I have spent a while looking around at different solutions for this because I don't like posting duplicate threads on any forum, but I think my problem might be unique.
I have a B2 pi, with the official screen (obviously). I removed all peripherals in an attempt to find the root of the problem.
Basically the problem is that my screen will not function at all even though it has functioned perfectly before the events that I will relate at the end. I ran these commands (with a clean sd card flashed with Raspbian Stretch 4.14 )
dmesg didn't output anything
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dmesg|grep -i ft5406
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pi@raspberrypi:~ $ lsmod Module Size Used by fuse 110592 3 cfg80211 573440 0 rfkill 28672 2 cfg80211 snd_bcm2835 32768 1 snd_pcm 98304 1 snd_bcm2835 snd_timer 32768 1 snd_pcm snd 69632 5 snd_timer,snd_bcm2835,snd_pcm uio_pdrv_genirq 16384 0 uio 20480 1 uio_pdrv_genirq fixed 16384 0 i2c_dev 16384 0 ip_tables 24576 0 x_tables 32768 1 ip_tables ipv6 425984 24
All ribbon cables are seated properly, there's no obvious damage (it might be a good idea to use a magnifying glass to look closer), the pi and screen are being powered separately from powerful enough power supplies and the pi works with the hdmi (but not when the 7" display ribbon cable is plugged in).
I am planning on using this in a Bluetooth speaker that will be powered off a 15.2V LiPo battery (I have a few reasons why). I am using dc to dc buck converters to step down the voltage to 5 volts and that's where the problem comes in. I was setting each converter to the proper voltage and started at the amplifier (https://www.adafruit.com/product/1752). At the time I didn't know this, but I was stupid and soldered the capacitor on backwards. Because of this, or perhaps something else, I pretty much blew up two converters (the main ic split with a loud pop). Two others were acting strangely, one wouldn't go much higher than 2.6 volts and the other I was able to set at 5 (but it took far too many turns to get it to change even a small amount). I figured out finally what the problem was with the amplifier and fixed it. Because I only had 6 converters (before I ruined any), I decided to use the that I was keeping for the screen and replace it with the slightly broken one, I believe this was where the mistake happened. The amplifier worked perfectly with the new converter, but the screen wouldn't turn on with it's new converter. I tested the output again and discovered it had gone down to ~2.6v. When I came back the next day (after ordering more converters) I plugged the screen into a normal power supply, just to see if the rest of my project worked, I discovered that the screen wouldn't turn on. I have gotten it to flicker white once right before the pi booted up, but that is the closest I've gotten it to functioning.
I'm afraid that somehow the broken converter sent a full 15.2 volts into the screen and either tripped something or broke something. I can provide pictures and/or more information if needed. If someone could help me figure this out that would be greatly appreciated!