D50MKii doesn't read due to laser potentiometer

kaosun

D50mkii is an excellent player with great sound quality. However, the laser it uses: KSS110G has a flaw (design flaw) potentiometer. The potentiometer causes lots of issues of laser current, which could not provide a stable laser current to support reading ability.

To get enough RF pattern: commercial disc 1.0-1.3Vp-p, CDR: 0.8-1.0Vp-p, the potentiometer in this D50mkii should be adjusted to about 345 OHM. For the 4.7KOHM original potentiometer, the contact point of the arm is very close to the end of the resistor body to get 345 OHM. Because the impedance of the potentiometer is 4700 OHM, the contact point shold be adjusted no more than 10% (10% is 470 OHM) from right end to get 345 OHM. Due to oxidization, bad contact and low resolution, it is quite hard for the old potentiometer to maintain at 345 OHM. This valve varies by time. In case the value getting smaller, laser current increases, reading improves; while getting bigger with smaller laser current, reading weakens with lots of noise/skipping.  The best solution is placing the contact point at the middle part of a potentiometer to get stable and reliable impedance to provide a constant laser current which is the best way to maintain a good reading ability.

1：Found the issue of non-reading disc, restore the reading by adjusting the pot a little bit to bring laser current, RF 1.0Vp-p on CDR (regardless of noise which up to 1.084Vp-p).

2: The laser current can be measured by the voltage across two 39 OHM resistors(connected in parralle, equals to 19.5 OHM). Current: 1.045/19.5 = 53.6mA. However, the pot connect point is almost reachs the right end of the pot body.

      

      

3: The pot is replaced by a 1K OHM brand new one, adjusted to its middle to obtain RF 0.8Vp-p on CDR (regardless of noise which up to 0.954Vp-p). Now the current is 1.033/19.5 = 53mA. Comparing the original current printed on the label which is "553" (means 55.3mA), the laser does not show any aging by my opinion. Theoretically, due to aging of semi-conductor inside of the laser diode, the laser efficiency decreases over time. To maintain the original reading ability(laser strength), the laser diode consumes more current (manually increase the current by reduce the power pot 10% every time) to restore the original brightness. If calculated current 65mA for example, we can assume ( 65-55.3 )/ 55.3 = 17.5% aging rate. For this case, I can tell there is no aging observed, the laser diode is still in excellent working status. The reason causes the laser emitting issue is the pot. Fortunately, the pot is replaced and hopefully the player can serve the owner for quite long time.

      

     

4: Same pot issues are found on all KSS110/KSS110G lasers, pls replaced them all if possible.