Author Topic: Nikko QSR-4040 Receiver Distortion Repair  (Read 322 times)

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Offline Arty30Topic starter

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Nikko QSR-4040 Receiver Distortion Repair
« on: April 17, 2024, 09:43:46 pm »
I'm starting a new thread since it's been quite a while and want to start from the top. In summary I received this receiver which has distortion on both front and rear left channels, coincidence?, I have since re-capped it in an attempt to resolve the issue as I have done in the past. this was unsuccessful though an there was still distortion. I did some more tests including testing the power amp separately from the rest of the powertrain. It appears only the power amps are the culprit. For this thread I am focusing on the rear left channel which exhibited popping at 'idle', possibly low volume, and bass distortion. distortion was only present at higher volumes or bass frequency. my next target was transistors, I had a pioneer that had it's issues solved this way. This unit has 2 C853, one C545, and one C945 as well as a pair of C1061 main amp transistors for each four channels. I replaced them with BD139, BD140, and 2SC1815 respectively. upon testing there is still distortion on the bass frequencies and/or at higher volumes, but it sounds more uniform if you will. I plugged in a function generator on the input and a scope on the output to view a sinewave input. what i found was that as I increased the volume on the unit, the top of the sinewave would be cut off, and by the time I reached full volume, it resembled more of a square wave. This leads me to believe I have made a bad choice in transistors or I need to adjust something somewhere. I will link a YT video I recorded of this setup first on the Left (bad) channel from low to high volume, then on the right (good) channel. they seem to exhibit similar behavior, but the left channel seems to saturate sooner. I am under the assumption that it should be a smooth sinewave all the time unless I am just oversaturating the amp with too much amplitude on the input. At this point I am beyond my knowledge and I am open to any tips, tricks, advice, etc, and would greatly appreciate it.

YT video:

P.S I don't have a schematic for this unit, if anyone has one or can get a copy, I would be in your debt.
P.P.S I'm also looking for a schematic for a Harman-Kardon sr-300b receiver.

Thanks, Arty.

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