Tascam 688 with no audio signals in or out

Discussion in 'TASCAM DIY Repairs and Mods' started by John Mackenzie, Oct 29, 2017.

  1. SkywaveTDR

    SkywaveTDR Well-Known Member

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    I can't read all of this but the BA6209 is a motor driver chip not related to audio output at all. The CD4066 is a very common IC in many decks and equalizers and they blow out with regularity. Of course they are CMOS so that may be why. In my repair of units I put signals in like a 1 KHz -10dB sine wave and follow the path to where it stops. I use a scope for this. It becomes pretty obvious after a time where the problem will be.
  2. rohmal

    rohmal New Member

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    Yes My Senior Sam. You are quite right. I made a mistake by just looking at the boards visually. As John's unit is dead should start from the power supply it self. I think your experience can guide John to safe waters. I let John know if he didn't follow the thread. Thanks again for Your contribution to the thread. Any advice about what we will miss on the way will be appreciated. Have a good day.
  3. John Mackenzie

    John Mackenzie New Member

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    Update on 688 audio problem.

    I fitted 14 pin DIL socket, nice and neat. Inserted new CD4066B chip, after checking power supply voltages were OK , on the main AC supply from transformer. I also checked voltages at various points around PCB where indicated on the board. all looked correct.
    After switching on again, I still have no Audio signals on meters, either from signal going in from input jack (line) or coming from tape. The new chip "looks" OK, no visible damage like the previous one. I need to investigate the output section I think, if I can find the relevant parts. I need to take more time to study the schematics before I continue with this, to try and work out the most likely culprits to examine. I don't have a lot of experience with solid state electronics , mostly limited to replacing output transistors/capacitors/ etc on guitar amplifiers. I have a good understanding of valve amplifier technology, with point to point wiring etc, but IC's and processors in the digital domain are a bit more challenging for me unfortunately. I will keep you posted with any further progress.
  4. rohmal

    rohmal New Member

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    Dear John, Find all IC data for pin numbers and voltages. Check all for voltage first. If all OK then have a look at your scan page 055 to start. Select all channels for line inputs. Put a signal on line B input and trace the signal according to drawing step by step for each channel. You can use a generated frequency for input and a O;scope or a signal tracer to follow it. Lets say the signal is present on IC U1 (2/2) (UPC4170HA)pin 6 and nothing on pin 8 means that part is faulty. If it is present at pin 8 it should next reach pin 4 of IC U2. If not cap C24vis faulty. If U2 IC (TC9164N) has responded to your input channel selection to line command U2 pin 4 should be connected to pin 6 and you can find the signal on pin 6(main In). that leads to EQ section cap C12 like. In some lines I see the signal path marked with a small arrow. So carry on. Take time and be patient. Do a systematic trace. You'll get use to the technique. See how yo'll do and post.
  5. John Mackenzie

    John Mackenzie New Member

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    Thank you again Rohmal, I shall try to proceed.
  6. John Mackenzie

    John Mackenzie New Member

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    Hi again Rohmal. I still have not had enough time to get much further with this. However, I did find out something today. I do have tape output on all 8 tracks, just using "tape out" sockets on the 688. I realise that this method of output misses out the mixer section of the machine, but at least I know my tape deck is working OK. I have a friend with test equipment who has offered to look at it for me. At least I have the schematics to give him to make his task easier. He is a highly qualified electronics engineer and also has some respect for older analogue type equipment . I still have hope for this !!
  7. SkywaveTDR

    SkywaveTDR Well-Known Member

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    It doesn't matter what kind of Engineer he is, now we will see what kind of Technician he is. The second does not automatically come with the first. When I was in Engineering school most if not all of the guys had no Technical background. Then in some classes I was with Fermi lab and bell Telephone people so they did know. One Instructor was from Bell Labs.
  8. John Mackenzie

    John Mackenzie New Member

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    Hello again Skywave TDR. Thank you again for having an interest in my Tascam 688 problem . I understand your comments completely . Practical skills and technical understanding of a particular product are a lot more useful than "academic qualifications", when it come to repairing faults. However, as I have said in previous posts, I no longer have access to the types of test equipment needed for this investigation. My friend has only offered to do this work for me as he does have the gadgets needed and the knowledge of practical electronics which I lack.
  9. wkrbee

    wkrbee Active Member

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    My question:Are you familiar with the routing aspects of the 688?
  10. John Mackenzie

    John Mackenzie New Member

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    Yes wkrbee, I have had many years experience using a 688 . Is there something that I can help you with? It is highly versatile recorder/mixer, but can be a bit complicated for a new user.
  11. wkrbee

    wkrbee Active Member

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    Hi John.The only reason I asked was because most problems people had was with understanding the 688 routing .
    John Mackenzie likes this.
  12. John Mackenzie

    John Mackenzie New Member

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    yes WKRBEE, I realised that I had replied in the wrong manner, without seeing your other posts on this forum. Your previous information has turned out to be the most useful "symptom" and possible cure. My 688 goes to visit a long time served Tascam technician tomorrow. He promises me that he understands everything discussed on these forum pages and will proceed logically to indentify and fix my problems. I can only hope now. ??
    I really want to thank all the people who have given their time to advise me on this . I am very grateful for all the inputs I have had. I shall post the results of "repair" as soon as I get this machine back. ( if it is fixable ! ).