I have an old Pioneer SX525 a friend gave me when he was moving cross country. I restored a lot of it back when I got it, replacing old failing capacitors and swapping out some of the failure prone transistors. Recently the right channel started becoming very noisy, so I fixed it. This is a brief log of that process.
The noise only occured when using the phono and radio inputs. It did not occur when using the aux input. The phono and aux inputs use the head amp while the aux input bypasses it. Therefore, I assumed that's where the problem was. Furthermore, since it only happened on one channel, I had another channel to compare against.
I started by just measuring the noise on phono mode with nothing connected to the receiver and the volume at ~50%. You can see below that the right channel has significantly more noise than the left.
Next I confirmed the problem was in fact in the head amp. I measured the left and right channel outputs of the head amp and confirmed the right channel was still significantly noisier.
Thankfully the head amp is pretty simple. Each channel uses just two transistors and switches the feedback network to switch between FM de-emphasis (pins 2 and 7) and RIAA equalization (pins 3 and 6). I had previously replaced all the capacitors in this section, so I suspected the transistors were to blame. I thought I had also swapped the transistors out previously, but upon opening it up I discovered I forgot the right channel.
I swapped the old transistors out with some 2n5088 transistors. I chose the
2n5088s for their low noise and high gain. They're popular in preamps for these
reasons. 2n5089s would have been even better, but they have a VCE
MAX of just 25V, which is lower than the circuit's supply voltage.
After replacing them I tested the output again on the oscilloscope and confirmed the noise was gone. After hooking everything back up it works great.
