Hello people. I am repairing a Sansui G4700 receiver, only one channel has a random fault, it only amplifies the positive part of the signal, this problem appears and disappears by itself. It is not a problem with the preamplifier, only in the power stage. The funny thing is that it can be working perfectly for a week or more, and suddenly the fault appears, lasts a few minutes and then disappears. Check all the welds, solve the ones that seemed suspicious, change some components affected by glue, but the problem remains. It is not false contact. The duration of the problem, does not allow me to make many measurements, in a short time it normalizes. I suspect some transistor, for the moment, I decided to exchange a group of transistors with the channel that works well and leave it in test to see if the fault persists or moves to the other channel. Any idea or suggestion is welcome.
Long shot.
Check R9, R10, 2.2ohms 0.5W on base of outputs maybe stressed. Sansui had problems with other amps (6.8ohm 0.5W) power rating was a bit low, resistors had a chalky appearance.
Check R9, R10, 2.2ohms 0.5W on base of outputs maybe stressed. Sansui had problems with other amps (6.8ohm 0.5W) power rating was a bit low, resistors had a chalky appearance.
The base resistors are a long shot, seen a few in vintage gear where they've failed, would be match for only +ve half amplification. Yeah, 99.9% of time resistors will fail open. Suggestion is to take a quick look at the resistors for signs of stress/chalky appearance or discoloured bands.