Located on the right (or sometimes center) of the console.
After 30+ years, the electrolytic capacitors in the PSU are likely nearing the end of their life. Replacing these is the single best way to lower the noise floor of your console. Amek 501 Langley Manual
The 501's standout feature is its early digital-to-analog hybrid control system: Amek Technology Group--Big by Langley - Loc Located on the right (or sometimes center) of the console