1D Mark II N doesn't power on

[forcanon]

Hey,
I have a problem with my 1D Mark II N.
Can you help me please?
The camera doesn't power on.
The original Canon NP-E3 battery seems to be dead. I tested it also with a voltmeter - it seems to be dead.
So I bought a new battery - but an off-brand (not Canon). The camera doesn't turn on with it.
Then I bought another off-brand battery (not Canon). The camera doesn't turn on with it either.
Both new batteries are good/charged (I tested them with a voltmeter).
When these batteries run low on power, the "low battery" sign is blinking on the top display of the camera - so the camera seems to get power (I also replaced the time-date battery - CR2025).
Also, when I fiddle on purpose with the lens mount, then the Err99 is being displayed on the top display - so the camera is getting definitely power from the NP-E3 battery.
When the battery is placed in the camera with the power switch ON - the voltmeter detects voltage on the flash connection pins.
And when the power switch is turned OFF - there is no voltage on the flash connection pins.
I have read on the net, that a firmware update can influence (or that it can cause) the (faster) "dead" of the mainboard.
I updated the firmware about 3 years ago. And now since about 2 months the camera doesnt't turn on.
Can this be the issue? How can I resolve it?
When I connect the camera to the PC, it doesn't get recognized.
What is the name of the mainboard (with the firmware saved on) ? Is it maybe CG2-1255 (the DC/DC converter) - or maybe CG2-1218 ?
Does anyone have an original Canon NP-E3 battery and live in Nuernberg in Germany and is willing to let me check if my camera works with this original battery?
I have found a post here on this forum with a similar problem ( https://community.usa.canon.com/t5/EOS/1D-Mark-2/td-p/215241 ) - but there is no solution provided.
What should I do?
I disassembled the camera - but I can not locate the problem.
I would be thankful for your help.

[jrhoffman75]

Depending on which is in better condition you can hve a working camera and a supply of parts.

Keh Camera.

[ebiggs1]

"Those older full frame digital sensors ARE kings"

However the 1D Mk IIn is not a FF camera but I do agree it makes glorious photos. No matter if it working with only 8MP. You may be thinking about the 1Ds Mk II.

BTW, the 1D Mk IV is not FF either. I don't believe a FF was ever made on the Mk IV design.