Sunday, 3 August 2014

capacitor - Amplifier overheating - how to find out if A671 PNP transistor broken?


I have a beautiful old Wega V 3840-2 amplifier. However, it has suddenly given out. I have some basic tools (soldering tools + multimeter: I'm a mechanical engineer, so I'm afraid I don't have my own oscilloscope), so I was wondering if I could somehow nurse it back to life.


The problem is that while listening to some music, the right audio channel suddenly gave out, and is instead playing a nice and soft 100 Hz hum, regardless of any volume or input. At the same time, I can feel one of the right channel main amplifying transistors heats up significantly within a few seconds, despite being attached to a massive heat sink. So, I know have two scenarios: either the 'hot' transistor broke down, or something upstream of the transistor is. The latter scenario is supported by the fact that sometimes, after a few seconds the transistor springs back to life with a slight crackle on the speakers. This would suggest a broken capacitor somewhere long the way, as far as I know.


My question is how do I determine where the problem lies: in the transistor or upstream?


Some additional details: the main amplifying stage seems to consist of a two-stage amplifier: a C1061 and A671 (PNP) transistor. The latter transistor is the one that heats up significantly. No other components seam to heat up. No components have visual indications of damage.




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