Instrument for repairing a PC motherboard

For repairing a PC motherboard , is a Oscilloscope or a logic analyzer more suited ?

I wonder where you would connect a LA at the motherboard? A general question is which faults you expect to be detect- and fixable on a motherboard. I think, CPU core supply is likely to fail and almost easy to fix. Possibly peripheral interfaces. Oscilloscope and multimeter can help.

In MB in various situation some logic like UART, SPI are coming inside so there we can use LA ?

Most of these signals is routed between external connectors and huge multi-IO-chips on recent boards. What do you want to see with a LA? I also wonder in which situations MB repair actually pays.

For sure you can inspect motherboard visualy for parts defects, such as capacitors leaking, MosFETs and other power parts exploding or cracking, melting parts, bad pins in power connector sockets,... also you can replace the broken parts with complete new or with good parts removed from some other motherboards. For scope usage and other signal logic instruments, you need additional knowledge, you need to know when and what signal you should see on what point on motherboard. For good professional repairmans this is not problem, because they pass good training for that or have many years of expirience, even they can have special service informations about board.

In any case you can take some old PC and try something. But keep in mind that you can lose lots of time on this. Lots of things are solved based on expirience, even without scopes and other equipment.

There is no single instrument that I use. You need a good post card reader, good multimeter, frequency counter. o-scope,, also some good soldering- desoldering equipment. A hot air machine is also a must have.And good schematics are very helpfull.