This is just a quick post regarding a blown 5A fuse on the AVR board in a Yaesu FT-726R.
I switched my radio on the other day and it lit up for about a second before everything went dead. I opened the radio and found the 1 1/4″ 5A fuse on the AVR board had blown violently.
FT-726R AVR Unit Circuit Diagram
I discharged the supply through a 100 ohm resistor and checked the resistance between the 13.7V DC feed to the radio and the chassis; it was a dead short. To cut a long story short, it turned out that a tantalum decoupling capacitor in the HF module had gone short circuit and blown the fuse. Once I’d replaced the tantalum capacitor, the radio powered up perfectly from an external supply.
I checked the internal power supply before installing a new 5A fuse and was astonished to find around 24V where I expected to find 13.7V. I assumed that the series pass transistor in the power supply had failed.
I unsoldered the red 13.7V wire which feeds the radio to check things out.
I did some basic voltage checks which revealed no voltage on the Q02 regulation part of the AVR circuit. On looking at the circuit diagram again, I realised that the Q02 regulation circuit components were on the FUSED side of the circuit…blown fuse, no regulation and full output. There’s a link to the AVR unit page in the manual here.
The power consumption of the radio at full output is around 4.5A from the power supply. The original fuse was NOT a time delay fuse, so I replaced like for like; I couldn’t find anything to tell me if the fuse should be a time delay fuse or not.
Once I replaced the fuse, the power supply worked as normal, so I reconnected the red 13.7V wire and checked the radio…it worked perfectly.