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I'm kind of upcycling an old radio with radio, Bluetooth and cast features.

The Bluetooth module and the amplifier for the speakers work with 12V so I chose a 12V power supply. It works well. When I tried to install the FM radio module, it did not work. The module turns on but the audio output is just white noise. I tried the module on a 5V power supply and it worked fine. For the Google Home Mini, which is modded to have a jack output, it doesn't even turn on. It just bootloops, but, like for the FM receiver, it works fine on an external 5V PSU. I tried multiple 12V to 5V buck converters and none of them worked.

EDIT: I checked the voltages after the multiple converters: I have 5V. I'm a kind of stuck right now. I tried searching on Google but couldn't find any solutions. Probably I'm too ignorant about my subject.

Here is a diagram of my situation:

Diagram of my electrical + audio setup

EDIT 2: If I connect directly the Google Home Mini to the 12V power supply (through a buck converter, of course,) it works just fine. If I connect the Google Home Mini with an external 5V power supply to the amplifier (which is connected to the main 12V,) it works too.

The FM module doesn't work with or without the Google Home Mini. It just works with an external power supply.

I'm using the jack output of the FM module, the L,R and ground output of the CSR8645. All my inputs are connected to a passive RCA selector with some RCA cables (which are cut when needed, for example for the Bluetooth module.) The amplifier takes an L,R and gnd input and outputs the same.

Here is a link to an identical in appearance FM receiver.

Here is a comprehensive datasheet for the CSR8645.

EDIT 3: Thanks to @AndrewMorton, I found that I have a 10V AC on the supposed 5V DC output. I tried removing the audio amplifier, which is a TPA3116D2, and bingo, and everything works fine.
So now I have to find out how to clear this 10V AC...

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    \$\begingroup\$ Can you verify tht you have indeed 5V after buck ? \$\endgroup\$
    – InfiniteLooper
    Commented Apr 23, 2023 at 13:20
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    \$\begingroup\$ Yes I checked and I have 5V. I Edited my post to add this info. \$\endgroup\$
    – Charles
    Commented Apr 23, 2023 at 13:52
  • \$\begingroup\$ maybe you have some AC on those 5V that prevents your device to work properly. I let expert help you. \$\endgroup\$
    – InfiniteLooper
    Commented Apr 23, 2023 at 14:03
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    \$\begingroup\$ Do you have an oscilloscope? \$\endgroup\$
    – Math Keeps Me Busy
    Commented Apr 23, 2023 at 14:30
  • \$\begingroup\$ @MathKeepsMeBusy, Unfortunately not :/ I know that this is what would be needed to visualize the problem but I don't have one and I have no way to access it :/ \$\endgroup\$
    – Charles
    Commented Apr 23, 2023 at 15:48

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