The guys have already given you good advice. The reason you have to press the heads down is because the main drive belt is slipping. This deck employs a servo drive system. The mechanical action of engaging and retracting the heads depend upon the energy provided by the rotation of the flywheel assembly. This mechanical action also imparts a momentary drag/resistance on the rotating assembly, which with a properly functioning belt is easy to overcome. However, if the belt is slipping, then the drag might be too much to overcome before slippage and the mechanism gets stuck in the last position. If the belt is structurally OK and simply suffers from deterioration of the surface layer, you might be able to restore function by cleaning the drive belt with isopropyl alcohol. To do a proper job, you should remove the belt and scrub it vigorously by a IPA soaked q-tip and/or tough paper towell, or something that will rub off the slick/powdery outer layer. You will notice your q-tip and/or paper towells turning black. When you are satisfied that the surface texture has restored some amount of "grip" you can reinstall it. Deterioration of belts is a very common issue. Sometimes, they turn into tar-like goo. Sometimes, they get hard/brittle and sometimes, they just lose their rubbery composition. There are other products you can try such as belt conditioner, but in my opinion, they are just a temporary band aid and if a cleaning with IPA doesn't restore it, then the belt should siimply be replaced instead of applying foreign compounds to the belts. Think of old car tires... they simply don't have the flexibility or grip they once used to have. The RX-5150LS would have differences in the electronics although I suspect that the mechanism is the same. Regardless, you should't need it as service manuals rarely provide step by step help in rebelting anyhow. For this service, good mechanical aptitude will get you far.