Sony KDP-51WS550 Training Manual page 30

4. AX-1 Protection Circuits
Q2006 monitors for a positive DC voltage condition and Q2005 monitors for a negative DC voltage condition on
the audio output lines. If there is a positive DC voltage present, Q2005 will turn ON causing the base of Q2008
to become negative with respect to the emitter, turning it ON. A high is now applied to the Main Microprocessor
IC0001/pin50, placing the unit in protection mode.
If there is a negative DC voltage present, Q2005 will turn ON causing the base of Q2008 to become negative with
respect to the emitter, turning it ON. A high is now applied to the Main Microprocessor IC0001/pin50, placing the
unit in protection mode.
In the protection mode the unit will turn OFF the AC-Relay and shut down with an 8X Stby/Timer LED flash
sequence.
Zero Detect Circuit
The zero detection circuit is technically not a protection circuit, but it will shut the unit OFF if proper AC Input is
not detected. The main purpose of this circuit is to detect, and along with the Main Microprocessor, calculate the
zero crossing of the AC input signal. The basic function is the same as the Inrush relay circuit in previous
chassiss, to eliminate power ON current surges, therefore eliminating undue stress on the AC-Input relay and
Power Supply components.
The zero detect circuit achieves this end by first monitoring four complete AC signal input cycles (frequency and
period are monitor) for zero crossing before power ON the power supply circuits.
A 120KHz full-wave rectified AC signal is applied to the base of Q6110. Each time the signal drops below 0.6V
(basically the zero crossing point) at the base of Q6110, Q6110 turns ON. Q6110 then applies a High to the base
of Q6111, turning it ON. Q6111 then applies a Low to the base of Q6112, turning it OFF, which pulls IC0001/
pin19 Low.
This process continues causing pulses of 3Vpp at 120KHz to be applied IC0001/pin19. The Main Microprocessor
IC0001 calculates the exact time of the AC signal zero crossing using the input pulses at IC0001/pin19. With this
information, the Main Microprocessor can precisely time the power ON event to coincide with the zero crossing
of the AC input signal. This produces a gradual increase in current and power to the power supply circuits, as
opposed to the unit powered ON at a high level of AC input causing an excessive current surge to be applied to
the power supply circuits at the power ON event.
The zero detection circuit must detect four consecutive AC signal periods within 500us of each other at a frequency
of 90Hz to 132Hz in order to calculate the zero crossing. If this condition is not met, the AC input is remove and
the unit shuts down with a 9X Stby/Timer LED flash sequence.
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