Dell 7920 Tower, 2 amber 4 white, have changed memory with no effect...ideas?

I have a 7920 Tower with a Silver Xeon 4110 CPU in.

I'm getting a 2 amber, 4 white error message when booting.  I've now tried 4 different memory pairs without a solution and have checked the memory in another machine and it seems OK.

Is there any way to 'force' the 7920 to do a BIOS reset before POST, so that it tries to reread the memory and resets it's internal 'what have I got installed' table?

I've read that I should be able to hold the Fn key and apply power to enter Pre Boot assessment

https://www.dell.com/support/manuals/en-us/precision-7920-workstation/precision_7920_om_pub/dell-enhanced-pre-boot-system-assessment-epsa-diagnostic-30

but my keyboard doesn't appear to have a Fn key as all the Function keys are single purpose (not dual function).  Sorry, just to add, I'm using an old PS/2 style keyboard as I'm assuming that a USB keyboard won't be recognised pre POST?

Any suggestions or ideas gratefully received.

Edit added - Found the CMOS jumper so going to try that.

CMOS didn't work, so looking for suggestions again.