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In a large number of models of SONY audio equipment, usually on the same display the following sign "PROTECT - PUSH POWER", which indicates that the computer is in protected status because one of its routes not working correctly.
This protection can be activated by different causes such as high temperature, overload, mechanical power output stuck and short circuit, the latter is the most common cause of these, basically when the output stage is short-circuited this sends the voltage power supply to loudspeakers, and obviously these coils are burnt in a short time thus the protection is activated to prevent this.

To detect this anomaly are used basically two NPN transistors as shown in the image below.

These transistors are usually found on the same board of the power output. When the power integrated circuit is placed STK short circuit, the transistor Q506 receives 0 volts at its base fixed by the resistive divider formed by R523 and the thermistor TH501, the transistor will turn off and leave the entire positive voltage reaches the base the transistor Q505, which will cause this to ignite and put in its collector to ground potential. This is conducted through diode D502 and come to another stage where other protections converge on the main board located audio equipment, this transmission line is labeled "PROTECTIVE" as shown in the image below.
Low or ground state arrive at the base of PNP transistor Q823, which in normal conditions to get 11.9 volts via resistor R824 keeping it off. When an abnormality occurs then receives a low state at its base, this causes the transistor collector in place 11.0 volts, which are reduced to 2.4 volts due to the resistive divider formed by R826 and R827. This voltage is applied to the transistor Q829, which is placed on and 0 volts at its collector, came to this state under Terminal called "PROTECT" in the microprocessor of audio equipment, which will make this into protection and display on screen the phrase "PUSH POWER PROTECT-" that because the Terminal "PROTECT" the microprocessor in normal conditions should receive 4.7-5V.


                                                 Terminal "PROTECT"

RECOMMENDATIONS: As most of the time the fault lies in the power amp, you should take off the IC amplifier and audio turn on your computer without it. If the sign "PUSH POWER PROTECT-" disappears and the device turned on with all its functions, then most likely the amplifier IC is damaged, make sure to change before it reaches the correct supply voltage and peripheral components to the work properly, because otherwise the fault would persist even with the IC amplifier in good condition.

After the repair is advisable to make a cold reset "COLD RESET", which will clear any operation error stored in the microprocessor, in many models is done by pressing STOP, ENTER, POWER, simultaneously turning the equipment previously . This restarts and displays the sign "COLD RESET". This method may vary among different models of computers, so I lean on the service manual.

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