Sony STR-K740P Repair

The protect error is the most expensive fault to repair because it requires the whole amplifier to be taken apart in order to gain access to the transistors for soldering. It usually requires replacement power transistors, and current sensing resistors, which are expensive. This is a time consuming job, as the transistors require desoldering from the heatsink.

When replacing the transistors remove the dried up thermal paste and clean the surfaces. I usually use a high performance thermal compound when installing these transistors. These transistors also require a mica layer between their back and the heatsink. When tightening the screws pay careful attention to the torque applied. If the screw is too loose, then the thermal contact with the heatsink will be poor and therefore heat conductivity will be poor. If the screws are too tight, then one runs the risk of damaging the transistor and warping the package. The databook indicates that a precise torque between 0.686 N•m and 0.822 N•m must be applied.

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