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.
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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.
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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.
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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.