Here is my situation,hope someone will help me
1:falcon cb5761 orignand read with lpt (block 1A found at 3FF)
2:nand seems ok so flashed xbr,greenlight in the middle of rfunit no video audio with hdmi cable
3:reflashed orig nand after 2-3 sec of xbox logo-->E79(forgot to remove jtag wires)
4:removed diodes and all,xbox no powers on,rf unit shows red led and it is damned hot
5:rf unit broke,changed with an other one,xbox starts but no audio video or any led on rf unit,-->no sync
6:that's all now I have the original dump on it but still cant sync controller or see anything(hdmi/component)
dvd tray still open/close and I hear noise with fallout 3 original disc

it is southbridge related?gpu?I've bought it on ebay but before all this it started with old blade dash fine.
of course I have no cpukey

What wiring are you using?

I have the same system (same CB) I am trying to JTAG that is giving me all sorts of problems.

Anyways, I was in the middle of flashing different images and I lost my video output. I thought it was a bad flash so I dumped the nand and verified it matched the original. In the end it turned out the GPU just needed to be reflowed.

Your system was probably reflowed or x-clamped and sold. You can try holding RT and Y after you turn on the system to reset the video. Do you see any bars or lines or anything when the system starts?