Technics SU-VX800 amplifier

Discussion in 'Technical Repair' started by Marius83, Dec 28, 2017.

  1. MrChips

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    Oct 2, 2009
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    @Marius83
    I am inclined to go with RC4558P or NE5532AP.

    While you're at it, you should buy some 8-pin DIP sockets and some 8.2V 500mW zener diodes, 1N756 or 1N5237.
     
  2. Marius83

    Thread Starter Member

    Dec 28, 2017
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    Thanks! :)
    I will get the supplies!
     
  3. MrChips

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    Let's keep our fingers crossed that this is the last stop for supplies. But you can never tell. I am well aware that we still have many components to go back in and you also had an issue with smoking from the front panel.
     
  4. Marius83

    Thread Starter Member

    Dec 28, 2017
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    Hehe, yes you are correct about the front panel. Time will show :)

    I have now ordered supplies.
    We'll just have to wait again :)
     
  5. MrChips

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    I wonder how many folks are sitting on the fence watching this show. It would be interesting to know. This is as exciting as watching paint dry. Or waiting to see me hang myself.:)

    Maybe we should ask everyone following this saga to sign in just for the record.

    Where is @R!f@@ when he's wanted? Hope he's not behind bars.:eek:
     
    hrs and DbLoud120 like this.
  6. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    3850 views ... so, you can draw some conclusion. I've viewed it probably 3x the number of postings at a minimum.
     
  7. MrChips

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    First time drive-by visits don't count. I want to know who is anxiously waiting for the next episode to be aired, while following the cookie crumb trail with a printed copy of the circuit schematics in front of them, like trying on unravel one of Murdoch Mysteries.:D
     
  8. JoeJester

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    @MrChips, I'm following your adventure. Only difference is, I have that section simulated in TINA and about to put the next section into simulation.

    I'll have to go back and look at which component is smoking in the front panel ... I did come to the game late with only cursory views before page 3 turned into page 4.

    However, here is a "stitched" portion of the two pages so we don't have to work too hard on reading the circuits that may have been damaged by the C-E short .... Sometimes I just hate the way the diagrams are working these days.
     
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    Last edited: Feb 14, 2018
  9. MrChips

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    @JoeJester, I just cut and tape the sheets together and then I can mark up the paper as we go along. I am simulating on CircuitMaker 2000 as we proceed.

    Here is a summary of where we're at.

    1) C425 filter capacitor on the +60V line keeps popping. We've left it out for now until we get all the voltages correct and stabilized.

    2) LEFT CHANNEL was blown accidentally. I think we have that stabilized. Q507-Q509 complementary output pair are in place. Q515-Q517 pair have been removed for now. DC balance not yet adjusted. Will do that later.

    3) LEFT & RIGHT CHANNEL outputs from preamp have been disconnected. Inputs to power amp are currently grounded.

    4) On RIGHT CHANNEL, Q406, D412, Q416 identified as blown. Waiting for more supplies to arrive. The plan is to replace Q406, Q408, D410, D412, Q416, IC402.

    5) Still to come, replace Q420, Q424, Q426, Q428, Q430.

    6) Still to come, check DC levels from preamp.

    7) Still to come, check possible fault on front panel.
     
  10. Marius83

    Thread Starter Member

    Dec 28, 2017
    141
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    Finally my supplies arrived.
    1N756A DIODE ZENER 8.2V 500MW DO35


    What is our next step MrChips? :)


    [​IMG]
     
    Last edited: Mar 8, 2018
  11. MrChips

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    Nice.

    IC402 got subjected to 64V. We cannot be too sure about its status.
    I would go ahead and remove IC402. Note the orientation of that chip before removing.
    The simplest way to remove an IC in a DIP package is to clip all the leads with a small side cutter rather than trying to save it. Remove each pin one at a time.
    Clean the PCB holes with solder wick or a solder sucker. Install an 8-pin DIP socket, oriented the right way as per original IC. Install replacement IC, RC4558P.

    When new ICs in DIP packages are received, you will notice that the pins are not pointing straight down but are spread apart. You want to bend the pins so that they are at right angles to the body of the chip. I do this by pressing the one side of the legs against the top of the workbench. Turn the chip around and do the other side.

