Tascam 688 - Won't Record groups 3&4, won't play back Group 8

Discussion in 'TASCAM DIY Repairs and Mods' started by notbillcosby, Aug 3, 2018.

  1. notbillcosby

    notbillcosby New Member

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    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.
  2. notbillcosby

    notbillcosby New Member

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    Partially fixed: Track 8 playback returned by replacing the fader bank attached to channels 6-9.
  3. notbillcosby

    notbillcosby New Member

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    Fully fixed! I swapped out the PCB with the group faders and got 3&4 recording now too. Hoping I can replace op amps on that malfunctioning board to get it going again too.
  4. SkywaveTDR

    SkywaveTDR Well-Known Member

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    From:
    Chicago area
    Gear:
    Teac, Tascam, Sony
    I don't know- it sounds like a lot of guessing to me and board replacing does not help figure out where the signal is being stopped. I would just use a scope to trace the signal and most likely a bad solder joint or a relay contact may be at fault. The solder joints in all this equipment is wave soldered leaving very little solder on a joint. Combine that with these units being portable and able to be bounced around a lot and these joints will break. All the Teac equipment that I have worked on for years never make aerospace soldering specs. The older stuff is better than the newer stuff. I typically spend an hour resoldering joints on reel to reel decks to increase their reliability on units like the X1000R and X2000R but the cassette decks are not much different.
    Have a look and you may find the problem by visual inspection. Components that go bad usually show no sign of being bad 96% of the time. I have been doing these repairs for 45 years so far.