Re:"* It looks like the CPU is forcing the receiver into standby every
second or so (_standby voltage to power board alternates between 5v and
0v. as measured on the ribon wire), * the clicking relay is on the
standby board, there is no display when relay is clicking"
That is standard protect mode. When a problem, like a wrong voltage or a short is detected from CPU, the system is forced to standby. The clicking noise is the safety switch forced by CPU every tot seconds. there will be no operations apart standby when in protect mode. The CPU higher voltages get shut down to prevent further damage, only low +5V is kept on to show the diagnostic protect mode. The clicking is just a signal that there is a fault, and not the fault itself.
Re:"EXCEPT when multi button is pushed, then clicking stops and I can see display"
That is a better hint on what the fault could be. On multi room different speakers are used. start checking speakers connection, focus on the speakers that are eventually not used in multi room.
If speakers connections are OK, concentrate on terminal block (input output) circuits, chck the surround mode selector. Testing is mainly done at component level, ensuring that there are no shorts to chassis, cold solder joints (broken ternminal contacts), or faulty components in this areas.
Since the fault is not ocurring when in multi room mode, you can for example exclude circuits like power supply transformer and video board (if present), that are not affected from changing to multi room mode.
If there is no error in there the following troubleshooting will be on the complex audio section (from preamps to power amp final and terminal).
About not getting into service mode, that would be quite unusual. When a marantz is on protect, it still goes in service mode and displays PROTECT at start.
I do not have the manual for SR8500, but you can follow instructions on how to get in service mode from page 32 on the manual for SR4400, uploaded here.