SA6700 nasty turn on/off thump

Ray Gianelli

Super Member
A buddy of mine brought this integrated amp by. He showed me that when it is powered on or off the meters deflect a lot, and there's a loud thump from the speakers. Occasionally the transient was powerful enough to cause the peak lamps to illuminate. Once powered up the amp worked flawlessly, and met specs for output and distortion.

Since both channels were affected I focused on the power supply. All regulated outputs looked good but when I scoped the unregulated supply at the diodes I saw the transient when power cycling.

While all the diodes tested good with my Fluke, I replaced them and it cleared the trouble. I believe that if the diodes were tested at a higher voltage than the Fluke puts out I might find a bad actor among the diodes.

Just posting in case anyone else searches for something like this. And it's always a good lesson that even if a component tests good with a meter doesn't always mean it's good; it may fail under load. The oscilloscope showed the trouble, and thus its value as part of a well equipped bench. Oh, and experience helps... and things like this add to it.
 
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