Sony began producing tape recorders in the 1960s. By this time in electronic history, tubes were out, and transistors or, "solid state" units, were becoming more popular. As a result of Sony's efforts to stay cutting edge, all of its tape recorders were created with no "user serviceable parts." Unlike their American-made counterparts, Sony reel-to-reel tape recorders come in oiled Walnut boxes and are stapled shut. There is no safe way to open the unit without damaging the case permanently. Here is a less-invasive step to fixing the unit.
Plug in your tape player.
Listen to hear if there is any "grinding" sound coming from the case.
Watch the tape reel-mounts. If one of them is not spinning, or not spinning at a steady speed, you have a slipped belt. Having a slipped belt means that one of the belts that transfers motion from the motor to the reel mount has broken, the rubber has dried or the belt has come off its mounts.
Place some denatured alcohol on a Q-tip and rub the tape heads down until the Q-tip comes out clean. De-natured alcohol is of a stronger concentration than the 70 or 90 per cent you would receive with over-the-counter alcohol. De-natured alcohol will leave virtually no residue once it dries. Rubbing the tape heads with denatured alcohol will remove all built-up acetate (tape residue) dust or dirt.
Check your VU meters to see if there is any audio being sensed on the tape. You should see the needles in one or both meters pulsing to the right.
Check your amplification system is on, that the "tape" input is selected and that the volume is up. Sony reel-to-reels require an amplifier between them and your speakers. Non-powered speakers will not work. A suitable amplifier should have RCA cable inputs. If it does not, you will need cables that convert your Sony RCA outs to the required plug type.
If you do not see the VU Meter's needles pulsing when playing a tape. Experiment with a few tapes in case your test-tape may be blank.
If your device is making a "grinding noise" when you press play and the tape mounts aren't spinning, this means you have slipped a belt. There is no real way to fix your tape recorder unless you wish to crack it open, track down a specific rubber belt and swap it out. Sony has made it difficult to enter into its units and replace parts. You run the risk of electrocution or even further damage to the unit if you attempt to service it yourself.