I was handed a Fitbit Blaze for repair, which was stuck in a reboot loop; I don’t personally own one — I have a different brand!

Originally, I thought this problem might be due a bad update, or that the on-chip flash NAND was worn out and could no longer handle writes. However, the behaviour was somewhat inconsistent, sometimes it would boot and show the time, but either way, the touch screen would be completely unresponsive. The buttons seemed to work, but only some of the time.

The Problem

Nothing software-based would solve the problem, and, to my surprise, this device has no hard reset!

In theory, a non-functional touch screen wouldn’t normally cause a boot loop, but I guessed that bad digitiser (touch screen) data might cause this to be the exception. I set about following the iFixit guides and delicately dismantling the device, but there was no obvious damage inside.

Isolating the Digitiser

Fitbit Blaze Disassembly
Fitbit Blaze Disassembly

There are multiple sockets in the device. It is possible to work out what they are:

  • Top-left: not many conductors, so likely Bluetooth antenna and/or auto-brightness light sensor
  • Right: leads to heart rate monitor and barometer enclosure
  • Bottom-middle: very wide and lots of conductors, so almost certainly the display

That means the bottom-left port, close to the bottom-middle and next to the display, is likely to be for the digitiser.

In terms of what else can be seen, the terminals above the display port touch some pogo pins, which lead to the USB charger. The red and black cables at the top left are attached to the motor. The battery solder points are underneath.

Checking the Digitiser Connection

When unclipping/removing the tiny ZIF cables from the main PCB, there was a tiny amount of green residue in a socket. This appeared to be the problem.

I also noticed that the heart rate monitor resin window had a huge crack across it, which had apparently been there for years, and it was full of residue. There was another crack on the plastic base near the corner. I proceeded as follows:

The Fix

This was mostly a case of cleaning up and fixing any damage, but bear in mind the parts are very delicate. I:

  1. Ensured all the ZIF cables were detached, including the end attached to the heart rate monitor.
  2. Cleaned up all the tiny ZIF connectors with isopropyl alcohol.
  3. Removed the plastic rivets holding the heart rate sensor in.
  4. Removed the tiny heart rate sensor PCB.
  5. Removed all the broken resin.
  6. Cleaned up the heart rate sensor with isopropyl alcohol.
  7. Made a new rear panel out of a clear plastic lid.
  8. Superglued the outer edge of the plastic lid to the device, overlapping the broken heart rate monitor window.
  9. Placed the heart rate sensor back in its housing, and used melted hot glue to hold it back in where the plastic rivots were removed.
  10. Carefully superglued and held together the remaining cracks in the case until it was set.
  11. Reassembled the device ensuring seals were intact.

After cleaning up and reassembling, the device functioned properly, and there was one happy owner. It has been working ever since:

Working Fitbit Blaze
Working Fitbit Blaze