Onkyo ht-r500 no speakers attached goes directly into protect mode, standby light blinking..

Enter debug mode to see which type of protection causing amp to shut down.

The amp protection circuit has been activated. Remove the power cord from the wall outlet immediately. Disconnect all speaker cables and input sources, and leave the AV receiver with its power cord disconnected for 1 hour. After that, reconnect the power cord and reconnect an input and a pair of speaker wires onto the unit. If the receiver still does not turn on than you will have to bring it in for service. Turn up the volume and if the receiver doesn't shut down, power off the receiver and add another pair of speakers. Power up the unit again and turn up the volume. If it doesn't shut down, power off the unit again. Now connect all your input sources and turn on your receiver. If the unit shuts down again, your unit will need to be further serviced at an authorized Onkyo service center.

Most likely, you have DC Offset(a bad amplifier). The unit is shutting down to protect your speaker system, as well as protect the unit from any further damage. If you require more assistance, or would like to inquire about repairing your Onkyo, please visit my website at audioserviceclinic.com. You may contact me there. Thank you.

Looks like a short due to loose connection on one of the speakers.

Try disconnecting all the speakers and see if it does the same thing. If it doesn't, connect speakers again one at a time to find out which one of them is doing it. Make sure the speakers rating match the output of the amp.

Activated protection circuitry means that the amplifier is seeing conditions it does not like and is shutting down the power to prevent further damage.
Either you have a short in your speaker wires, in your speakers, or a blown amplifier.
Disconnect both speakers and turn on the amplifier. Does it still give the error message? If it does, your amp is blown. If it does not, carefully inspect your speaker wires and their connection. Make sure all looks good and reconnect the speakers and try it again. Still get the error message? You likely have a problem in your speaker cabinet(s).

If you call Onkyo directly, 201-785-2600, there is a procedure to "reset" the system, something like holding the power button on and holding down another button for a few seconds. Can't remember which, so I can't be sure, but try giving them a call.

I went back and found a loose cable/coax. Re-attached and now the receiver works as good as ever.

This Yamaha model has a "Thermal Protection Integrated Circuit", You can check the online manual at www.yamaha-uk/pdf/owner_manuals/rx-v2500.pdf