Harman kardon a 401 service manual

Page 2

PRECAUTIONS

Always disconnect the chassis from power line when soldering. Turning the power switch OFF is not
enough. Power line leakage passing through the heating element may destroy the transistors.

Never attempt to do any work on the transistor amplifiers without first disconnecting the AC line cord and
waiting until the power supply filter capacitors have discharged.

Replacement for output and driver transistors, if necessary, must be made from the same beta group as the
original type.

If one output transistor burns out (open or short) always remove all the output transistors in that channel
and check the bias adjustment, the control and other parts in the network with an ohmmeter before
inserting a new transistor. All transistors in one channel will be destroyed if the base biasing circuit is open
on the emitter end.

When mounting a replacement power transistor, be sure that the bottom of the flange, the mica insulators
and the surface of the heat sink are free of foreign matter, for they may cause transistor failure.

Silicon grease must be applied between the transistor and the mica insulator, and between the mica
insulator and the heat sink for better heat conduction.

Fuses must be replaced with size and type indicated. Use of other types can expose components to
destructive current levels.