Sony UX motherboard question

Hi, I am into laptop repairs as a hobby & have recently "stepped up" to attempting component level repair work. After a couple of "easy" no-backlight repairs, I have my first challenge. The computer is as follows:

model -Sony VGN-UX380N handheld pc
motherboard- Sony MBX-150
Intel 945GMS-ICH7M chipset
cpu - U1500 1.33Ghz core solo (soldered to mobo)
ram - 1GB DDR2 (soldered to mobo)

This computer does not POST. I have the power LED come on after hitting the switch, but nothing else. No status LEDS, no display (am using known-good LCD), no keyboard backlight. Same results on AC power or battery (pc will charge battery). Cooling fan will come on if I leave the system on long enough for temps to rise hi enough (is normal fan operation for the UX).

Normal POST behavior for this series is: power LED is on, keyboard status LEDs flash once, then you get splash screen. If you hit any key then the keyboard backlight comes on.

I have the Sony motherboard schematic for the UX series- so far I have tested every S5-S3-S0 power rail I can find (12 of them) & all are present when "on" & measure within 0.02V of nominal- including VCPUCORE.

I went through the power-on sequence chart starting with the +VRUN entries since I assume that previous signals must be present in order to activate the .VRUN rails. From there to the end (H_CPURST#) I scoped each signal one at a time & "all" were present (a couple were bga to bga so I couldn't check those, & I didn't scope every pin on the clock generator chip- just know that I have a 33.33Mhz signal at the CLKdebug pins).

I stole the bios chip to fix another UX board with a bad chip-that second UX worked so the chip wasn't the cause of this no-post problem. I then got a TL866 & programmed a new chip with a Sony bios .wph & soldered that in- no change.

At this point I am looking for suggestions or ideas for the next tests I should perform to continue troubleshooting this board.

thanks Chris