    Remove zener diodes D410 and D412. Note the orientation of anode & cathode (black band on body is cathode).
    Install replacement zener diodes, 1N756A.

    Install Q406, 2SC2631
    Install Q408, 2SA1123
    Install Q416, 2SA1123

    Power up. Check for smoke.:eek:

    Measure voltages at
    R428
    R430
     
  12. Marius83

    Thread Starter Member

    Dec 28, 2017
    141
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    I have a solder wick, it works very good, removing the IC with all legs were no problem. :)
    I noticed the orientation, and it were marked on the pcb how to place the socket.
    Same with the diodes, it's markings on the pcb showing which way to put it.

    SO, i put in a socket and replaced IC402, then i replaced D412 and D410
    At last i installed Q406, Q408 and Q416(new one).

    Power ON. No smoke ! :)

    Measured voltages:

    R428: +9,8 / +11,8 Volts
    R430: +7,8 / +5 Volts


    [​IMG]


    [​IMG]


    Overview of the amp as it is right now, i realised it was an unclear picture afterwards..
    [​IMG]
     
  13. MrChips

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    Oct 2, 2009
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    Ok. Thanks for that.

    I see C410, Q412, and Q416 are not installed.
    Install these and repeat the voltage measurements on
    R428
    R430

    I need a chance to see where we're at and I will get back to you. I will be away for some hours and may not respond until the next day.
     
  14. Marius83

    Thread Starter Member

    Dec 28, 2017
    141
    3
    The two pictures are taken at different points, if you look at the first one, Q416 is installed like i wrote.

    But you are correct about C410 and Q412, i can't remember you've said i should install them, but i may be wrong :)
    I will install them and see what happens :)

    No problem about beeing away, plenty of time
     
    Last edited: Mar 9, 2018
  15. Marius83

    Thread Starter Member

    Dec 28, 2017
    141
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    So i installed Q412 and C410

    Power on!
    No smoke!

    Measurings:

    R428: + 2.6 / + 7.1 Volts
    R430: - 0.3 / - 8.2 Volts


    [​IMG]
     
  16. MrChips

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    Oct 2, 2009
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    A bit of an imbalance there.
    Let us install the transistors from Q418 - Q430 and see if the voltages balance a little better.

    Q418 - in-place?
    Q420, 2SC3311
    Q422 - in-place?
    Q424, 2SC2631
    Q426, 2SA1123
    Q428, 2SC3944
    Q430, 2SA1535

    Make sure you toss out the transistors that were tested as bad.

    Measure voltages at:

    R428
    R430
    R464
    TP502
    R514
    R570
    R572
     
  17. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    I'd test them before installing, to ensure a bad one doesn't slip into the mix.
     
  18. MrChips

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    If you have not done it yet, hold off on installing Q428 and Q430.
    Let's try and get those two zeners D410 and D412 balanced.

    Measure voltages at
    D410
    D412
     
  19. Marius83

    Thread Starter Member

    Dec 28, 2017
    141
    3
    I have just done it...:)

    Q418 and Q422 has been installed the whole time.
    Q420 has also been installed the whole time but this i visually damaged.

    So i have:

    Replaced Q420 with a new one.
    Installed Q424, Q426, Q428 and Q430 only (Q430) was tested ok earlier, the three others are new.
    I have marked the bags with the bad transistors, ensuring no bad one's getting in the mix.

    Power On, no smoke!

    Measurements:

    R428: - 2.5 / + 7.2 Volts
    R430: - 4.8 / - 7.9 Volts
    R464: - 2.6 / - 2.4 Volts
    TP502: - 3.7 / + 2.5 Volts (at the + pin)
    R514: - 3.2 / - 4.6 Volts
    R570: 0 / - 4 Volts
    R572: - 4.2 / - 3.9 Volts
     
  20. Marius83

    Thread Starter Member

    Dec 28, 2017
    141
    3
    Measured voltages at the zener diodes:

    D410: - 0.7 / + 7.3 Volts
    D412: - 8.8 / - 0.5 Volts
     
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