[Sideband]

Most simple tape recorders like this one only had two heads. One (usually the first one) would be the erase head. The second would be the record/playback (or sometimes referred to as the read/rewrite head) head. The erase head simply functions as an electromagnet on record and erases the previous recording just before the record/playback head records new information on to the tape. Some very simple and cheap machines had a permanent magnet in place of the erase head and this was mechanically moved away from the tape on playback and then pressed against the tape on record.

One reason that your machine may not be erasing/recording is that the circuit referred to as the 'bias oscillator' may not be working. This will cause no erasure of previous recordings and also any new recordings will will sound distorted and weak and will be 'mixed' with the old recording.