My Rhodes electric piano has a hum in the signal coming out of the effects loop jack but not out of the preamp output. What is causing the hum? The piano was recently fully serviced

A hum in your Rhodes electric piano's effects loop, but not its preamp output, points to an issue specific to the effects loop circuit or its interaction with external gear. Even after a recent service, several common culprits can introduce unwanted noise in this part of the signal chain. Here's how to diagnose and address the hum: Steps: 1. Isolate the Effects Loop and Test Cables: First, bypass any external effects pedals or devices. Connect a short, high-quality, shielded patch cable directly from the "send" jack to the "return" jack of your Rhodes' effects loop. If the hum disappears, the issue is likely with your external effects units, their power supplies, or the cables connecting them. If the hum persists, the problem is internal to the Rhodes' effects loop circuit or its jacks. 2. Check for Ground Loops: If you're using external effects, ensure all connected gear (Rhodes, effects pedals, amplifier, audio interface) is plugged into the same power outlet or a high-quality, isolated power conditioner. Ground loops are a very common source of hum, occurring when devices are grounded through multiple paths. If using a multi-amp setup or connecting to an audio interface, a ground loop isolator may be necessary. 3. Inspect Effects Loop Jacks and Internal Wiring: For late-model Rhodes suitcase pianos, the effects loop jacks (often labeled "Accessory 1" and "Accessory 2") have internal switching connections. If these connections become faulty or dirty, they can introduce noise. Try cleaning the jacks with contact cleaner. If the hum persists even with a direct patch cable in the loop, there might be a loose solder joint or a failing switching jack that needs professional attention. Watch-outs: