Hitachi AX-M140 Digital Radio

Hitachi AX-M140 digital radio component system can also receive FM and has RDS facility. There is a single connection at the back for the aerial. I bought this pre-used and the previous owner mentioned that the DAB radio unit was not switching ON. Since it was not working, I decided to start there and have a look. The DAB unit receives DC voltage from the control cable, and this voltage then goes to a 5 V voltage regulator.

A basic cursory measurement using a bench power supply indicated that the regulator was working and producing 5 V. Another SMD regulator on the PCB side was also working. However further investigations revealed that the control signals were missing. Pin 22 of the control cable brings the unit out of standby, and pin 23 carries the DAB ON/OFF control signal. These signals originate from the system controller IC in the DVD unit.

With the DAB unit disconnected, I decided to see if the rest of the units worked without it, and they did! The standby light in the amplifier unit came ON, and the music bank hard disk began to spin. I installed a Glenn Miller CD, and it played fine for a minute. Then suddenly, there was a smell of something burning, so I pulled the mains plug out, and noticed that the smell was from the DVD unit. It definitely did not like Glenn Miller I thought. Opening the DVD unit, I noticed a burnt resistor, and following that trail, led to the system control IC on the DVD board.

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