The service manual is available online, such as hifiengine.com.
If you are looking for simple fixes, you should exercise each audio switch/button, especially any multi-pole switch for RECORD mode. In other words, operate each button several times, and put it into RECORD mode several times. I am unfamiliar with this model, but it is likely that it uses a multi-pole switch to change the circuitry from play to record mode, and these commonly get dirty and corrode over time.
Check that the tape recording is good and the correct track format, and the heads clean and at least appear aligned correctly.
Check the line outputs to determine if the problem is in the tape machine or the amplifier/speakers.