As the title says. I have done some troubleshooting already that should hopefully let someone help me narrow down where the problem lies. I would guess it's just a few transistors or ICs that need to be replaced. I CAN record to groups 1, 2, and 5-8. When I play back the tape on another machine, Track 8 will play fine. This is just a playback issue with track 8. If I record to tracks 3 and 4 on another machine, they will indeed play back without issue on the malfunctioning 688. It's not a routing thing. I've got that part of the 688 figured out pretty well. I have opened the machine up and done a visual inspection of the bottom-most board, seated in the base of the mixer side of the unit, and have determined which section is for which group but didn't notice any obviously burned out components. I found the Playback Level trim pot for track 8 and sprayed it out and worked the pot back and forth plenty, and this didn't solve anything. I can hear some quiet playback (crosstalk?) when listening back through track 8 on the mixer, and the pan and level controls do adjust the sound, so I know it isn't an issue with the channel in the mixer. Is there an IC that controls the rec amps and playback amps for each channel, or a specific cap that may have caused these problems? My limited electronics knowledge tells me that's probably the kind of thing I'm looking for. I have no qualms with replacing board-mounted components, so if anyone knows what might need to be taken off of here I'm ready for it.