AX-M140 Amplifier Power Supply Repair

A failing AX-M140 power supply board can result in all sorts of havoc including the system remaining in standby mode. On a well-used unit, the board will have heat damage, and therefore the components involved in dissipating the heat are well worth checking to make sure they are within specifications.

On the AX-M140 power supply board 1204D2-11, there is heat damage caused by Zener diodes DZ1100 and DZ1101, which regulate the 30 V, and 28 V rails respectively. They dissipate a lot of heat and raising them above the board can help. In addition, there is marking around the following resistors.

• R1102 (10 kΩ)
• R1103 (560 Ω)
• R1100 (10 kΩ)
• R1101 (1 kΩ)

Interestingly, the power rating of resistors R1102, R1100, and R1101 is 1/8 W, whilst R1103 is 1/4 W. An eighth of a watt seems woefully under specification considering the power they dissipate, therefore I would replace these with higher wattage ones.

R1103 (560 Ω) works in conjunction with Zener diode DZ1100 to provide voltage drop for the 30 V rail. R1102 is also part of the resistor network for this circuit. R1101 (1 kΩ) works in conjunction with Zener diode DZ1101 to provide voltage drop for the -28 V rail. R1100 (10 kΩ) is also part of the resistor network for the -28 V rail. If you are planning to get some more mileage from this hi-fi, and prevent further heat damage to the board, then it might be a good modification to replace them with higher wattage resistors raised above the board.

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