Janochytron
Active Member
Hi,
I got this nice amp as faulty, the description was, shortly tested, one set of speakers only working.
I opened it up before plugging it in for a quick visual. This amp has the power transistors not soldered but plugged in sockets. So I took out the heat sink with the 4 power transistors out, tested them, all good. I turned the amp on without them, all good, relay clicked.
Now I put them back in, turned it on but the bulb tester lit up like it was Christmas. Full on. And the bulb is 150 watts. I did not test it before removing, so can't be 100% sure it was not me doing something, but the heatsink has 2 screws and can be put in in only one way. I have not removed the transistors off the heatsink at this stage either, so couldn't be put in wrong way around.
Because it's easy to swap the transistors around I have tried different combinations. It draws too much current whenever a pair of them is plugged into any channel (or both).
Without them, voltages are good and it plays into headphones fine.
Now the manual lists pair of 2SC252/2SA1075. But the amp has 2SC3546/2SA1389. But they don't look to have been replaced, they look to be put there in the factory. From what I could find they are compatible (correct me if I'm wrong here)
As it's happening on both channels independently I was suspicious of power supply. I checked the big capacitors, even replaced them just in case. tested smaller capacitors. the rectifier bridge looks ok. Also all of these work fine without the outputs in.
I checked for bad solders, there were few loose ones, but fixing them did not make any difference.
Any ideas what could it be?
service manual is here:
https://www.hifiengine.com/hfe_downloads/index.php?pioneer/pioneer_sa-940_service.pdf
here are some pictures of how the heatsink assembly looks and works
I got this nice amp as faulty, the description was, shortly tested, one set of speakers only working.
I opened it up before plugging it in for a quick visual. This amp has the power transistors not soldered but plugged in sockets. So I took out the heat sink with the 4 power transistors out, tested them, all good. I turned the amp on without them, all good, relay clicked.
Now I put them back in, turned it on but the bulb tester lit up like it was Christmas. Full on. And the bulb is 150 watts. I did not test it before removing, so can't be 100% sure it was not me doing something, but the heatsink has 2 screws and can be put in in only one way. I have not removed the transistors off the heatsink at this stage either, so couldn't be put in wrong way around.
Because it's easy to swap the transistors around I have tried different combinations. It draws too much current whenever a pair of them is plugged into any channel (or both).
Without them, voltages are good and it plays into headphones fine.
Now the manual lists pair of 2SC252/2SA1075. But the amp has 2SC3546/2SA1389. But they don't look to have been replaced, they look to be put there in the factory. From what I could find they are compatible (correct me if I'm wrong here)
As it's happening on both channels independently I was suspicious of power supply. I checked the big capacitors, even replaced them just in case. tested smaller capacitors. the rectifier bridge looks ok. Also all of these work fine without the outputs in.
I checked for bad solders, there were few loose ones, but fixing them did not make any difference.
Any ideas what could it be?
service manual is here:
https://www.hifiengine.com/hfe_downloads/index.php?pioneer/pioneer_sa-940_service.pdf
here are some pictures of how the heatsink assembly looks and works