Hi Everybody,
Long post ahead!
I've had a very nice NAD 514 CD Player for a while and it's been working very well. It sits in my home theatre as part of a stand alone stereo music setup.
Over the last six months or so it's been "skipping" during playback. After a bit of searching there were two things that were identified as the potential culprit. 1) The Laser Assembly, and 2) the circuit that drives the Focus Coil in the Laser Assembly.
The NAD 514 uses a Sony KSS-212A Laser Assembly. Aftermarket versions are still readily available so I ordered one and swapped it out (remembering to remove the solder bridge protecting the laser diode). Sadly the problem remained. I did notice that the wires from the Laser Assembly to the main PCB didn't look original. The previous owner didn't know about any work being done on it, so this "repair work" may have preceeded his ownership of the CD Player.
I then turned my attention to the Focus Coil section of the CD Player in particular Q102, a 2SD1858. My finger immediately told me something is wrong, it burnt a transistor shaped mark into me! It was VERY hot! Searching showed a few posts on hifi fourms revealing this transistor does run hot but few as to why and no real solutions. One post did say that the CD sits at the wrong height in this players mechanism, forcing the Focus Coil circuit to continually apply an offset to the lens to keep things happy. One suggestion was to change the transistor to a 2SD669 which is electrically equivalent and being a TO126 package can have a heatsink affixed or mounted to the chassis (with insulation) to help dissipate the heat.
I ordered both 2SD669 and 2SB649 transistors to replace both drive transistors for the Focus Coil, Q102 and Q103. These arrived yesterday and were dutifully swapped out. I formed the legs of the 2SD669 to allow a bit of space to allow a small heatsink to be fitted (see attached). I was happy to find that a CD would now play fine with no hints of the skipping issues experienced before, BUT the 2SD669 still rose to an an extremely hot temperature during playback. The heatsink is quite modest, but much better than the original 2SD1858 could handle. Tests show the voltage applied to the Focus Coil is a fairly consistent 670mV during CD playback (at TP105). I've not yet measured the current being drawn by the Focus Coil during operation, but the DC resistance of the Focus Coil is around 7 Ohms. The positive DC voltage on the collector of Q102 is almost 15 Volts. Even without writing it down on paper you can see that a transistor circuit that drops 15 Volts to 670mV and to drive a 7 Ohm load would need to dissipate the rest of the voltage as heat! This seems to be somewhat of an Achilles Heel in this design.
I'm wondering if anyone in here has had any experience with repairing/servicing the NAD 514 or CD Players with a similar Focus Coil circuit. Have you found the transistors in the Focus Coil running at elevated temperatures?
I could just mount Q102 to the chassis and accept that it runs VERY hot during operation, but I would like to find out if something else could be causing the issue. I will attempt to go through the adjustment procedure as detailed in the Service Manual. I will need to build a "Special Circuit" (described in the Service Manual) and get hold of a Sony YEDS-7 test CD.
One other thing that I noticed while inside the 514 is that if I handled/touched the wires going from the detector diodes in the Laser Assembly to CN103 on the PCB playback would skip! If I touched them too long playback would stop. I found this interesting.
Michael.
Long post ahead!
I've had a very nice NAD 514 CD Player for a while and it's been working very well. It sits in my home theatre as part of a stand alone stereo music setup.
Over the last six months or so it's been "skipping" during playback. After a bit of searching there were two things that were identified as the potential culprit. 1) The Laser Assembly, and 2) the circuit that drives the Focus Coil in the Laser Assembly.
The NAD 514 uses a Sony KSS-212A Laser Assembly. Aftermarket versions are still readily available so I ordered one and swapped it out (remembering to remove the solder bridge protecting the laser diode). Sadly the problem remained. I did notice that the wires from the Laser Assembly to the main PCB didn't look original. The previous owner didn't know about any work being done on it, so this "repair work" may have preceeded his ownership of the CD Player.
I then turned my attention to the Focus Coil section of the CD Player in particular Q102, a 2SD1858. My finger immediately told me something is wrong, it burnt a transistor shaped mark into me! It was VERY hot! Searching showed a few posts on hifi fourms revealing this transistor does run hot but few as to why and no real solutions. One post did say that the CD sits at the wrong height in this players mechanism, forcing the Focus Coil circuit to continually apply an offset to the lens to keep things happy. One suggestion was to change the transistor to a 2SD669 which is electrically equivalent and being a TO126 package can have a heatsink affixed or mounted to the chassis (with insulation) to help dissipate the heat.
I ordered both 2SD669 and 2SB649 transistors to replace both drive transistors for the Focus Coil, Q102 and Q103. These arrived yesterday and were dutifully swapped out. I formed the legs of the 2SD669 to allow a bit of space to allow a small heatsink to be fitted (see attached). I was happy to find that a CD would now play fine with no hints of the skipping issues experienced before, BUT the 2SD669 still rose to an an extremely hot temperature during playback. The heatsink is quite modest, but much better than the original 2SD1858 could handle. Tests show the voltage applied to the Focus Coil is a fairly consistent 670mV during CD playback (at TP105). I've not yet measured the current being drawn by the Focus Coil during operation, but the DC resistance of the Focus Coil is around 7 Ohms. The positive DC voltage on the collector of Q102 is almost 15 Volts. Even without writing it down on paper you can see that a transistor circuit that drops 15 Volts to 670mV and to drive a 7 Ohm load would need to dissipate the rest of the voltage as heat! This seems to be somewhat of an Achilles Heel in this design.
I'm wondering if anyone in here has had any experience with repairing/servicing the NAD 514 or CD Players with a similar Focus Coil circuit. Have you found the transistors in the Focus Coil running at elevated temperatures?
I could just mount Q102 to the chassis and accept that it runs VERY hot during operation, but I would like to find out if something else could be causing the issue. I will attempt to go through the adjustment procedure as detailed in the Service Manual. I will need to build a "Special Circuit" (described in the Service Manual) and get hold of a Sony YEDS-7 test CD.
One other thing that I noticed while inside the 514 is that if I handled/touched the wires going from the detector diodes in the Laser Assembly to CN103 on the PCB playback would skip! If I touched them too long playback would stop. I found this interesting.
Michael.
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