NAD C 520

Posted on Feb 4, 2014

The LCD display on my NAD C520 compact disk player has become unreadable. The audio and CD deck are operating fine but operating the unit without a legible displayis difficult. If you look at the LCD under the right external lighting you can see that the basic LCD displayis still functioning so the backlighthas died for some reason. This machine is somewhat more

NAD C 520
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complicated than a toaster so the the first thing to do is try and find a schematic diagram and some service information - otherwise you will spend an age tracing out the schematic for the backlight circuit by hand. The first thing I discovered from an Internet search was that the backlight is an electroluminescent (EL) panel and that these have a limited service life of perhaps 3, 500 hours. It appears that many folk have had similar problems. I also found a copy of the service manual with a schematic, a fault diagram (which won`t help with this particular problem), board layouts and component identification. Then I checked the EL backlight power supply starting at the AC fuse and progressively working through the rectifier and regulator to the EL high voltage AC test socket. You can do these checks with a simple multimeter (just keep yourself physically well-away from the mains supply on the AC power board). The actual EL output voltage isn`t specified in the service manual but the inverter is switching regulated 9 V DC through a 5:85 turns ratio transformer so under no load we should see about 150 V AC at the output terminal, adjustable through a variable current-limiting potentiometer. This estimate of the drive voltage seems typical of EL panels, with a frequency of around 400 Hz. If you must tweak the potentiometer (R708) then tweak it right back to its factory set position because we have insufficient information to...

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