Specifications

Australian Monitor Service Bulletin
AMIS120 Zener diode problem
We have encountered a higher than acceptable failure rate of zener diode circuit reference D15 in AMIS120 amplifiers.
The Zener diode is part of the thermal protection circuit in the amplifier and in time may go short circuit and cause the amplifier to mute all audio.
The problem occurs after the amplifier has been in use for many months and appears to be caused by excessive current dissipation in the diode
caused by higher than expected low fan speed running current draw.
Airway obstruction, dust build up on the rotor blades or in the bearings may cause the fan to run below speed and draw enough current to cause
thermal degradation to the diode over time.
This problem is limited to AMIS120 amplifiers manufactured between April 2008 to July 2009.
The solution is to replace the 15 volt 1 watt zener diode with a 20 volt 1.3 watt or higher (two 1.0 watt or similar) zener diodes. The diode should
also be raised 5.0mm above the board for improved heat dissipation
Original diode 15V 1W zener.
Replace with 20V 1.3W or 2 of 1W.
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18 August 2009