[Test-Equipment] Battery backup for Wandel & Goltermann PS-19

[Gollum]

I have a Wandel & Goltermann Level Generator model PS-19.
I know that it has a rechargeable battery for the backup of the settings.

In my unit the batteries show the sign of the age, they keep the
settings not any longer than 24 - 36 hours.

I opened the unit to replace them, but I have been unable to find them !
The unit is built with the customary W&G style, robust as a tank, but
utterly difficult to service, unless you are very, very familiar with it.

And the situation is aggravated by the fact that I have been so far
unable to find a service manual.

Does perhaps somebody know what must be dismantled in the PS-19 to be
able to reach those batteries, to replace them ?

Many thanks in advance

Alberto  I2PHD