Hi to all
I managed to serial jailbreak my kindle voyage with this board:
https://www.amazon.de/gp/product/B0178HVEH0 (removed the jumper - did not use a voltage level shifter)
Just soldered part of a floppy cable to the serial ports RX/TX and taped a cable for ground on one of the metal plates.
I had a little problem - could not get into diags via bootm 0xe41000 but I copied the factory firmware to the kindle in advance and when you boot up serial at some time there is a message "Press ENTER to open menu" or something like that where you could select to upgrade/downgrade the Voyage with a bin file from /mnt/us. After that I was able to use the installHtml jailbreak method.
Thanks to all
rola