Dead Yaesu FT-450

Symptom :

 Yaesu FT-450 can not be powered on with no response from any button.

Diagnotics :

Since the on/off power is controlled by the main CPU, then the CPU is the first thing to be checked. The supply to the CPU should available and in the right value.

There are 4 voltage regulators in the Control Unit board. They are for 8V,  5V and 3.3V supplies, plus one 5V standby that continuously supplying voltage to the CPU.

Check all the voltage output from those regulators gave good results. So, it is not the regulator that fails.

The CPU needs to be reset when DC power is connected to the radio. The reset is provided by PD GEN circuit. The way it works is when the power applied, /RESET line will stays low (0V) and after some delay, it will go high (5V).

The IC Q4002 sense the input voltage in pin 2 to determine if the input voltage from power supply is good enough for operation.

Problem Cause :

 A surge voltage from power supply, might damage the IC. But in this case, it was found that C4045 has gone broken and shortened ! Leaving the pin 2 voltage always 0V and thus preventing the CPU from leaving reset state.

Solution :

Replace bad C4045 and the radio can be powered on.

Symptom :

The radio can not be turned off and must be unplugged from power supply.

Diagnotics :

 DC from power supply is controlled by relay. The CPU control the relay via POW_ON line. Check on POW_ON gave correct reading as CPU is trying to turn on and off the relay. But the relay still activated and make the radio can not be turned off.

Problem Cause :

A surge in the DC input causes failure on D5415, it become having high resistance and Q5402 failure shorting CE pins.

Solution :

Replace bad diode and transistor. After replacement, the power relay works as expected and radio back normal again. 

The radio ready returns to the owner ..