Asus Prime B450M-A starts only when 12v 8pin cpu plug disconnected

[arabex21]

Hello everyone. I have been reading this forum for a long time - I really appreciate the amount of very interesting materials and information in the field of electronics.

I have a problem with this configuration:
ASUS PRIME B450M-A
RYZEN 1300X (I replaced it with ATHLON 200GE - the same behavior, the processors are functional, tested on another board)

When the entire set is installed (CPU, RAM, ATX connector and 12V 8 pin CPU plug), the board starts up for a while (about 1-2 seconds), the CPU fan starts and the board turns off immediately. When I disconnect the 8-pin CPU power connector, the motherboard boots up. Interestingly, i can start it using SW_POWER_ON but I cannot turn it off by holding SW_POWER_ON longer.
Could it be the BIOS's fault, or should I focus on the CPU power section? I measured the board for shorts on the ATX connector and there are no shorts, as well as on the 8-pin 12V CPU connector.
I looked at the mobo carefully under a microscope - there was no mechanical damage. Unfortunately, I don't know the history of this motherboard.
Maybe someone had a similar case?

This is my first post on this forum. Greetings to you.

[DynaxSC]

Hi what do u mean with " ...boots up ..."? Without the 12V ATX Power the CPU gets no VCORE so the board can't boot normally. I guess u mean "the board powers on, but does not boot".

In this case the issue must be in the VCORE PWM. Check all mosfets, resistors and mlcc caps. Also the VRM controller and phase doublers, if any exist, can be the reason. BIOS can't have anything to do with it. Also the CPU can be broken, have a short. For CPU check if the board powers on without CPU, but ATX 12V connected (but not all boards do it by design). Also check the PSU cable for continuity.

[arabex21]

Thank you for your reply. Yes, of course the board does not boot, but it does start, sorry for the lack of precision. Only when the processor is in socket, the board's LED backlight lights up and there is a response to PS_ON+GND. The processor is functional, it works with another motherboard (it was also replaced with another one). The power supply is ok tested with another mobo's. The board without a processor does not respond at all, regardless of the ATX 12V connection. I'm waiting for the shipment with the TL611pro, then I will be able to check the POST (the connected speaker does not make any sounds). Once again, thank you very much DynaxSC for the tips, i will take measurements in the CPU power section and update post.

[DynaxSC]

Hi, get better the TL631, its never, TL 611 is old stuff. Regarding starting w/o processor it may be correct behaviour, some boards check whether socket is occupied. On Intel boards there is a pin called Socket Occupied in the cpu socket, it is routed to the SIO, which checks the signal. It can be tied to GND, then the board will start w/o processor. But I'm not sure how it is done on AMD plattform.
BTW the TL6xx will not be of big help when board does not start to boot, it will show only 00 at the beginning, as long as the processor will not run the Bios program. You should first check the power sequence where It stops. The diagrams are quite similar between different boards, so if you don't find the one exactly for your board, try with some other with same or similar chipset. You also will definitely need then the boardview for your mobo.
On the Bios chip you can check with a scope, if the data is beeing read by the processor, eg. on pin 2 (data). On AMD platfirm the Bios is connected directly to CPU, whether on Intel it is read by the chipset and it's firmware OS (Management Engine), then routed to the CPU.