No Output on Right Channel

Discussion in 'Electronics Repair' started by keat63, Jun 8, 2011.

  1. keat63

    keat63

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    Guys.

    I have an electronic drum kit and yesterday discovered that it has no output on the right channel, I've ruled out the speakers, as it does it also on headphones. I've ruled out the output socket, as there is a stereo output also, which has the same issue.

    So my electronics knowledge is very basic (and I mean very).
    I'm not bad with a soldering iron, and have undertaken a number of simple repairs in the past, but I have no electronics diagnosis skills. I can identify certain components, but my success rate is based purely on the odd cap or fuse blown. End of.

    No doubt, this would be an easy fix for someone who knows what they are looking at, so I'd like to ask a really sill question.
    If I were to post a photo of the PCB, would it be easy for the guys with extensive knowledge to take one look and simply say "it's probably that component"

    I could probably buy a new unit for £35, but a fix would be more satisfying, and probably more cost effective.
     
    keat63, Jun 8, 2011
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  2. keat63

    Resqueline

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    Just shoot & post. If your photographic skills are up to snuff I'm pretty sure we can help you.
    We might not be able to tell right away though, but may need you to measure some parts.
     
    Resqueline, Jun 8, 2011
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  3. keat63

    davenn Moderator

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    hi there
    weclome to the forums :)

    yes, post some pics top and bottom sides of the circuit board. Make sure thay are sharp and clear so many post such blurry pics its next to impossible to view them. size them to ~ 640 x 480 ease of viewing

    cheers
    Dave
     
    davenn, Jun 8, 2011
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  4. keat63

    davenn Moderator

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    ha Res you posted whilst I was still typing ;)

    D
     
    davenn, Jun 8, 2011
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  5. keat63

    keat63

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    Thanks Guys.
    I had a quick look yesterday, there is nothing obvious.. if it helps, I think my kid has had a pair of powered PC speakers connected to the headphone output, not sure if this could cause any damage.

    I'll upload some photo's this evening when I get home from work.
     
    keat63, Jun 8, 2011
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  6. keat63

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    It's unlikely that connecting PC speakers to the headphone output would have damaged anything
     
    (*steve*), Jun 8, 2011
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  7. keat63

    shrtrnd VIP Member

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    I don't know. My kid put the wrong speakers on his audio amp, and blew the speaker
    protection fuses. If the fuses weren't there, he probably would've taken-out the output
    amp transistors.
     
    shrtrnd, Jun 8, 2011
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  8. keat63

    keat63

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    Hopefully, you should see a series of images. I hope these help.
    If it helps, the unit in Question is a Legacy DD502 drum module.

    Incidentally, these pictures are sharp on my PC, but a little blurry on here !!!
    Maybe if you require a better closeups, I'll take some at a higher res.

    Anyway, Iv'e had another prod and a poke this evening, and actually, if I listen through the headphones, there does appear to be audio on the right hand channel, but it's about 10% of that from the left.
     

    Attached Files:

    Last edited: Jun 8, 2011
    keat63, Jun 8, 2011
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  9. keat63

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    So are you getting no output on the right channel through the speakers and a reduced level in the headphones?

    Do you hear the right channel through the headphones, or are you hearing crosstalk from the left channel? You can only tell the difference if there is different signal in the left and right channel -- if you can, set one instrument to be left channel only and another to be right channel only and see what you actually hear.
     
    (*steve*), Jun 8, 2011
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  10. keat63

    keat63

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    I'm not sure what you mean crosstalk. ?
    The unit doesn't have functionality to set any sort of mixing between channels.
    If I wear headphones, with the left side away from my head, I can definately hear audio from the right hand side, but like I say its about 10% of that from the left.

    There are 3 outputs:

    Right (no audo)
    Left/Mono (no audio on right)
    Headphones (low audio on right)


    The headphone output has some sort of amplification, as it's much louder than the output from Left/Mono.
    I hear nothing from right. (even through headphones)

    I guess it's tricky to tell with speakers, as it's not really possible for me to isolate the working one. (PC Speakers)

    I've noticed this evening that the volume pot, has 4 wires, so it may control each channel independantly?
    I've never experineced a pot to pack in without some sort of warning.
    And even when theyv'e gone, you can usually get something out of them with a wiggle.
    Might look at this tomorrow evening.
     
    keat63, Jun 8, 2011
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  11. keat63

    Resqueline

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    Crosstalk is when a signal on say the left channel crosses over into the right channel.
    If the left & right sources carry similar signals then you can't differentiate between crosstalk and volume differences.
    If you apply say 400Hz to the left channel and 1kHz to the right, and you hear 400Hz in both channels - only with low volume on the right channel; then that's crosstalk.
    If you hear 400Hz on the left and a low 1kHz on the right then the right channel has a different problem.
    Take a closer shot of the chip (U8?) near the jack connector. That's probably the audio output amplifier. Also write its part number for clarity.
    You might try to increase the resolution to 1024 x 768 - while trying not to exceed 100kB by too much.
     
