I've been using this device for more than 8 years. Firmware and maps were updated in 2017.

Yesterday, quite suddenly, it stopped working/booting. All I see are colored stripes when booting it up. After like 30 seconds, the screen just goes dark. The buttons are still illuminated white (which does not match the "random color theme" I set in the settings and have been using for all those years) and the device heats up, so theoretically it's "working"/powered up, but not doing anything useful.

I'd rule out any direct physical damage. Summers tend to get quite hot, though, so it's possible that the device was just literally grilled to death. The fuse has obviously not blown, although the fuse connectors do look... a bit strained. Probably nothing to be worried of, though, since it's been drawing quite a lot of power for so long.

I definitely rule out a software issue, since I haven't been playing around with any files whatsoever for years. As far as I remember I haven't even dismissed the start up notice for the past few weeks.

So far, I've cut power to it and will try a cold boot tomorrow. Also, while at it, I'd like to start it in test mode. As far as I've understood I just have to put the super test mode key file into an SD card and boot up, right? [1] Luckily, inserting and ejecting SD cards is still working (even though through the failsafe mechanism that slides the display down when pressing the eject button for more than 3 seconds). Maybe the internal flash drive is somehow hosed.

If I cannot boot into test mode, is there any way to boot from the contents of an SD card instead of reading files off the internal flash drive, in case that really broke? If yes, what files should I take/boot up? The 2017 map update? [2]

If that's not possible, do you think that exchanging the main logic board could help? [3] To that end, is there any documentation of the internal layout? Like an exploded view with descriptions of the modules? [4]

Also, I've found a used unit that might be helpful for my case. The owner says that bluetooth is not working for him, but that could be anything from outdated firmware, or phone hardware incompatibility to a broken BT module. Do you know if the BT module is on a separate board? [5] If yes, I could just rip out the main logic board, or, in the worst case, replace my device with the other one after swapping BT modules.

Here's the Z110-BT and F10BT Service Manual. Should be the same hardware as your F20BT.   Z110-BT Service Manual

Wow, thanks a lot!

That definitely helps. The BT module seems to be located in the monitor assembly, which sounds odd at first, but makes sense: you want to have as little interference as possible.

I guess I can get it working again with that.

Unit is working again. The defective part apparently was the NAVI unit. The power supply parts were probably hosed on this board - and it is the second-in-chain that receives and passes power down to other units.

To answer my own questions:

On 8/3/2019 at 11:04 AM, Ionic said:

Also, while at it, I'd like to start it in test mode. As far as I've understood I just have to put the super test mode key file into an SD card and boot up, right? [1]

Yes, mostly. Inserting a card with the super test mode key will make the device automatically reboot into test mode if it's working correctly.

On 8/3/2019 at 11:04 AM, Ionic said:

If I cannot boot into test mode, is there any way to boot from the contents of an SD card instead of reading files off the internal flash drive, in case that really broke? If yes, what files should I take/boot up? The 2017 map update? [2]

No idea/didn't check. The CC unit that includes the flash memory wasn't broken so the device booted up correctly once the power issues vanished.

On 8/3/2019 at 11:04 AM, Ionic said:

If that's not possible, do you think that exchanging the main logic board could help? [3]

According to the service manual, the CC unit includes the flash memory, so yes.

On 8/3/2019 at 11:04 AM, Ionic said:

To that end, is there any documentation of the internal layout? Like an exploded view with descriptions of the modules? [4]

See RonS's answer above. Additionally, there is an incomplete service manual for the Z120BT/F20BT series. It's not as useful as RonS's complete service manual for the Z110BT/F10BT series, but it does include part numbers and some exploded views, so it can still be a handy document when salvaging parts from other sibling devices (i.e., to check whether other parts would fit for instance).

On 8/3/2019 at 11:04 AM, Ionic said:

Do you know if the BT module is on a separate board? [5]

BT module is in the display assembly and can be exchanged if needed. According to the manuals, the BT module is connected using a BtoB (floating board to board) connector, so it's easy to remove. The tricky part is disassembling the display assembly, though. Note that SOME parts cannot be disassembled - doing so would break them. The illuminated keyboard part being the prime example. I personally haven't disassembled the display assembly since it wasn't broken (other than the BT module in the other device, but I'm not going to get a spare part that is being sold for $245 (!) at pacparts).