I was setting up a mobile power supply for my Raspberry Pi 2 consisting of: - a 12V Battery - a step down power converter that converts 7-24V to 5V/3A

It was working fine until I connected the battery to my power supply for charging while the system was running. That shouldn't be a problem but I was so dumb to first connect +battery with +supply while the power supply was switched off. As expected, the RPi was rebooting due to power loss but since then HDMI output does not work. Everything else seems to work just fine.

I did that failure again to measure what happens. I figured out all that happens is that VUSB was grounded very shortly and must have been back to 5V after approx. 500us.

Is there some special component as part of the HDMI circuit that might be broken and can be replaced? What could it be?

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I know its been a while but did you get anywhere with this?

I have a second hand Model B which refused to output any HDMI - from checking with a DMM I found that Pin 4 on the HDMI connector was shorted to Ground which, as Wikipedia shows is a signal data line. Checking with the schematics (see top right page two) for my type of model I can see that most of the HDMI signal lines go via clamps or direct to the BCM-2385 {yours would be very similar I believe}. Either the clamp diode to ground on that line has failed short-circuit to ground - which is a possibility from static damage IMHO - or the output from the CPU/GPU has suffered the same - or indeed both!

Fortunately for me, I can get by without that display (the unit is to be run headless apart from a 16x2 LCD status panel) and can use a composite output to a TV if I need to. Which is just as well as even replacing the minute clamp diode circuitry is not a feasible operation and obviously the central processing core is not replaceable.

For you it might be useful to carefully check with a digital multi-meter for shorts to ground on the signal lines on the HDMI connector, but try and always ground yourself by e.g. touching a bare metal pipe to a central heading radiator before touching any connection on the Pi again - static damage can be quite insidious in its effects on ICs so it is best to remove any excess from yourself first.

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