Tascam 644 Midistudio - Solved - No Right Channel Input Signal

Discussion in 'TASCAM DIY Repairs and Mods' started by RNRS, Aug 10, 2019.

  1. RNRS

    RNRS New Member

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    I have a Tascam 644 that I purchased new in 1991. It has been in storage since about a month of use during that year.

    I just had the battery replaced (August 2019), and all functions appear to be working with the exception of not being able to pan any input source or signal to the right. Only left channel signal is shown in the output LED display. Panning any input source in any channel to the right just gradually lowers the left channel input in the output LED as expected, but no reciprocal level ever appears in the right channel meter or monitors.

    All playback options and panning functions work normally.

    If I pan any input signal fully right, there is no level in the monitor speakers or headphones. Pressing the Mono switch returns sound to both channels if pans are set to left, or center, but that level fades as I continue panning right and disappears completely when set fully to the right.

    So, can I assume the best and hope that some circuit board connection was disturbed when the battery was replaced, or should I be prepared for some kind of component failure? I have the service manual and may be inclined to get inside and check connections it I knew which board and connections were likely to be the problem.

    Hard to tell, I know, from this brief description, but since this recorder has had so little op time, I'm hoping it's something simple. I think I've ruled out any routing and settings issues, but I'd love to know if I've missed any.

    Thanks for any thoughts or suggestions.

    -rnrs
  2. SkywaveTDR

    SkywaveTDR Active Member

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    The use of a scope will be helpful in this venture. Since no channel has right channel output then you look for a switch or part in the circuit that is common to all. It might be a op amp used to buffer the channels output to the buss. The other question is there anything coming out of the pan pot of each channel. I have not come across a thing like this so far. Maybe a connector that was disconnected to open it up is not restored back to original position.
  3. RNRS

    RNRS New Member

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    Thanks for the help, Skywave TDR,


    I've made some progress. I have found ways of achieving the kind of panning behavior I'd expect. (For instance, if I route the same input through the Dual path, the Dual pan pot works the way I'd expect the channel pan to operate).

    I can also assign a second input through Group 2, which gives me a result exactly opposite of the Group one input. ( that is, no *Left* channel when fully panned left instead of no *Right* as described earlier.

    According to the manual, when using the "Scene 1" preset, the channel pans should all respond to input and positioning "normally".

    On the plus side, pushing and chasing the input signal around is giving me a better understanding of what happens when various choices are made, and how similar output can can result from several different paths.

    When using recorded material in the tape transport as the source material, the channel panning works normally. For instance using Preset "12" routes the output of a 1-4 channel recording through channels 1-4 with none of the issues described when using the hard ware inputs.

    In summary, the channel pots all function normally when working with anything but hardware inputs.
  4. SkywaveTDR

    SkywaveTDR Active Member

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    Well the more they digitalize these things the more complicated the routing can be going through analog switches and that. Maybe there is a burned out 4066 if they used them. I can tell as my computer does not have the schematics of the unit on it. It seem that the service manuals of these later made units is very rare to come by. Well at least you got it going some of the way.
  5. RNRS

    RNRS New Member

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    Thanks yet again.

    I'll keep at it.

    If you'd like to have a look at the circuitry:

    Here's a schematic that shows the two pan options at the top of the page. the one near the box labeled "Dual" works fine with the same input sources. the one at the top doesn't feed the right channel (or left if it's assigned to "Group 2 instead of 1)

    https://www.manualslib.com/manual/1355797/Tascam-Midistudio-644.html?page=49#manual

    And here's the simplified diagram from the same User's Manual.

    https://www.manualslib.com/manual/1355797/Tascam-Midistudio-644.html?page=9#manual
  6. SkywaveTDR

    SkywaveTDR Active Member

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    What I see on the amp after the channel fader is that the pan send signal to odd and even channels and so that has to be taken into consideration. If when panning left the group 1 or 3 is enables then the output should arrive at those outputs the group 2 and 4 will have the right channel outputs. In troubleshooting a scope is used to detect a sine wave signal at the output of the pan pot and then applied to a buss and then traced all the way to the output. If a Buffer amp has gone out the that is what you will see is input to the amp and no output which will indicate that something went wrong in that stage. It is just a matter of tracing a signal and finding where it stops- I have done this on dozens of mixers both in Broadcast and home studios. Do you have a scope to do this with?
  7. RNRS

    RNRS New Member

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    I don't have an oscilloscope, but a recommendation would be helpful. I'd be starting with very little experience and cash.
  8. RNRS

    RNRS New Member

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    Well, I seem to have figured out the panning issues. Thanks for your help suggestions over the past few days.

    In this case, it was a Scene Routing issue. Both Group 1 and Group 2 must be selected for panning to work in a conventional manner.

    I was using the Scene 1 preset by chance, and it was set up with Group 1 only.

    The last two links below include diagrams of presets 1 and 5. The upper left image of each page shows the routing set up for Left only (Scene 1, where only Group 1 is selected) and Scene 2 on the next page, where both Group 1 and 2 are selected for each channel. Selecting both groups allows the channel panning to behave "normally".

    The intended use of both of those presets is noted in a description above the images.

    This exact issue came up on this forum a year or two back here:

    http://www.tascamforums.com/threads/644-panning-issue.4563/



    https://www.manualslib.com/manual/1355797/Tascam-Midistudio-644.html?page=54#manual

    https://www.manualslib.com/manual/1355797/Tascam-Midistudio-644.html?page=55#manual