My pioneer VSX-D704S won't turn on. Only The standby light turns on.
Your unit being stuck in standby mode is an indication that its having a power problem. Remove its plug from the wall and fix it back into the wall, then turn it on again. If its still stuck in safe mode, then the power supply board in the unit is the cause of the problem.
The power supply board might be defective and maybe that's why the unit won't power up. Its receiving power but it cannot transfer the current to the necessary part of the unit because a capacitor, diode, etc may have been faulty. Until these bad components are replaced, the unit might not come on.
To actually find out what's wrong, you need to disassemble the unit with a screw driver and check what's wrong. You will be needing a meter in order to test the components on the power board, once you detect any bad component, you can make necessary replacement at any electronics accessories store withing our vicinity. This is the only way by which you can detect the problem.
Of course, testing the power board with a meter will require someone that has the ability. So it you can't, then you might be needing the help of a repairman or you can call a friend that has the ability to help you out.
I wish you the best of luck.