"The master bus is humming."
The Langley PSU uses a connector. The manual explicitly states: "Do not mate or un-mate the PSU connector while the unit is powered on." The backplane has no suppression diodes; hot-plugging will blow the master rail fuses and potentially the BA-440 op-amps.
A built-in sine wave generator (typically switchable between 100 Hz, 1 kHz, and 10 kHz) used for calibrating the console meters against external recording decks. Maintenance, Troubleshooting, and Calibration Amek 501 Langley Manual
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 was established in the late 1970s with the specific goal of creating high-quality mixing consoles that were more reliable and easier to service than the American competitors of the time (such as MCI and API). "The master bus is humming
The Complete Guide to the Amek 501 Langley Mixing Console: Operations, Maintenance, and Legacy
Swaps the channel input and monitor input paths. This allows you to bring your tape/DAW returns onto the large faders instantly for mixing. The Complete Guide to the Amek 501 Langley
Scratchy audio controls or intermittent signal dropouts are usually caused by oxidation inside the switches and rotary potentiometers.
The HF and LF bands are shelving, while the two mid-bands are peaking with adjustable frequency centers. The "EQ In" button allows for quick A/B testing of your processing. Master Section and Monitoring