Roksan Kandy amp - speaker relays switch sometimes with delay

Hi Folks,
my Roksan Kandy KA1 Mk III has a small issue. When the amp is not used for some days and then switched power on, the speaker relays take about 15 seconds to switch through. After listenning for a while, switching off and then immediately switching on, the relays switch through within 3 seconds as used. In short: the issue happens when the amp is cold. What can it be ?? Otherwise the amp is working/sounding good.
 
One solution someone told me but I cannot imagine that this works. He told me about a battery in the amp that could be empty. Just had to set in new battery. I opened the amp but where the hell is here a battery ? Anyone an idea ?