Sony STR-DE485 Hum

Occasionally amplifiers can exhibit a 50 Hz audible humming, which in electronics is a noise component within the amplifier circuit. To troubleshoot this issue you have to determine the source of the noise. Is this noise present on all the inputs or only one specific input? Does the hum increase as you increase the volume, or does it remain the same level. If you turn the volume right down and this noise is still present, then the power supply and the DC offset are the places to investigate. Usually, this form of noise is at a constant level, and disappears when the input signal increases, however, when listening to quiet classical music, it is noticeable.

The main audio power supply uses two 6300 uF / 63 V Nichicon electrolytic capacitors and people often focus on these first, however they seem very good quality, and usually there are never any problems with these. However, this amplifier is getting on and the capacitors may have deteriorated. Before replacing them, a quick check on the oscilloscope usually indicates the presence of a small ripple that may be worth investigating further.

I remember one amplifier that would indicate a continuous ripple and noise; it was tracked all the way to the bridge rectifier. The diodes in the bridge are usually under load most of the time, and in this case, one of them had partially failed.

This amplifier uses a large conventional transformer marked Bando 1-437-633-11, and manufactured in Indonesia.

Many "engineers" who have more money than sense will change all the electrolytic capacitors, and it is actually not necessary to do this. If you use an oscilloscope, you can track the source of the noise, and it is probably better to start in the power supply section. This amplifier also has split-voltage rails for the operational amplifiers, and therefore it is worth checking these as well.

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