I have a TIU Rev. L that only power-up through the USB port. All the fuses are good no visible damage
to the internal components. The led will flash once and stay off when i apply power to the "Aux power input"
with a 18VAC power supply, the same thing if I use the "Fix voltage in 1".
When it's powered with the USB cable I am able to do back-up, upgrade, etc. It was sent to MTH for repairs some years ago.
Any suggestions on what the problem ?