and 5.02V. This comes from a Traco Power TSR 1-2450 which delivers 1A at 5V, so should be a suitable regulated supply.

The power LED is now brightly illuminated, but the board otherwise appears to be dead.

At this point I have the HDMI connected to a TV set, and a Sparkfun FT232R breakout board attached to the 3 pins for UART0 (this is set to use 3v3 IO). At first connection the LED on the breakout flashed briefly.

Note that I cannot use the Olimex debugging cable because I cannot get the Prolific PL2303 driver to work on my laptop, so the FT232R is an attempted substitute for this.

After a short period of time I noticed excessive heat in the power supply. Which melted the plastic surface the board was resting on. I removed the power!

I then see that the diode marked on the schematic as SMB6.0A is no longer attached to the board.

I plan to resolder this and try again.

Has anyone else had this happen?

Is resoldering it likely to work?

I have now looked at the board more closely. At present both D3 and D4 are missing.
There is a slight heat mark at the ground end of D4, so this is still component that fell off.

There is a high resistance between the pins where D3 should be.
Should D3 be populated?

Further measured in either direction the resistance of the detached D4 is about 16.6 ohms.
You might suspect my meter tries both directions so I tried a small signal diode which is different in each direction.

Does this mean that D4 has broken down, and should be replaced with a new one?

Should this breakdown only occur if more the 6V is applied to the power jack?