Amplifier Protect Error

If you have a Sony amplifier that flashes "Protect" and then switches OFF, then you will need to do some troubleshooting to figure out what has gone wrong. Almost all Sony amplifiers have the capability to display this error on the front display panel. Many other makes also have this capability because they often have similar protection systems.

Power transistors, being a semiconductor material, are very expensive to replace. It is the most expensive component of an amplifier, and if they fail by going short, then essentially you have a dead amplifier that is worthless. The speakers are also expensive and a high-voltage through their coil can easily render them non-operational. Therefore, it is vital to have some form of protection mechanism to protect these expensive components. The protect mode is designed to prevent just such damage to the amplifier.

Short circuit protection has come a long way over the years, and many engineers will remember a shorted output transistor taking the bridge rectifier and expensive parts of the power supply such as the mains transformer with it. With the latest short-circuit detection techniques; it is possible to isolate the power rails from the short very quickly, thereby protecting expensive components in the PSU circuitry.

Type 1 Overload Protection

The first stage of protection usually incorporates a single transistor circuit, which tracks the temperature of the power transistors. A potential divider with NTC thermistor circuit detects the temperature of the transistor, and the circuit trips the protect mode when the transistor overheats.

Amateurs who might connect multiple speakers to a single speaker output can cause this type of fault to occur. Using a 4-ohm speaker load where an 8-ohm is required can also cause this type of fault. It is not a short, but an overload of the output. Since there will be more current passing through the power transistors, it will cause them to overheat and tripping the protect mode.

If the volume is fully up and the amplifier trips into protect mode, then this indicates an overload condition. Normally, lowering the volume or switching OFF the amplifier to cool for a while clears this condition, until the next time it overheats.

In car audio amplifiers, a worn out speaker voice coil usually causes intermittent shorts. The damaged insulation varnish on the voice coil can intermittently short as it rubs against the magnet. This is a difficult fault to diagnose because the overload can occur intermittently and people often do not suspect the speaker.

Type 2 Short Circuit Protection

In addition to the thermal overload protection there is also a secondary protection circuit for guarding against hard shorts at the speaker output. If there is a hard short circuit at the speaker output, then ideally you want the power to the transistors to cut instantly thereby preventing damage to them. Low value resistors in a potential divider arrangement are usually used to sense the flow of current through them. There is usually a single transistor circuit biased to detect the current flowing through the resistors. When the current reaches a predetermined threshold value, it signals the microcontroller, which then controls the relays such as that shown below.

Usually, each transistor has its dedicated current sensing resistor in the circuit. In the event of a short circuit, these resistors go open to protect the voltage rails. These current sensing resistors are very low in value and typically around 0.22-ohm. A short at the speaker output causes these resistors to go open almost as a fuse would go open. There is normally a puff of smoke associated when they go open.

Power transistors can fail open or short. Some of the modern MOSFETS such as the SUP18N15-95 tend to fail short between all three of their leads. Bipolar transistors such as the MN2488 and MP1620 also fail short between all three leads. In any case, an electronics engineer is required to make a diagnosis and replace the faulty components such as the power transistors. The amplifier will keep going into protection mode until the removal of the shorted transistors.

Depending upon the design of the amplifier circuit, there can also be other collateral damage. Sometimes the main power supply including the voltage regulators and bridge rectifier can fail.

Only for the Experienced Engineer

If you are not an experienced electronic engineer, then do not attempt to repair an amplifier that displays the "PROTECT" message. Some of these amplifiers use state of the art Japanese protection circuitry and can be a challenge. Repair usually requires a lot of experience and simply being a member of the guild/fraternity/brotherhood, with a bunch of lofty over-inflated grades gained during the Blair education era, will not be sufficient.

Sell it on eBay

If you have a faulty amplifier showing this error, and it happens to be in a good cosmetic condition, then one option is to sell it on eBay. Providing you sell it as Spares or Repairs and are honest about the error you can get around ten pounds for it, however it depends upon the auction day and what you have.

Many faulty amplifiers go for around £10.00 and someone with the intelligence and plenty of time on their hands might buy it to restore it. If you are selling it on eBay, a reasonable starting amount is around £10. Remember that the buyer has to spend twice that amount for the replacement components and a day of his or her time to get it working again, which by no means is certain.

When selling on eBay, be honest about the condition of the item. Show all the different sides of the amplifier. Point out any dents and scratches. Mention any error messages that you have seen.

I love restoring amplifiers and sometimes repair amps for celebrities in the music industry simply because many are classic designs that are irreplaceable. Unfortunately, there is not much profit in repairing amplifiers. Some of the latest ones can take a whole day just to take apart and get to the power transistors, but it keeps away the boredom for a few minutes anyway.

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