    Resqueline, Jun 8, 2011
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  12. keat63

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Crosstalk is signal leaking from one channel into the other. For example if you have a stereo amplifier and you are only putting a signal into the left channel, you may hear it (at a much reduced level) in the right channel too. This is crosstalk and it's generally a bad thing.

    If your device is stereo (or has separate left and right channel amplifiers if mono) there is a chance that what you're hearing in the right channel of the headphone is crosstalk, not a real signal from the right channel. This doesn't affect anything if the device is actually mono, but in a fault condition it may mean that the fault is not in the output stage.

    If it's audible in the headphones and not at all with the speakers then you may have a separate headphone amplifier and the fault must (in all probability) therefore be before either of them.

    The board you've shown seems highly integrated, so it may be a challenge.
     
    (*steve*), Jun 9, 2011
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  13. keat63

    keat63

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    If I connect headphones to the headphone socket, the audio is much louder than that coming from the mono output, so based on this, I would suggest that there is indeed an amplifier for the headphone output.
    And if what I'm hearing is cross talk, then maybe the problem is not with output, but with the audio processing for the right channel ?

    I've taken close up shots of U8 (which is in the middle of the board) and U18, which is close to the output jack if it helps. However, they are quite large, so if its allowed, I've dumped them on my server.

    http://www.cabletiefan.co.uk/u8.jpg
    http://www.cabletiefan.co.uk/u18.jpg

    Chip numbers are as follows:
    U8= 74HC3740
    U18= TDA7308 (i think its a 7)
     
    keat63, Jun 9, 2011
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  14. keat63

    keat63

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    Guys

    I think I may have been a little hasty with my diagnosis.

    If I connect headphones to left output... I can hear audio through the left headphone.
    If I connect headphones to right output..I can hear audio through the left headphone.

    Maybe not as strange as it sounds considering I'm using 6mm to 3mm Jack converters (tried 3 different ones), I have stereo headphones, but could be mono outputs.

    The outputs are not as loud as the headphone output, i guess they never have been as they are meant to feed an amp.

    So maybe, its the headphone amp which is lazy ?
    Could anyone identify what this may be.. if it's U8, then these only cost about 80pence
     
    keat63, Jun 9, 2011
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  15. keat63

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    In the first 2 cases (headphones connected to the outputs) does one sound louder than the other?

    Do you normally use the (amplifier) outputs or the headphone jack? (i.e. is this problem exhibiting itself because you are using the headphone jack, or was trying the headphone jack something you did to try to diagnose the issue)

    edit, you may be correct in suspecting the headphone amplifier
     
    (*steve*), Jun 10, 2011
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  16. keat63

    keat63

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    I always noticed that the amp outputs were a lower volume, but can't say I noticed that they were mono. However, this makes sense really. They are both at the same level.

    Personally I use the headphone outputs, and up until last week, both channels were working, and equally balanced.
     
    keat63, Jun 10, 2011
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  17. keat63

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    OK, what I'd do next is look at the input levels to the headphone amplifier (it seems you located the chip).

    If these are OK (i.e. they are the same level) then the problem realy does look like the headphone amplifier.

    In that case, I still stand by my original comment that plugging in PC speakers to this *shouldn't* have damaged it, but maybe they have. My argument as based on the fact that headphone amplifiers are typically very low power amplifiers and are fairly tolerant of abuse of their outputs. But maybe not so in your case :(
     
    (*steve*), Jun 11, 2011
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  18. keat63

    keat63

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    Sorry. I'm now lost.
    My electronics knowledge ends at soldering and identifying the very basic of components.
    Could anyone confirm that U8 is indeed the headphone amp ?
    I'm not sure how i would check the input levels.

    But I could possibly change the chip.

    Thanks
     
    keat63, Jun 11, 2011
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  19. keat63

    poor mystic VIP Member

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    Hi keat63
    I found the following link by a google search: http://www.okdatasheet.com/datasheets/ST-MICROELECTRONICS/TDA7308.html. this link does describe the TDA7308 as an amplifier.

    If there were any doubt, I have put the attached, zipped, mp3 file together (using audacity and lame). The left channel just says left, left... and the right channel also identifies as right, right... So if you hear both "left" and "right" as words in your phones both channels are working, but if you only hear "left" then the right channel is dead.
     

    Attached Files:

    Last edited: Jun 11, 2011
    poor mystic, Jun 11, 2011
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  20. keat63

    Resqueline

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    You have identified U8 as being a high-speed CMOS digital chip. Remember the ? behind U8, and the parentheses? The pic was too unclear to do anything but guess.
    U18 clearly is an audio chip.

    The board should accept zip files, but only up to around 100kB in size.
     
    Resqueline, Jun 11, 2011
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