Hello,
Just got a Pioneer from a friend sitting on my table. He complained about a crackling noise on one channel, some days after listening for a while, some days no problem at all. He has had it checked out by professionals a few times who changed some capacitors and the output transistors to 2SA1301/2sc2380 instead of the 2SA1075/2sc2525 that were the original ones.
When I first powered up the amp I immediately got a lot of heat coming from the output transistors. checked for bias and had about 160mv on one channel and 120mV on the other and over 150mV of dc on the outputs.
I guessed that nobody took care of readjusting after changing those output transistors. I was able to set dc to 0mV +/-2mV but even with the two trimpots turned completely counter-clockwise I still had over the 77mV of bias that the service manual aims for. I have had this problem with some other amps after changing the output transistors but there was a simple vbe multiplier that I could slightly redesign to get where I wanted. In this case I don't really see how to get there.
Any suggestions would be welcome, as well as a possible cause to the crackling (that is his description, I never managed to hear it with my own ears).
Just got a Pioneer from a friend sitting on my table. He complained about a crackling noise on one channel, some days after listening for a while, some days no problem at all. He has had it checked out by professionals a few times who changed some capacitors and the output transistors to 2SA1301/2sc2380 instead of the 2SA1075/2sc2525 that were the original ones.
When I first powered up the amp I immediately got a lot of heat coming from the output transistors. checked for bias and had about 160mv on one channel and 120mV on the other and over 150mV of dc on the outputs.
I guessed that nobody took care of readjusting after changing those output transistors. I was able to set dc to 0mV +/-2mV but even with the two trimpots turned completely counter-clockwise I still had over the 77mV of bias that the service manual aims for. I have had this problem with some other amps after changing the output transistors but there was a simple vbe multiplier that I could slightly redesign to get where I wanted. In this case I don't really see how to get there.
Any suggestions would be welcome, as well as a possible cause to the crackling (that is his description, I never managed to hear it with my own ears).