TM-6900 HD COMBO will not boot

I've been given a dead one which I got working by replacing 5 bulging caps in the psu.
Last night I installed the last up to date Phantom patch I could find via a USB stick .
All went well " write ok" displayed on the TM display but the box didn't reboot so I left it like that for 20 minutes and still no reboot .
I turned it off via the back switch ,gave it 10 seconds or so and switched back on --- nothing -- no blue light - no display.
I've tried the TM Loader ver 1.8.8 with null cable but still nothing on switch on .
I know the cable is good as I used it on my TM 1500 CI+ with Darkman loader for loading keys.
Tried holding in the menu and select keys while powering on -- nothing .
The friend who gave me the box has an identical one and he successfully loaded the same patch as me onto his without any problems .
All voltages coming out of the psu are exactly what it says on the main board power input plug

I think I have a doorstop unless anyone knows any different please .
Thanks

Just a quick follow up -- a power supply from a known working 6900 makes no difference so the power supply isn't the problem .
An annoying mystery of what has gone wrong and how to fix iit .
Anyone know any points on the board to bridge to force a memory clear/reset please?
Thanks

There are 2 different power supplies on that model use the wrong one and it won't work all nos. should match on the board.

Thanks fred fickle -- I checked all the numbers printed on the connector to the main board and they matched .
Something went **** up during the software update - it's resting in my attic until a winter project !