Repairing a NAD 502 CD Player
My trusty NAD 502 CD Player, which I bought in 1996 from Richer Sounds, developed a fault a few months ago. The display went completely dark, so although it would play CDs perfectly, it was tricky to operate. Luckily it was not a failure of anything more complex than the backlight, which turns out to be a couple of wire-ended small capsule lamps.
This CD player has a fantastic specification and had great reviews at the time, so I was not in a hurry to dump it for a newer model just for the sake of some tiny and cheap lamps in the display.
This really is built-in obsolescence because any incandescent lamp will eventually burn out, and these are not even replaceable without soldering. I suppose it was a cheap option at the time, and maybe the design life of the CD player wasn't more than 10 years. To last 23 years is doing well in consumer electronics. NAD can be forgiven for not using white LEDs since they didn't exist until 1996 or so and were very expensive at that time.
Instead of just replacing them with the same capsule lamp, for example part number 606-CM7220 from Mouser, as suggested by CPL593H on head-fi.org, I decided to upgrade to LED. For this sort of application, you want a wide an angle LED as possible, to create an even backlight, something like 90 degrees is sufficient. I found some through-hole white LEDs with the specs I wanted (120 deg) from RS. Don't be tempted to use higher brightness but narrow angle LEDs because these will just produce spots with a dark surround.
The lamps are supplied from the 0V rail with current going into the -15V rail from the 7915 regulator, so there are two sets of 0V pads and two sets of -15V pads. I wired a 1k resistor in series with two LEDs in series to give an adequate brightness, and this draws less current from the regulator than the capsule lamps. Whilst you could reduce the resistor to increase the brightness and draw more current, I wouldn't want take more than the original design for thermal reasons, although those regulators are on a big heatsink.
The tricky part is getting the LEDs into the right position, which I achieved by bending the wires on the components. They are stiff enough to stay in position. Care should be taken not to short the LED wiring to the link wires that sit below it (due this being a single sided PCB), using a piece of insulation tape on the link wires if necessary.
If you are testing this with the case open, be very careful of the top left corner where the mains comes in and is exposed on two big pins near the transformer!
NAD haven't used a fuse between the incoming Live and the transformer, so they are relying on the mains fuse in the plug. The only internal fuses are on the main board, so they are on the secondary of the transformer. From an LVD compliance point of view, I am not happy with this construction. Soldered wire mains connections can break off, and that cable tie does not prevent one mains connection from touching the other one if that occurred. Also note that there is no incoming earth core in the mains cable, so internal connections to the metal chassis would use the signal screen through to the amplifier as a fault current path!
A couple of other points about the general construction. The "heavy lifting" part of being a CD player is being done by a set of Sony chips, all surface mount and on the underside of the PCB. I expect this will be due to Sony, with Philips, being the originators of the CD technology, so there wasn't that much choice. The analogue filtering and output buffering is using NE5532, which seems to be the opamp of choice for audio and that hasn't really been improved on significantly since.
The display is now back in service and should last many more years.
The completed unit in use: