Thanks.
8 V seems to fit from what I read out for the Electronic. No problem to make 8 V out of a 9 V battery, simply ad a silicondiod in serie. The slicondiod use 0,7 V so that it it.
I have one With no battery so I need to find out the Voltage on the battery. Anyone out there who knows?
I could also like to have a servicmanual because the one I have is form a later Version of the unit than the one I have.