Having recently completed a major restoration of a Beocenter 7700...

Your Beocenter is likely to have many, many faults.  But in particular, it would be a good idea to check the main power circuit-opening relay RL1 on board 3.

This relay is a double-pole single throw normally CLOSED   that interrupts main power when the system in in standby mode.  It is likely to have failed on at least one of the poles, which can cause either the +30v supply (which powers the 15v and 12v systems) or the -30v supply to misbehave.

To test the relay, first plug the system in while listening closely. The relay should "click" upon inserting the plug, as the system goes into standby.  If the standby button still operates, the relay should click as the system is turned on and off (standby).  Clicking of the relay indicates the 6.5v standby power is working, and the relay coil correctly receives power.  

If the relay fails to click, it could be a fault in the standby circuits, or a failed relay.  The original relay has a removable clear plastic cover, so the contacts can be inspected without disassembly. The contacts are made of a fairly toxic metal, so you might want to avoid opening it. If the contacts appear dirty or burnt, the resistance is high, or the plastic is melted, you will need to replace RL1. Schrack makes compatible relays, a 6vdc DPDT RP2 or RT2 will fit.

This is of course, just one of many likely failure points, among many more possible ones. 

Keep at it though, as a working 7700 is an exceptionally nice looking system that can also sound magnificent.  I'm running mine with Beovox 45.2s, which seem better matched to the system than RL60s....

Good Luck!