HP 4285A "CORR DATA CSUM ERROR" boot message

[rhb]

I just received a 4285A from ebay and at power up it displays the message in the subject line.

Does anyone know anything about it?  I've queried the HPAK list on groups.io and gone through all manuals without learning anything useful.  I  surmise it's a NVRAM battery failure, but locating the part is a bit of a challenge as there are no components visible when you take the skins off. So considerable disassembly is required to play hunt the wumpus.  I've not been able to find a service manual with component level data.

From the manuals I've read a short-open-load kit is used to generate the lost coefficients.  That is listed in the maintenance manual as HP 16342A, though there seem to be others.

Reg