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YAMAHA AST-C10 no audio

Discussion in 'Troubleshooting and Repair' started by HellasTechn, Jun 26, 2017.

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  1. HellasTechn

    HellasTechn

    1,523
    212
    Apr 14, 2013
    Hello everyone and have a nice day.

    This time i am working on a Hamaha ast-c10. It will power on but the output relay will not click and no audio is heared from the output.
    I have found a few bad capacitors around the power amplifier chip and replaced them but still not working. I can not scope the output section to see what is going in because all cables are short and tight so i can not plug it in while the pcb is out.

    I have the service manual but cant figure it out. Any help appreciated.

    THANK YOU.

    P.S.
    here is a link to where i downloaded the service manual from:

    http://www.owner-manuals.com/ASTC10-service-manual-YAMAHA.html
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,908
    2,100
    Nov 17, 2011
    Kalimera Constantine,
    did you check the output voltages from the power supply?
    Any fuses that need to be replaced?
    Does the headphone output work? This would indicate that at least the amplifier section up to this pint is o.k.
    Just asking...

    The output relay is obviously controlled by a supervisory circuit which monitors the output for a DC signal (service manual p. 21). Can you measure the output of the amplifier at the input of the relay to check whether there is a DC signal present? If there is a DC signal, the defect is in one of he previous stages. If there's no DC, then the DC detection circuit max be defective or the relay driver is out of order.
     
  3. HellasTechn

    HellasTechn

    1,523
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    Apr 14, 2013
    Guten Morger Sir !

    Not yet :)

    True, i see the transistor controlling the relay and an other transistor that drives the relay transistor. they are both good. Now about the chip that actually controls the transistors, i am not sure. i will read p.21.

    I can not because all cables are too tight and the board has to be mounted in the chasis so i have no room for a probe or anything... In other words i can not have it horking with the board removed...
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

    9,908
    2,100
    Nov 17, 2011
    Remove the board, solder sme test leads to teh necessary points and reassemble the unit. Lots of work but this way you can probe the board while built-in.
     
  5. HellasTechn

    HellasTechn

    1,523
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    Apr 14, 2013
    Good idea thank you !

    See that is why when many brains work on the same project it's better than one brain !
     
  6. Harald Kapp

    Harald Kapp Moderator Moderator

    9,908
    2,100
    Nov 17, 2011
  7. HellasTechn

    HellasTechn

    1,523
    212
    Apr 14, 2013
    I had no luck on this one finding the fault...
     
  8. Harald Kapp

    Harald Kapp Moderator Moderator

    9,908
    2,100
    Nov 17, 2011
    When you bridge the output relay (short circuit the contacts, do you hear audio from the output? Can you measure with an oscilloscope whether ther is DC present (it should not)?
    Just to verifiy that it is the relay control circuit that fails. Otherwise the relay control (protection) circuit may work just fine and the fault lies within the amplifier outputting DC - which it should not.
     
    HellasTechn likes this.
  9. HellasTechn

    HellasTechn

    1,523
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    Apr 14, 2013
    The problem lies right there !

    Thako you so much Harald.
     
  10. 73's de Edd

    73's de Edd

    2,848
    1,209
    Aug 21, 2015
    Sir HellasTechn . . . . .aka Constantine . . . .

    Well here I am now, somewhat delayed by finding that all 3 of the available freebie schematics of that unit were all messed up in the bottom right quadrant, but I pieced together the best ones.

    Also, I see that the two major operative clues have ALREADY been given by “Der Kappster”.

    My routing of an “extender” test point wire, from CRAMPED / or / OTHERWISE INACCESIBLE areas is either done by using some of my OLDE TYME . . .75 year olde telephone solid 22 gauge Cu wire , with its four, GREEN, BLACK, RED and YELLOW wires .
    Or the lighter, solid 24 gauge Cu wire, being found in my 1000 ft reel of CAT-5 wire.
    That gets solder tacked on at the point to be tested . . . . and labeled at the other end, if several tests are involved at the same time.

    Check BOTH potted amp output pins . .11 and 13 . . . ,as one would certainly expect only one amp section having gone bad and bleeding DC out onto the AF power output line .

    Referenced points of interest are:

    Initial presence of the power supply’s approximately balanced + 34 and –34 supplies in PINK.

    Your dastardly relay . . . which I think is being a " blurred " RLY601 . . . with its disrupting contacts placed on both the R and L channel outputs . . . it's markup . . .being boxed in YELLOW.

    It's coil is being schema far left, bottom left quadrant, in its YELLOW box.

    The BLUE transistors are its drivers.

    Going back to the output stages, the ORANGE-YELLOW-RED transistors are being sensing / sampling transistors that are monitoring for :

    • Excess static DC voltage presence on the output line of either power amp.

    • Or excess current consumption of a channel

    • Or excess peak AC audio excursion on an output line.
    (Like you had no speakers connected or a speaker wire came loose or was broken . . . then you run the volume up with no output loading then being present. )

    Any / All of those fault conditions will tell RLY601 to open the connection to the speaker lines.

    Last markup is the GREEN, pins 11 and 13 of the STK4142 II potted amp modules.

    CAVEAT . . . . . STK4142 II . . . . .check all pins . . .VEWY-VEWY CAWEFULLY . . . .a la Elmer Fudd . . . to see if a skimpy flow soldering / wetting job, back at board manufacturing time, might now be showing up as a FLOATING pin connection .

    Le Schematique . . . .”Unitized” . . . .

    [​IMG]


    73’s de Edd

    .
     
    Last edited: Jul 9, 2017
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