The CD player finally plays music again, but still has trouble with the higher tracks.
Fourth Problem: Worn out Laser
Let's find out how worn out the laser actually is. I soldered a wire to the RF test point and connected it to a scope.
The service manual recommends to use a special test CD, but you can use any CD for the RF test. I found out that older CDs gave better results, probably because modern production methods are cheaper and less reflective. Do not use CD-R, as these discs use a dye and are not as reflective as standard CDs.
According to the service manual, the signal should have an amplitude of about 1.2Vpp (+0.2V, -0.4V).
The actual amplitude is just above 600mVpp, far below the required minimum of 800mVpp.
The first thing one should try now is to clean the lens. I used a cotton bud and 100% isopropanol for that. After that, the amplitude was a little better with about 880mVpp, but still at the minimum.
I could now recalibrate the player and try to get the best out of it.
But honestly, a new optical pickup module for this player can be ordered at the Bay for less than €20. It's not genuine, since Sony is not producing the KSS-240A any more. But I am curious how good that remake actually is. If it should be worse, I could still continue to use the original one.
The optical pickup is very sensitive to static discharge. If you try the replacement at home, use an antistatic wrist strap and other ESD protective measures while handling it!
The replacement of the pickup was much easier than expected. Only a plastic clip is holding the metal shaft of the sled in place, so it could easily be pulled out. After that, I could remove the old pickup.
And while I was at it, I thoroughly cleaned the shaft and the plastic parts from the old grease again, and reapplied a bit of fresh silicone grease. I also applied a little bit of grease to the gears that are moving the sled.
Then I inserted the new pickup and fixed it with the shaft. The first test with the new pickup brought good news, and bad news. The good news is that the replica pickup is working, and the RF amplitude is even above the 1.4Vpp upper limit now.
The bad news is that the drive was making a loud and awful whistling noise now. So I recalibrated the player as described in this restauration project of a similar player.
The calibration was successful. The whistling noise is gone (except on track changes), and the player plays all the tracks again. I am sure an experienced technician can calibrate it a lot better, but I'm happy with the result.
And thanks to my proper cleaning, the sled noises are also much better. By all means good enough considering the age of the player.
I'm almost done! There's just a tiny problem left: The crackling volume pot.