I recently found this Pioneer SX-450 receiver at a garage sale in my neighborhood. It was mostly functional, but the signal was somewhat intermittent. I decided to give it the royal treatment - new transistors (the originals are infamous for getting noisy), new switches (dirty or intermittent tape monitor switches are the most common issue I see in old stereos like this), new electrolytic capacitors, and cleaning the pots and input jacks.
This receiver is constructed nicely - when you open it up you have access to the top and bottom of the PCBs without having to remove them, so it's very quick and easy to replace parts.
Replacing the electrolytic capacitors is fairly straight forward. I did double check some component values using the service manual (courtesy of Hi-Fi Engine). One section of the PCB was covered in melted wax, I guess someone left their candle unattended.
I replaced the four pushbuttons with C&K F-series DPDT switches, along with the matching four position mounting bracket. It was a bit tricky as the old switches had much longer legs than the new ones, so I had to extend all the legs with solid core hookup wire. A bit tedious but it worked well. This solved the intermittent signal issues the stereo was having. Over the years, switch contacts frequently start to degrade and cause the signal to cut in and out.
I also replaced all the transistors in the audio sections of the amplifier. Conveniently, the circuit board had transistor symbols printed on it to show where the emitter, base and collector of each one went, which makes life a bit simpler, especially when using new transistors with different pin outs. The transistors in a stereo like this can usually be split up into a few categories - small signal, drivers, and output. You'll usually need both NPN and PNP transistors of each type, although that depends on the circuits specific topology. Here's a run down of the original transistors and the replacements I chose to use:
2SA725 replaced with BC556
2SC1775 replaced with KSC1845
2SC1318 replaced with KSC2316
2SA720 replaced with KSA916
2SD313 replaced with MJ15028 (these are the four output transistors)
2SC1222 replaced with KSA992
When replacing the four output transistors that are mounted to the heatsink, you'll want to clean off the old heatsink compound and apply a fresh coat to make sure there's adequate heat transfer there. It's also generally a good idea to replace the mica insulators that sit between the transistors and the heatsink. Those things can get pretty brittle over time and if one cracks and creates a short between the heatsink and the transistor, it will be a major problem.