See:
https://ibb.co/QPhK0BP
Remove LEDs from the board.
The very low voltage of the Kindle's serial port disrupts the current used to drive the LEDs.
You can find that information in Geekmaster's post on building a 5v (or 3.3v) converter to 1.8v
Use "prefix" set to "tools", click "show threads", then find there (second thread on list):
"Serial jailbreaking your fw ..."
https://www.mobileread.com/forums/sh...d.php?t=267541