How to Fix Sony Amplifier Protect Mode Error

The STR-K740P protect error usually occurs in the form of "PROTECT" flashing on the display, and this article looks into how to fix this error and bring the amplifier out of this mode. The purpose of the protect error is to protect the amplifier components from damage should a short circuit occur. The most common form of short is usually user error when they short the speaker terminals, or overload it by attaching too many speakers. The protection mechanism usually places the amplifier into protect mode when it detects a short, and during this time it prevents power to the amplifier circuitry, thereby preventing harm to the speakers and expensive amplifier components.

When you see this message, the first thing to do is to switch OFF the amplifier immediately, and disconnect the speakers from the terminals, at the amplifier end. Let the amplifier cool for 15 minutes and then switch ON to see if the message still appears. If the message goes away, then you need to make sure the speakers are not short and correct impedance. However if the protect message keeps appearing all the time and without the speakers attached, then it is likely that there has been permanent damage to the internal components, and the unit will require repair.

This is a complex piece of Japanese electronic engineering and it requires a fully qualified and experienced electronic engineer to repair it, therefore do not attempt to repair it. In the following sections, we look at what sometimes goes wrong when such an error appears permanently.

If we look at one side of the push-pull stage, we see that if Q703 goes short between its collector and emitter junctions, the voltage rail would flow through the speaker and ground, however, there is R716 in the path, which will go open. When R716 is open and Q703 is short, Q705 will detect current through its base junction which starts the process of flagging the protect error.

Usually, a Class AB push-pull configuration has a ceramic resistor within the power path, which the Sony service manual calls non-flammable resistor that requires checking. When the transistors go short between their collector-emitter junctions, it creates a path from the positive voltage rail to ground -- or negative rail to ground, in a split voltage system -- and the only resistor in this path is this non-flammable resistor, which goes open. In this channel example, the non-flammable power resistor is R716 which is a twin resistor package consisting of a pair of 0.22-ohm resistors in series.

The audio power output design is class AB push-pull configuration, consisting of MP1620 and MN2488 pair of Japanese transistors manufactured by the Sanken Electric Company. The MP1620 is a PNP type, whilst the MN2488 is its complementary NPN transistor. These are hardy transistors with collector current (Ic) rated at 10 A. It would require a hard short across the speaker terminals in order to damage them. This together with a state-of-the-art Japanese electronic protection mechanism ensures that they will survive for many years.

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