Introduced more than 10 years ago (versions between 2003 and 2006 from what I found online), the Logitech Z-2300 provides 200 watts of total RMS Power, 2 x 40 Watts into eight ohms for the satellites and 120 watts RMS into eight ohms for the sub-woofer.
All considered, this system was for the price a good set that provided really decent sound…. only this one was producing hum, even with inputs shorted.
To find out what caused the hum, I started with a simple app (Spectroid) on my smartphone to identify the hum frequency. Spot on, it was 100 Hz. For sure the power supply capacitors needed replacement.
Opening the box starts by removing the front grill (search for online videos) removing and disconnecting the sub-woofer. Only then are the cables long enough to remove the back plate.
And then what do you find : goo, a lot of goo, glue what else sorts of mastic they had on hand to stick things in place. Connectors to back-plate, capacitors to PCB, and thermal grease (a lot, again) on heat sink and power chips. Looks like goo is too cheap, considering how much they used.
The replacement caps I selected are Elna, 10’000 uF, 35 VDC 105 °C. The fit in place nicely, and the PCB holes were large enough to accommodate the snap-in types.
Re-solder all cables, re-place 31 screws and connect it to find out it works, and the hum is gone. Completely silent without input signal. Let’s call it a success, the unit is salvaged, the listener will probably be happy and all in all for 10 CHF of capacitors. Now on to clean the stack of dry, stone-hard glue pieces spread all over the lab.