I had a problem with my old Pioneer PD 9700, the disc wont play at all. The first thing I did was to check the lens on the pickup assembly and to my surprise it had fallen off it's sitting base. I carefully glue it back using a fine drop of KRAZY glue after blowing off any dust inside where the optical prisma reflector is allocated. The Infra red beam comes from the diode which is a double setting diode, one for the PD (photo diode) and the other for the LD (laser diode). The latter is the one which detects and reads the CD first at a wavelength of some 780 nm or so and a 1mA output. If the CD is not properly sat on the disc tray then the beam will not actuate the second diode (PD) which in fact is the one reading the disc bit stream content. Some CD players have a test mode setting to correct any issues concerning alignment, focusing and so on (check the service manual of your device), essentially used when you are replacing the whole pick up assembly or when you are certain there is an alignment problem (i.e. the disc wont play correctly or there is no information about the disc on the front displayed panel. However, it is extremely rare a diode fails unless you use your CD player 24 hours a day!, therefore it is worth checking first, both alignment and any marks or dust on the lens. You could also check the red light coming out of the lens using your smartphone camera, therefore avoiding eye naked exposure. The way you should do it is turning on the device with your camera pointing to the lens. Press play and the LD will send the first beam light through the lens for about a second or two. If you see light through your camera then the LD section of the diode is ok. It is worth noting that the infrared beam could be diverted if the lens are scratched or dirty, rendering the IR light becoming out of focus. If the lens is opaque, then you should simply clean it up using ordinary windscreen liquid and a piece of cotton wrapped on a tooth stick. Avoid any unnecessary pressure on the lens as this is sat on a balancing housing, simply rub it up gently with the cotton wrap tooth stick dampened with windscreen liquid . Most of the times, dust or finger marks on the lens causes CD players not reading discs properly or playing it at all. The second thing to check up is the CD tray compartment, especially if your CD Player is old. The parts to look at first are the screws, washers and most importantly lubrication on the conveying section. Remember, if the disc tray is not properly sat or lubricated then the IR beam may be out of focus or unable to read the disc. In my case, my Pioneer was manufactured in 1991, so it is quite old. Upon checking the washers which are rubber kind, I noted there was one showing wear and cracks on it so I replaced it straightway. After careful adjustment of the set of screws holding the CD in-tray section, the old Pioneer came to life again!
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