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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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  • \$\begingroup\$ I would remove power from the system, then use the multimeter to check for short circuits to ground around the "hot" parts. There may be some valid ground contacts around there, but maybe you'll find one which does not look right. Next i'd replace the chip which gets hot. Good luck! Also, this site is not kind to those looking for help repairing electronics.. \$\endgroup\$
    – KyranF
    Oct 28, 2015 at 15:32
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    \$\begingroup\$ You have left and right - do comparative voltage measurements at the same circuit points. \$\endgroup\$
    – Andy aka
    Oct 28, 2015 at 15:39
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    \$\begingroup\$ If it comes and goes, suspect a dry solder joint. Look for grey granular ones and re-solder them. \$\endgroup\$
    – user16324
    Oct 28, 2015 at 15:40
  • \$\begingroup\$ @KyranF I had a sneaking suspicion that this question might be considered off-topic (I wouldn't be personally offended by a close vote), but I tried to keep it general by asking how to determine whether a transistor is faulty so that others can apply the answers given to their own problems. We'll see :) Either way, the comments you and others gave will give me something to work on. \$\endgroup\$
    – Sanchises
    Oct 28, 2015 at 15:52
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    \$\begingroup\$ I would say the base connection of the PNP transistor is having a bad time, and until it heats up and reflows, the collector-emitter is just passing current (and heating up) while oscillating at 100Hz which you hear while it's in this condition. \$\endgroup\$
    – KyranF
    Oct 28, 2015 at 16:17

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