Intermitent sound each 5 seconds - Harman Kardon AVR 100

Hi,
What we are doing, in this case is looking for a shorted out speaker system, or wiring to speakers, or cables or components that are connected to the receiver that have a short circuit. This is what I would do if I were called to your house to debug your sound system.
Follow this procedure and see what happens:
1 Disconnect one set of speakers at the back of the receiver and turn the power button on again. See if it stays on. If not, go to step 2.
2 Remove the next speaker and keep repeating the test. If you don't find it, go to step 3.
3 Disconnect all of the equipment connected to the unit, one piece at at time.
Somewhere along the line, the receiver may not shut down. If that is the case, go to Step 4. If it is not the case. Your receiver will need professional servicing.
4 Identify the the offending cable(or component)and replace or service it.
I hope this solves it for you.
Best Regards,
Mark