So tonight I had a chance to start on the refurbishment of another Pioneer SX-838. This receiver was sent to me for restoration work since the owner described crackling sound from the balance control. While it is here it will be tested, cleaned up, electrolytics replaced, and any noisy transistors replaced before being tested again. This receiver does have the dreaded 2SC1451 on the amp board that should be replaced regardless of condition.
Overall, the receiver is in great shape and will really shine from a good detailing. The inside is dusty as to be expected and all original. First things first a quick bench test to see how the amp performs.
A complete bench test was planned. However, after starting with a 1KHz sine wave on both channels I saw enough to stop testing. As the amp was brought up in gain the waveforms were obviously uneven in amplitude. The left channel appears a bit stronger then the right. When output was tested the amp could not be brought to clipping. At maximum volume the amp was only pushing 47w on the left and 38w from the right. I have a quick picture of the waveforms below and you can see the difference in amplitude. The balance control is definitely dirty as it caused distortion when used but is not causing the channel imbalance.
As I imagined this receiver is not meeting original factory spec and needs a good tune up. A few lamps are out as well and will be replaced. Below are a few shots of the receiver being tested as well as an inside shot.