I acquired an vintage amplifier and started the journey of giving it a new life. I have managed to repair most of the damage and have come to a working device.
But there is one problem still to solve. There is a DC offset on the output side of about 600 mV. And it's only visible when there is an input signal present. The AMP has a DC bias adjustment, and it works fine 2–5 mV on the output. I traced the problem to the preamp board; the NPN side Q412 (2SA968) is not showing this behavior. Only the PNP side Q411 (2SC2238). I have 2 of the same amplifiers; the other one has the exact same problem, but only a 200 mV DC offset. I need to mention the original Transistors on the preamp board were damaged and i needed to replace them.i checked all the parts and everything seems fine,Maybe there is a biasing problem on the PNP side, i replaced the transistor with an equal China copy. Furthermore, I really don't know. I hope someone will be able to point me in the right path for solving this technical problem.
Original source for the documentation (HARMAN KARDON HK-775 SM-2 service manual & repair info)
https://elektrotanya.com/harman_kardon_hk-775_sm-2.pdf/download.html
Original source for the documentation (2SC2238 Datasheet) https://alltransistors.com/transistor.php?transistor=12929
Original source for the documentation (2SA968 Datasheet) https://alltransistors.com/transistor.php?transistor=9123
(This is the PRE-DRIVER board.) Pink marks are all the components on the PRE-DRIVER board.
Left (The Signal at the Transistor output Q411 2SA968) PNP Side DC offset is visible
Right (The Signal at the Transistor output Q412 2SA968) NPN side has no visible DC offset.
DC voltage at the terminal output when a 1Khz sine wave signal is applied
DC voltage at the terminal output when no signal is present
Don't hesitate to ask for more info or measurements data, i will be more than happy to provide it.
But there is one problem still to solve. There is a DC offset on the output side of about 600 mV. And it's only visible when there is an input signal present. The AMP has a DC bias adjustment, and it works fine 2–5 mV on the output. I traced the problem to the preamp board; the NPN side Q412 (2SA968) is not showing this behavior. Only the PNP side Q411 (2SC2238). I have 2 of the same amplifiers; the other one has the exact same problem, but only a 200 mV DC offset. I need to mention the original Transistors on the preamp board were damaged and i needed to replace them.i checked all the parts and everything seems fine,Maybe there is a biasing problem on the PNP side, i replaced the transistor with an equal China copy. Furthermore, I really don't know. I hope someone will be able to point me in the right path for solving this technical problem.
Original source for the documentation (HARMAN KARDON HK-775 SM-2 service manual & repair info)
https://elektrotanya.com/harman_kardon_hk-775_sm-2.pdf/download.html
Original source for the documentation (2SC2238 Datasheet) https://alltransistors.com/transistor.php?transistor=12929
Original source for the documentation (2SA968 Datasheet) https://alltransistors.com/transistor.php?transistor=9123
(This is the PRE-DRIVER board.) Pink marks are all the components on the PRE-DRIVER board.
Left (The Signal at the Transistor output Q411 2SA968) PNP Side DC offset is visible
Right (The Signal at the Transistor output Q412 2SA968) NPN side has no visible DC offset.
DC voltage at the terminal output when a 1Khz sine wave signal is applied
DC voltage at the terminal output when no signal is present
Don't hesitate to ask for more info or measurements data, i will be more than happy to provide it.
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@romeoleung Yes i adjusted VR401 for +/- 10 mV. I uploaded an image that shows 1-2 mV at the terminal output.