Hitachi AX-M140 Amplifier

The AX-M140 amplifier uses the TDA7294 power ICs, which are DMOS audio amplifier ICs in multiwatt-15 package for class AB amplifiers. According to the documentation of the IC, it can provide 60-watts minimum with 8 Ω load and ±35 V supply. According to the amplifier documentation one can expect 50-watts per channel with 8 Ω speakers.

This amplifier unit contains a toroidal transformer that provides the main voltage rails for the power amplifier and the necessary voltage rails for all the other units that connect to it. Since the unit is remote operated, they also have to include standby circuitry that remains powered all the time. Since they used a single transformer to provide all the voltage rails including the standby voltage, it remains powered all the time and this is its proverbial Achilles heel. In addition, to the standby power, the control cable also carries -28 V, +42 V, +5 V, and +12 V voltage rails for the connected units. Due to these voltage rails, the manual warns not to install or remove the control cable whilst the power is ON. You cannot just pull the cable out and install anyhow because this is not a modern plug-and-play system. Many people who ignore this warning will end up frying the main system controller IC in the DVD unit. Once this IC fails, the whole unit becomes useless.

Unfortunately, the unit I managed to get already had a burnt system controller IC, and in addition, the courier had kindly introduced some more damage to it by throwing the parcel around and parking a lorry over it. Consequently, two of the units came squashed and the boards destroyed. However, I was able to take them apart and straighten the chassis for some decent photographs.

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