Recently my FT-7800 decided to act strange. It would turn on and receive fine, but as soon as the PTT was pressed it would shut down.
OK, so what has this to do with the FT-7800 not turning on?
Well, the answer is quite simple. Although the symptoms are different, the cause and cure are usually the same.
To diagnose the fault I had the radio on the bench and powered it up as normal. This was followed by some voltage checks with the aid of the service manual.
Using the schematic diagram for the radio, I discovered that Q1043 (a 2SB1301 transistor) forms part of the power switching circuit. Checking the voltages around the component revealed that when the unit shut down, the voltage on the collector dropped down to 3v. Normally, this should be about 0.02v below the supply voltage, thus when using a 13.8v supply it should be in the region of 13.7v.
Locating the offending part on the FT-7800 PCB can only be done with the service manual. The reason for this is that very few components are silk screen marked directly on the PCB. The service manual includes PCB layouts which identifies the component positions.
If you have little or no experience of surface mount device (SMD) soldering then I would suggest you leave the repair to someone experienced. To illustrate just how tiny the 2SB1301 SMD transistor really is, this picture shows the faulty one beside a cotton bud (q-tip).