An Avionics Technician expressed concern regarding the passenger overhead video monitor electrical control Tapping Units being manufactured with a poly switch; instead of a fuse as in the older units; causing the power transformers to fail; melt; and smoke in their A320 aircraft.

Narrative

Aircraft tapping units are being manufactured with a poly switch in place of a fuse on the older units. This creates a circuit which is inadequately protected against shorts; and causes the power transformer to fail; melt and smoke.Callback conversation with reporter revealed the following information: Reporter stated electrical tapping units are called just that; because they are used to 'tap' electrical power; which is then used for other circuits. Power for the A320 passenger overhead monitor and video signal comes from electrical Alternating Current (AC); while Direct Current (DC) power is used as the controlling circuit. DC power and some AC are drawn from the same Tapping Unit; which controls two monitors. Reporter stated the Tapping Units are continuing to fail; because the real problem of the DC fuse blowing; or the resettable poly switches in the Tapping Units failing; is caused by a capacitor leaking and shorting out; in the DC control circuit in the monitor itself. He has demonstrated the failure analysis on their test stand; in their shop; to a manufacturer representative; who agrees. Reporter also stated the manufacturer had previously issued a Service Bulletin (SB) that is accomplished during an overhaul or repair of the monitors and includes replacing some capacitors. These electrolytic capacitors are not life limited; will leak and fail; generally within four or five years; and short out; causing the fuses in the older Tapping Units to blow; or the resettable current limiter switch to fail and short out. Reporter stated he has also suggested that a fuse be installed in the DC control circuit; inside the monitor; and down stream of the Tapping Units that have a fuse or resettable switch.