Dear Vutt,
nice report . I have the U4941, which is very similiar to your model.(Without TV-Card)
I don't know if it helps, but mine had a lot of capacitors that leaked.So, please check the capacitors on the TV card and elsewhere.My main power supply on the Backside started delayed or not at all, a brown electrolytic capacitor inside the black box near the pulse width IC had leaked.
I noticed that your DC board, it provides 8V,12V,-12V,-8V,5V (the right one of your picture) is missing a shield box, my card is completely enclosed by the sheet metal.
Did you get schematics?
My one has error 900 lock det, the troubleshooting guide here from the U3661 does not help, since different RF board.