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.

[ebiggs1]

"Capacitors of that age are the most likely component to fail ..."

Been subject to that recently.  Not a camera but I did have two old cameras that simply died sitting on the shelf. A year or two ago I picked up my old 1D, yup the very first, and it was dead. It made the best portrai, I mean, and just to be clear it only had 4MP to work with.

[jrhoffman75]

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

Keh Camera.