This good classic stereo amplifier with 2SA1302 / 2SC3281 pair had no sound at all. Power ±42V OK. Speaker relays don't activate. It's some kind of protection. I check power transistors output and I find -42V at Left channel. Right channel normal (0V). So it's a DC protection going on.
No transistors shorted, neither finals or preamps.
I check for cracked solders (long term thermal stress). I find some suspicious ones, but believe me that I know when a solder is really broken, and in this case none seemed completely broken. Also, no changes when knocking the board.
Then I start to measure things randomly at L channel, diode test mode, specially diodes and low value resistors. I reach R247 (100Ω 1/4W) that reads higher. I double check and it's about 650Ω, but it's not burnt or bad looking at all. Same for surrounding components. I replace it without great expectations but surprisingly it's fixed!
Very, very random failure.
No transistors shorted, neither finals or preamps.
I check for cracked solders (long term thermal stress). I find some suspicious ones, but believe me that I know when a solder is really broken, and in this case none seemed completely broken. Also, no changes when knocking the board.
Then I start to measure things randomly at L channel, diode test mode, specially diodes and low value resistors. I reach R247 (100Ω 1/4W) that reads higher. I double check and it's about 650Ω, but it's not burnt or bad looking at all. Same for surrounding components. I replace it without great expectations but surprisingly it's fixed!
Very, very random failure.
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