One of my RB433 doesn't see any mPCI card.
They're not vissible neither in Interfaces nor in System -> Resources.

It worked perfectly for 1,5 year and suddenly stopped
I've tested few types of mPCI cards - any results.

It's not a software problem (I've updated RouterOS via Netinstall), it's a hardware issue.

Do you have any idea which part should be replaced to repair that board?

Do you test all mPCI slots? Try push and hold CPU when you turn on RB and booting RouterOS

The Atheros processor.
Unrepairable.

Not exactly :))

Not exactly )

could you give us more details

Depend what symptoms you have. Write to support@mikrotik.com I hope he can give you complete solution.