Raspberry Pi Display Board Schematic?

[GeorgeIoak]

Anyone seen one out there or someone who's repaired one? Had a bit of a brain dead moment and connected +5V/GND backwards and it immediately blew the board. I can't find anything that obviously damaged yet I' getting 0 ohms between +5V and GND. Most of the parts on board don't have part number marking so that makes it hard to repair if I don't know what to buy.

[frozenfrogz]

I need to admit I do not fully understand what broke down where. If you are referring to the schematics for Raspberry by main boards, they are all available on the official Raspberry documentation site.
If you are talking about one of the various displays you can buy for these boards, you might want to be more specific in order to get usable answers...

Regards,
Frederik

[GeorgeIoak]

Sorry for the confusion. Not the actual Raspberry Pi, the "Official Display Board"

[frozenfrogz]

I would suggest that you ask in the Raspberry Pi forums directly. There you might get input from the developers / tech support directly. Taken from the documentation, you need a PSU capable of delivering 2.5A - are you sure you blew something, or is it possible the power supply you are using is to weak?
I do not know if there is any reverse voltage protection - usually one would have designed the board with "stupid" users in mind, but of course it might or might not be there. If there is no reverse voltage protection, you could have already blown the display interface bridge and replacing those VFBGA64 0.5mm packages is not the easiest job. Everything else is at least QFP.

[GeorgeIoak]

I searched the Raspberry Pi forum and didn't find anyone talking about repairing a blown board so I thought this forum might be better.

It's not a power supply issue, I'm using a bench power supply at the moment. I have the capability to replace anything so that's not my worry, it's identifying the parts on the board since most just have part marking that has nothing to do with the actual part number (unless you have a datasheet).

I measure almost zero ohms between the 5V and GND pins on the board so definitely something blew and shorted.

[frozenfrogz]

Can you post some hi-res pictures of the front and back from the display PCB? I could not find the backside of the PCB via google images.

[GeorgeIoak]

These aren't the best but I think they'll do. The back side is almost exclusively caps and there's nothing under the FFC cable.

[frozenfrogz]

Do you have a flatbed scanner? If so, please remove the FFC cable and do a scan of both sides of the PCB. Without good pictures there are little to no options to remotely help with this board. Also, please mark one edge or corner so that the orientation of the board is explicit. Since there are no schematics available the only viable way to keep track of what might be going on is by identifying components and tracing the circuit manually.

[GeorgeIoak]

I do but I think the focus might be off because of the different depths of the parts. Here's link to the original picture, see if that works for you, https://www.dropbox.com/s/71z6dpwcxnwurie/2017-12-28%2014.42.06.jpg?dl=0

[mbless]

I do but I think the focus might be off because of the different depths of the parts. Here's link to the original picture, see if that works for you, https://www.dropbox.com/s/71z6dpwcxnwurie/2017-12-28%2014.42.06.jpg?dl=0

The scan is so we can see the trace routing.

[GeorgeIoak]

The USB connector on it is too tall so everything on the board is not in focus when I put it on the scanner. Even the bottom side doesn't focus because of the FFC connector.

I understand that seeing the traces is needed to make a schematic but if I can't determine part numbers I won't be able to replace what's broken and that's my primary goal. I might be able to get an X-Ray of the board tomorrow and that would certainly be better than a flatbed scanner picture.

[DBecker]

It's likely that you blew out a component when applying reverse voltage.  Basically a chip overheated, turning from a set of MOSFETs to a conductor.
If you are lucky, it's only a single inexpensive chip rather than all of them.

Figuring out which chip may be as simple as seeing which one gets hot.

My guess is U1, which appears to a 5 pin (so23-5) switching buck converter that probably converts the 5V to a 3.3V or 1.8V supply.  That's the part of a circuit that is the first against the wall when things go reversed.

[GeorgeIoak]

I'm positive a chip has blown, I've worked with electronics long enough to know when I've done something bad. I did some additional internet searching yesterday and ran across someone who actually did reverse engineer U1 back on an earlier version of this PCB. It's actually the backlight driver and if I'm lucky it's that chip so I'll pop that off the board today and see if the resistance between 5v and ground pins increases. The details of that circuit is here

U3 is a TI 65101 (based on it's part marking) and it's a LCD power solution (generating VCOM). Looking into this datasheet I see that pin 6 is used to drive a NPN transistor to generate a 3.3V rail. Right out of the datasheet it calls for a BCP54 if you want a rail with 337mA capability and I can see Q2 is connected and has a marking of BCP54.

Those are probably the 2 main power circuits on the board so I've got a good place to start. It would be nice to try and determine some of the other parts on the board but just going off of part marking makes it rather difficult.

[Aodhan145]

Try powering it up with a current limited source and see what gets hot. Try putting isopropyl alcohol on the board and see which chip evaporates it off as there are no components that are obviously blown. Just power it with the usual voltage and not high enough current to damage the board. It shouldn't damage anything extra just heat up the shorted chip.

[DBecker]

I wouldn't expect a hefty NPN to be the first to short.  U3 would failbb first to protect it ;-)

[GeorgeIoak]

I usually use my bench supply and use it in current limit to help protect against boards totally blowing up. I'm not so sure about using your method to look for the failed part. If one chip is blown that could then lead to other chips getting blown by powering up again. I wish I had a FLIR as I could use that with the meter at current limiting and see which chip heats up first.

I have had other boards with TI regulators and in my experience they can tend to be rather finicky so I would suspect U3 (TPS65101) as well but I think I'll start by removing U1, measuring the resistance and if it is still zero I'll remove U3 and see what it reads. Hopefully it's one of those 2 chips.