Hello friends, I need your help to find the fault in a Kenwood KA 405 amplifier, which I enter with the left channel burned and very neglected. The amplifier, after completely disassembling it and making the relevant restoration, repaired the left channel power stage that had the two burned output transistors (2sa1105 and 2sc2580) and a 2SA794 driver that replaced it with a 2SA 1358 as a direct replacement. All transistors I was lucky to get them originals from the same manufacturer, so I rule out that it is one of them. I tell you that the fault it produces is only in the left channel and from what I have noticed it is only when it is demanded from 50% of the total volume, I estimate that if I raise it more the volume will fail more quickly. The sound is clear and without distortions until the moment that for some reason only the sound of the left channel is cut off and a very special sound appears, as if it were a discharge or lack of mass, I repeat, only on the left. a few seconds until the protection chip is activated and the audio is cut off. I turn off the computer, let it rest for a few minutes, turn it on and work again until the problem happens again. If I set the volume to approximately 25% for 1 uninterrupted hour it has not done so, only sporadically it is heard as a mid-peak blow on the baffle, but the discharge does not appear. I comment on the tests I did to rule out possible failures: In one of the discharges, while this was happening, disconnect the preamplifier from the input of the power stage and the continuous noise as if nothing were installed until the protection jumped (preamp discarded) I changed all the electrolytic capacitors to that stage channel to test if it was any of them. The continuous fault. How to get the service manual, connect a tester continuously to the quadruple diode (I think it controls the resting voltage or varies the voltage according to the temperature since it is seated in the heatsink of the power transistors). I measured positive voltage with respect to mass that has to be 1.17V. I estimate that this value varies a little according to the work that the stage is doing. What I do notice is that at the time of the discharge that value jumps to about 2.7V and begins to descend slowly, the protection jumps and the value continues to decrease until it is 1.17V which is at the moment the protection is disconnected and returns to normal and returns the sound. What could be the problem? The quad diode? The heatsink is just beginning to get warm, so I rule out temperature problems. Will it be one of the driver transistors? The complement of the one that burned and did not change because its measurements and gain gives me correctly? I also changed one of the two resistors that go in parallel and in series with the quadruple diode (STV-4H) because I saw it with a little bit of rust on one end and although it gave me the correct measurement, I changed it for safety. I hope you can help me and if you need more information I will gladly give you more details. Thank you. PD: Under the manual for any colleague who works for you hfe_kenwood_ka-405_4055_service.pdf [4.29mb]