head unit wont turn on

Hi all, I own a '93 Toyota corolla hatchback that recently had to be jump started. Since then my American HIFI-510 head unit will not turn on. The manual that came with the head unit does not mention any security code that needs to be entered after being disconnected from power. I have checked every fuse in the drivers side kick panel and under the hood, all appear to be fine. I have also checked the fuse that is attached in-line to the power cable of the head unit, again it appears fine. I have noticed that when attached to the car (ie being powered by the battery) the red LED behind the volume knob lights up, leading me to believe that the unit is receiving power. However the display remains blank and buttons are unresponsive, including the "reset" button. Any advice would be greatly appreciated

Try to disconnect power from radio, and then plug again.

Pull the radio out and use a volt meter to verify you have power on the red and the yellow wire, and ground on the black wire going to the radio.Kevin Pierson

Thanks Kevin, With the car battery on, I tested the wires you suggested for DC voltage The yellow line has no power Likewise for the red line (referred to in the manual as Memory ACC) The ground, tested with one contact on the yellow wire, one on the black, reads 12.5V

If you had one meter lead connected to the Constant 12v wire of the radio, and you had the other lead connected to the black wire of the radio, and it read 12.5v, It does have constant power. 

Turn the key to the ON position, touch and hold the black meter lead to the chassis of the radio, then probe the red wire.  What voltage does that read?

i am an idiot wrote: If you had one meter lead connected to the Constant 12v wire of the radio, and you had the other lead connected to the black wire of the radio, and it read 12.5v, It does have constant power.  Turn the key to the ON position, touch and hold the black meter lead to the chassis of the radio, then probe the red wire.  What voltage does that read?

Tried this, I get a reading of ~12.6V. Also tried probing the yellow wire which gives a similar result

Have you unplugged the wire harness out of the back of the radio and checked the fuse that is located under that plug?

Did you touch the black lead to the chassis of the radio?  If so, unplug the harness from the back of the radio, and check the fuse that is probably located under the harness.   If the fuse is good, your radio is broken.

I did touch the black lead to the radio chassis. I have checked the fuse that is inline with the yellow power cable of the loom/harness, and it is still good. So this means that the fault is with the head unit itself? Good to know. Thanks for all the help.