Yaesu FT-857 Totally Dead

 

Yaesu make a great little portable radio that covers the 160-10 meter bands and also VHF / UHF. Ideal for carrying in your backpack, with quick, easy setup so that you are on the air with a minimum of fuss.

The old man, who owned this radio, had accidentally connected the plus and minus battery leads the wrong way and now the radio was totally dead. He realized that a mistake like that, could be terminal and he may have to use it as a plant stand instead.

I put the radio on the bench and tried to simulate the effect he was seeing. Correctly hooked up to my lab power supply, I could see that the background light in the display was coming on for around half a second after pressing the power on key and then going off again. Other than that short moment of hope, the radio was back to being dead again. Time to crack open the case.

A quick look around inside, did not show any signs of nasty damage. There was hope to be had yet. A quick test around with the probes, showed that there was no power getting to any of the main components, so most likely the main board CPU was detecting a fault and shutting down right away. Not a really good sign to be honest, as trying to find faults like that can be difficult and time consuming. The first thing I decided to do, was to take a look at the circuit diagram and see where the Vcc, Power Switch and HW reset pins were on the CPU.

Then the CPU was located on the main board and the pins probed.

Vcc did have full battery voltage on it and I could find the power on signal and reset pins too, so the oscilloscope was hooked up to see if the hardware reset was coming shortly after the initial power on signal. 

This is an important instant in time for the boot up of a CPU. The reset must come shortly after the signal to start-up. If that signal does not appear, the CPU will not boot and immediately power down again.

The digital oscilloscope was set to catch the first rising flank and stop and store the picture. Here we can see the latching mechanism working correctly. So the main CPU seemed to be good. The next step was to do the same thing for the head unit CPU. 

Once, the head unit starts up, it will communicate with the main board CPU to tell it which band to select, which frequency, which modulation mode, etc. If that serial communication does not start to flow, the main CPU will also shut down to protect the other radio components.

It was at this point that I noticed something was not right. I was not seeing the same signals on the head unit, as I was on the main board before the jumper which connects the two units together.

Somewhere we were losing connectivity between the headset and the main board. I unplugged the RJ45 cable that connects the two units and buzzed out all the pins 1:1 on the cable. Quickly, I found one pin that was not making a connection. Looking at it under the microscope, I could see signs of burning. The connector was cut off and a new one crimped back on. Immediately, the radio jumped back to life as if nothing had ever happened.

It would appear that the reverse polarity in the head unit was enough on this pin to burn it out before it popped the inline fuse in the battery cable.

The old man was very lucky that the protection mechanism built into these radios was able to isolate most components from power until the initial handshake had taken place between headset and main board.

Another happy customer.