3B+ Power LED regular flashing sequence, no Green activity LED

Hi,

Just tried powering up a 3B+ using three different wall plug PSUs (5v, 2 - 2.5a) and a Manson HCS-3202 bench PSU.
With all of them, the red power LED continuously goes through a sequence of four long and three short (might be 4 and 4, not sure) flashes.
The green activity LED never lights. These PSU's all properly power other Raspberry Pi 3's (and their LCD panels) that I have. All PSU's have been checked against a meter and seem to be operating correctly (good V and A, within tolerance with dummy load). I know from previous postings that a flashing power (red) LED is usually down to a problematic PSU. But those never refer to a regular sequence of flashes. Is this something new to the 3B+? I can't find any documentation on-line for it, if it is new. In this case I don't think the issue is a problematic PSU, could be wrong but I don't think so. Any ideas?

Please help.

I didn't find the right solution from the Internet.

References:-
www.raspberrypi.org/.../viewtopic.php
Operation Management Service Video

Thanks!

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