Question about Yamaha DSP-AX1 10-Channel Amplifier

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Unbalanced LEDs> amplification problem, or mere graphic drop out?

My newly acquired JVC AX-1 Amplifier does not show as much power level indication on the left LED. cannot really hear a difference when panning between channels, in stereo nor in mono modes. but i wonder if certain frequencies are not being amped as much on the left channel, or if somewhere in the circuit something is not as it should be. is it possible that the left LED is merely "lazy" and not as sensitive as the right LED or is what arrives definitely what is being shown on the level indicator? in which case there is not as much power arriving to the left channel?

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  • seehandler Apr 12, 2009

    i plugged in an external tape deck (actually a 4 trk) and measured the outgoing signal with it and it checked out as balanced. (i switched the left and right input signals from the cd player into the amp to check as well). after trying to sum the L and R channels by switching to MONO on the amp and not noticing any difference, i realized that the input from cd to tape player (through REC out) might not even be going through the amp. in any case i did not feel as confident with the result as i did before trying the mono signal. but at this point i am also not as confident in the things i thought i might not be hearing in the left channel. good to hear that it is possible the metering circuit is a possibility and that it does not necessarily mean the actual power level on the left channel is lacking.

  • seehandler Apr 13, 2009

    i downloaded the repair manual (which shows the circuits) from this website, but am not fit enough to be sure of what i am looking at. checking the meter reading externally does not do any good if when monitoring through the tape out (which is the way i did it) the amp is bypassed, which is my suspicion since the MONO function on the amp had no effect on the readings on the external UV meters, which it should. this is similiar to the fact that tone adjustments on the amp do not usually alter recordings being made onto tape (on consumer products). realizing this, i thought that getting a reading externally through the line out is perhaps not the best way. cannot think of another way at the moment, save measuring individual components on the circuit board which is beyond me. also, my interest is to find out if the meters or the amp itself are broken. i do not want to use it constantly if it is, which would be the equavalent of walking around with a patch punched with holes over one eye. the balance could obviously help adjust the left channel, but would alter the stereo image produced on some reordings.

  • seehandler Apr 13, 2009

    no sorry, i downloaded the manual from here:http://diplofix.com/thread-0/114470

    sorry!


  • seehandler Apr 13, 2009

    better yet herehttp://diplofix.com/forum-JVC/A-X1

  • seehandler Apr 13, 2009

    the 2nd link above is for the service information, which is all i have. it shows diagrams of the different circuits. thanks for your input up to now..............

  • seehandler Apr 13, 2009

    ok still need to get to this, but
    A)R903 and R904 are listed as having 62 K, if i read from the list correctly. they are there and look the same, more i cannot say. four identical stripes on each of em,
    B)C904 does not look leaky, replacing it will be a task for me
    C)cannot find the TA7318 on the board, diagram, or in the list of parts. where is it located, approx?
    do not have an oscilloscope. might know someone that does, will take some time.
    D) the first thing i did notice when i opened the case was that around the C621 C622 area on the board there is a darker area of brown, which looks to be caused from heat. thought something on the other side of the board may have overheated, but checked and there is nothing down there! it looks fine from the other side as well. can the board discolor from regular wear after years of use? the amp does get warm after all. nothing in that area looks to be damaged and a quick check of some parts with a volt meter showed those parts checked to be ok.

    thanks again!


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This could be a defect in the metering circuit. In order to verify this, you should measure the output externally. Get that external measurement and post the results here. We'll continue from there.

Dan

Posted on Apr 12, 2009

  • 2 more comments 
  • ABRsvcs Apr 12, 2009

    Unfortunately the metering circuits are different for different manufacturers and models. Some take the reading from the pre-amp section others from the output section. In any case, these are run through resistors to drop the signal level and compared and divided to get the meter readings internally inside ICs. Unless you can actually hear a difference, use the centering indent on the balance control to center the unit and leave it alone.



    Dan

  • ABRsvcs Apr 13, 2009

    If you have measured the output from the tape-out connections and the balance is correct, you have eliminated part of the front end. The tape out is independent of the volume control and tone circuits.



    The link you provided will give access to the owner's manual not the service information. If you have a copy of the service information, please provide a link and I'll take a look and indicate where to test the balance.



    Dan

  • ABRsvcs Apr 13, 2009

    Ok there are a number of things to check:



    1) Compare R904 to R903. They should both be 27K resistors.

    2) Check C904 or just replace it. If it leaky, it could be the problem. This is a 1ufd 50V cap.



    3) If you have an oscilloscope, look at the signal levels when the unit is in Mono mode at pins 4 and 6 of the TA7318. These are the input pins and should read identical at this point.



    4) Check pins 2 and 8 of the TA7318. If these are different levels while pins 4 and 6 are the same, the TA7318 is defective.



    5) If #4 checks out OK, then the meter itself is the problem and you can try adjusting R908 to compensate.





    Keep me posted.



    Dan

  • ABRsvcs Apr 13, 2009

    Locate page 5 of the manual. Look up and slightly to the right for the TA7318 IC which is an in-line IC with 9 pins. You will probably see R908 first on the board as a round part with 3 pins. Check the items listed above from there.



    Dan

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SOURCE: unbalanced LEDs> amplification problem, or mere

This could be a defect in the metering circuit. In order to verify this, you should measure the output externally.

This is a repeated problem, please see initial proble for the complete solution.
Dan

Posted on Apr 12, 2009

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