I also have this unit, Pioneer PD-T505, and have had it since new in approx 1988, although it has not been used extensively ... maybe alot for 2-3 years, but then under 100 hours a year.
It has just started to skip ... what happens is that I put a CD into either tray, and it will "spin" the CD for a bit, which is rather noisy, but it will not play.
I opened the unit, and through trial and error found out that if I apply some pressure to the top arm that drops down the top center magnet, the unit plays perfect, and even the noisy spinning stops ...
I noticed a metal crossmember across above this plastic arm, and was wondering if there was some kind of spring mechanism that might be missing? I turned the unit upside down and gently shook it, and found no evidence of any loose parts or springs ...
This unit is old, but it worked PERFECT up until this point, as well as being the FIRST CD player I saved and bought myself while in highschool, so if I can extend it's life, it would be awesome!
Hoping for good news ..
Paul