I had the same issue with DX7II-FD (which has the same FM engine like TX802). It turned out to be a faulty OPSII chip: YM2604. After replacing the chip the noise issue has been fixed. Finding a brand new or even used YM2604 is impossible these days. My story was related to 10 years ago and I had access to an old Japanese Yamaha service center that had only one set of brand new EGM/OPSII. These chips are very sensitive to short circuit on pins, be careful to not touching them with your tools while TX-802 is turned on otherwise you will damage those in less than a second and make your synth useless forever.
