Ross Henning
Active Member
Hi Guys,
I am at my wits end with a Technics SA-300, and I'm hoping someone with more experience can shed some light on what I'm missing. This receiver exhibits distortion, even at low volume levels, and seems to be affected more by low frequencies than high. If I feed a 100hz signal, I see straight line clipping, both at the top and bottom of the sine wave, at the collectors of Q605 and Q606. The signal is clean at the collectors of Q601 and Q602. I've attached the schematic for reference. I've replaced all of the electrolytic and film caps on the amplifier board. I've tried replacing all three driver transistors on each side, including Q602. I've also changed out D601 and D602. None of this has made any difference in the clipping. I've checked voltages against the schematic, and they all match. If I turn the volume way down and watch the scope, the visible clipping of the sine wave goes away at a lower level. I've noticed that while listening to the tone through speakers, I can definitely hear the clipping, but I've also noticed that if I crank the volume knob quickly, there's a "Catch up" to the sound that appears to be happening. That makes me think there's a capacitor charging too slowly somewhere or something similar. My VM tells me that when I do this quick action with the volume knob, I lose a couple of volts off of the -35 volts that feeds Q606 at the Emitter. The voltage stays down until I bring the volume back down. I've spot checked resistors in the area of Q606 and didn't find anything out of place. Again, it affects both channels. That makes me think maybe it's something in the power supply, but I'm not finding any measurements out of place. I'm attaching the portion of the schematic with the amp board as the full file was too large to upload. The full schematic is available at hifiengine.com. I'd appreciate any suggestions to try. I'm a noobie, so it's very possible that I've missed something obvious. Thanks in advance!
I am at my wits end with a Technics SA-300, and I'm hoping someone with more experience can shed some light on what I'm missing. This receiver exhibits distortion, even at low volume levels, and seems to be affected more by low frequencies than high. If I feed a 100hz signal, I see straight line clipping, both at the top and bottom of the sine wave, at the collectors of Q605 and Q606. The signal is clean at the collectors of Q601 and Q602. I've attached the schematic for reference. I've replaced all of the electrolytic and film caps on the amplifier board. I've tried replacing all three driver transistors on each side, including Q602. I've also changed out D601 and D602. None of this has made any difference in the clipping. I've checked voltages against the schematic, and they all match. If I turn the volume way down and watch the scope, the visible clipping of the sine wave goes away at a lower level. I've noticed that while listening to the tone through speakers, I can definitely hear the clipping, but I've also noticed that if I crank the volume knob quickly, there's a "Catch up" to the sound that appears to be happening. That makes me think there's a capacitor charging too slowly somewhere or something similar. My VM tells me that when I do this quick action with the volume knob, I lose a couple of volts off of the -35 volts that feeds Q606 at the Emitter. The voltage stays down until I bring the volume back down. I've spot checked resistors in the area of Q606 and didn't find anything out of place. Again, it affects both channels. That makes me think maybe it's something in the power supply, but I'm not finding any measurements out of place. I'm attaching the portion of the schematic with the amp board as the full file was too large to upload. The full schematic is available at hifiengine.com. I'd appreciate any suggestions to try. I'm a noobie, so it's very possible that I've missed something obvious. Thanks in advance!
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