HP Z400 six beeps problem

I have an HP Z400 workstation with the problem described here.

It arose in the same manner: it was working, I changed the BIOS settings and in particular, the PCIe-slots to "compute". The consequence is the same, on power on it gives six beeps and halts without giving a chance to change BIOS settings.
As for the other user, the reset CMOS button has had no apparent effect.

I have left it unpowered, without battery, for weeks. The problem persists.

The other user found success using a PCI graphics card, but I do not have one.

I have tried the existing card in a different slot to no avail. I've added another, presumed good, card, to no avail. I have tried with no card, that didn't change anything either.

I suspect installing one or two cards supporting "compute" will fix the problem. I'd be happy to buy one if that's what it takes.

The workstation has a quad-core XEON processor, 2x SATA HDD, NVIDIA Quadro 600 1GB Graphics card from HP.



The manual is here. See p 74 for motherboard layout, and following pages for disassembly/assembly instructions. Tools are not required.
PP 123 and following describe removing and replacing cards.