Yamaha stereo receiver repair

One day I got a dead Yamaha RX-E410 stereo receiver that didn't turn on. The unit was sent to original service and their response was that the main micro controller M30620FCPFP was dead and needs to be replaced and reprogrammed. Of course replacing this IC isn't an issue, but you can't get the source code from anywhere. So unit is beyond repair.. Then it got in my hands.

Of course my first thought was the power supply, because micro controllers are the last thing that die inside a system (well in almost all cases). I measured all the voltages on power supply board and everything seemed fine. Also no signs of overheating, damaged parts. Everything in order. But still doesn't turn on. I slowly started to believe that maybe it really is the micro controller fault. But that still didn't stooped me.

I replaced the XTAL for the micro controller, but still no luck. Strange, but ok. Found the data sheet of the micro controller and started tracing the wires on what does what. Everything ok. Power button was connected to one of the micro controllers pin and it was ok. After some hours spent still no luck.

Then it came to me... I'll try to reset the micro controller manually (RESET pin). And as luck would have it, unit turned ON! Immediately I saw the this was the fault and that meant that the micro controller was OK. I traced the reset line and saw that it goes to the power board. There was some kind of a circuit based on unknown SMD components. After some thinking I realised that this was a POWER on RESET circuit (POR) that gives a micro controller the needed reset when the power stabilises. Because the IC's where unknown, I couldn't repair the circuit. No problem here. Let's make a new circuit that will reset our system.

There are many IC's that can do this, but I had none at my hand. So I decided to go with the good old NE555 timer. I found a great resource of circuits that can be found here. I needed the power ON delay circuit. I quickly drawed the schematic and create it on the breadboard.

 

The circuit works like this. When the power is applied, timer will trigger and output will be set to HIGH, which sets the RESET line to LOW (hex inverter), which holds the micro controller in reset state. When the timer runs out, RESET line is set to HIGH which puts the micro controller in normal state. 

The delay can be calculated:

Tsec = 1.1 x R1 x C3

Tsec = 1.1 x 47 kOhm x 10 uF

Tsec ~ 0.5s 

I chose 0.5 second delay, because it is enough for voltage to stabilise.When done, just connect the +5V, GND and reset line and CUT the wire reset that goes to old malfunctioned power on reset circuitry.

Here is a video of proof that this works. On the video you can see a LED that turns on, when RESET line is hold LOW. Later this LED was taken off.

So our expensive receiver lives to play for another day.

Any comments and suggestions are welcome.