Hitachi AX-M140 System Control Board 1204D2-20

The system control board 1204D2-20 for the Hitachi AX-M140 is in the DVD component unit. This board has the main system control CPU 3P9228AZZ, which is the heart of the whole system without which, the other component units will not work. Its primary function is to communicate to other component units through the serial bus. It provides the control signal to the other units to switch ON, and directs the audio switch IC (PT2314), to select the relevant audio signal for input to the power amplifier. It also conveys the bass and treble settings to the audio switch, which has the necessary preamplifiers built-in.

Removing L2002 isolates pin 5 from the rest of the circuitry. It might then be a good idea to check C2039 and C2040 to make sure they have not gone short. However, after that, there is not much else left except pin 5!

This unit had a strange fault in that the main system controller CPU 3P9228AZZ had gone short and its supply voltage (CPU5V) at pin 5 (Vdd) was short to ground. A short of this nature is likely to damage the voltage regulator IC2006 that produces the 5 V rail, and the power supply. Luckily, there is R2054 (47 Ω) resistor that feeds the voltage regulator, which burns up going open when the 5 V rail goes short. I wonder how the engineers knew that this would happen?

The markings on the sticker label are 1204D2, and 0x54A0. Unfortunately, I was unable to find a replacement CPU chip and therefore decided to leave the unit as it is until such time when Hitachi decides to sell this part. If you work for hitachi or know someone who sells it, then please do drop me an email. Since the CPU performs the tasks of switching ON the units when required, and selecting the audio signal, there may be a way to modify the circuit by installing a manual selector switch. A manual selector switch could provide the necessary control signal to switch ON each unit and select its audio output; however, it would be a very large multi-wafer selector switch. The other option I considered was to install each unit with its own dedicated transformer and power supply, manual power switch, and RCA audio output sockets. However, it would require extensive wiring and much of the digital convenience would be lost, including the use of the remote control. The power amplifier unit would also need rewiring with the inclusion of bass and tone circuitry, and RCA sockets for audio input.

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