Mac Plus Startup Sequence error

[Borgmac]

Hi,
A few years ago, I got help on this forum for a Mac SE startup issue. Thanks again @Kai Robinson and @Branchus.
This was actually my first post here (Dec 2021!) and I enjoyed since then numerous contact with members.
Today, I am back with a startup issue, this time for a Mac Plus.
Here is the video.

As for the SE, the grey background is there, but the corners are not rounded. It was at that time an ADB issue.
For this Mac Plus, what I understood from Troubleshooting_Your_Macintosh_1992:

1. Hardware Initialization is OK
2. The P.O.S.T.
   1. begins by testing ROM and other critical circuitry on the logic board. If all is well, the machine emits a medium-pitched (soft) startup chord that sounds like Bong! OK as I got the startup sound
   2. Next, the system memory is tested. if a Mac Plus or Macintosh SE has a RAM failure, it would be unusual to get audible tones. Instead you typically face the infamous Sad Mac. OK as I do not have a Sad Mac.
3. Operating system startup sequence
   1. Now that the hardware has been initialized and tested, we come to the final stage: the Macintosh operating system startup sequence. At this time the operating system software is loaded. Here's how it works: Your Macintosh computer searches all floppy drives for a diskette that contains a file called System File. KO If I connect a Floppy drive or Floppy EMU, it does not react.

Tests already done:

• Voltages on the Logic Board connector, all OK.
• Switch the logic board from this Mac Plus to a working Mac 128K, got the same issue, so Analog Board should be OK.
• Tested between working 128K and Mac Plus
  • TSM and LAG signals, OK
  • HSYNC at logic board J8 connector are similar
  • I have ordered new electrolytic caps to replace the old ones, but they look quite good.

Does anybody has an idea of what is stoping the startup sequence before getting the rounded corner on a Mac PLUS?
Thanks for any help or suggestion.