Technics SU-VX800 amplifier

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

  1. Marius83

    Thread Starter Member

    Dec 28, 2017
    141
    3
    @mjaa
    Thanks! :) I couldn't have done this if it wasn't for Mr.Chips.
    But i think it's really interesting.
     
    Last edited: Feb 9, 2018
  2. Marius83

    Thread Starter Member

    Dec 28, 2017
    141
    3
    Finally my transistors arrived.

    I got only one of Q420, C3311..
    But ten pcs. of C2631 and eight of A1123

    What is our next move, Mr.Chips? :)


    [​IMG]
     
  3. MrChips

    Moderator

    Oct 2, 2009
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    Ok.

    Install new Q406 - 2SC2631
    Install Q408

    Power ON and check for smoke, especially at R414.

    Measure voltage at
    R428
    R430
     
  4. Marius83

    Thread Starter Member

    Dec 28, 2017
    141
    3
    So i installed Q408, and a new one 2SC2631 at Q406.

    Power on.

    R414 smoked instantly.

    Measured voltage:

    R428: - 56,8 / -54 Volt
    R430: - 59 / -64 Volt
     
  5. MrChips

    Moderator

    Oct 2, 2009
    16,282
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    That's a bummer!:(

    Remove Q408 and R414.
    Install new R414, 100Ω.
    Leave out Q408.

    Power ON and measure voltage at R414.

    Edit: That was an error. I meant Q406. At this point, it doesn't matter any more.
     
  6. Marius83

    Thread Starter Member

    Dec 28, 2017
    141
    3
    Removed Q408, replaced R414 100Ω.

    Power on, no smoke.

    Measured voltage at R414: -64 / -64 Volt
     
  7. MrChips

    Moderator

    Oct 2, 2009
    16,282
    5,004
    Thanks.

    There is a short or bad component bringing -64V to R414. We've got some digging to do in order to get to the bottom of this.

    The first thing would be a close, careful visual inspection of both sides of the board, top and bottom around the following components looking for shorts or bad solder bridges:

    R414
    R416
    C410
    D412
    C414
    Q412
    Q416

    Next step would be to set you DMM to the lowest resistance range.
    With the power OFF and disconnected, see if you can measure resistance across
    C410
    D412
    These two are in parallel. Hence you will get the same reading.
    Measure resistance one way and again with the leads reversed.

    Later, we may have to pull some components and measure.

    (I will look at the photos already posted to show you the area we need to investigate.)
     
  8. MrChips

    Moderator

    Oct 2, 2009
    16,282
    5,004
    Can we zoom in on this section of the solder side of the board?

    Technics SU-VX800 solder side right channel.jpg
     
  9. Marius83

    Thread Starter Member

    Dec 28, 2017
    141
    3
    I understand, i have now inspected the board on the underside and overside carefully at these components you have mentioned, i cannot find any shorts or bad solder bridges.
    I took a picture of the area you wanted to look closer on, it is not a very good picture as i didn't have the best light available.
    Notice the PCB beginning to get a little worn at R414.. it doesn't take the heat very well always. I had to scrape off a little of the green "paint" to get a new point to solder R414, but that shouldn't be a problem.
    Just to mention why it looks like it does.:)

    I measured D412 and C410, and they have the same values as you mentioned.

    So i set my DMM to lowest resistance range:

    D412: With red nearest the heat sink: 80Ω
    With black nearest the heat sink: 79,5Ω

    C410: same as D412


    [​IMG]
     
  10. MrChips

    Moderator

    Oct 2, 2009
    16,282
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    Please remove C410 and measure its resistance out of circuit.

    Also, remove Q406 and test both Q406 and Q408.

    After doing that, set the meter to diode test.
    Test the diodes both ways, forward and reverse, while still in circuit, power OFF:

    D409
    D411
    D410
    D412

    For your convenience,
    J119 is the connection where D409 and D411 are joined.
    J124 is the connection where D410 and D412 are joined.
     
  11. Marius83

    Thread Starter Member

    Dec 28, 2017
    141
    3
    Ok, so i've done what you've said.:)

    I removed C410; and i get open circuit on any resistance measuring on this capacitor.

    So i measured the transistors again, out of circuit of course... has my new Q406 got killed? :rolleyes:
    Q406:
    RED probe to B.
    BLACK probe connected to E: 753 mV
    BLACK probe connected to C. 764 mV
    Reverse the measurements as follows:
    BLACK probe to B.
    RED probe connected to E. 1053 mV
    RED probe connected to C. Open circuit
    Connect RED probe to E.

    Record DMM reading with BLACK probe connected to C. 1015 mV
    Reverse the procedure as follows:
    Connect BLACK probe to E.
    Record DMM reading with RED probe connected to C. Open circuit

    Q408:
    Connect RED probe to B.
    Record DMM reading with BLACK probe connected to E. Open circuit.
    Record DMM reading with BLACK probe connected to C. Open circuit.
    Reverse the measurements as follows:
    Connect BLACK probe to B.
    Record DMM reading with RED probe connected to E. 884 mV
    Record DMM reading with RED probe connected to C. 852 mV
    Connect RED probe to E.

    Record DMM reading with BLACK probe connected to C. Open circuit.
    Reverse the procedure as follows:
    Connect BLACK probe to E.
    Record DMM reading with RED probe connected to C. Open Circuit.

    So the diode measurements:

    D409: 953 one way and open the other
    D411: 955 one way and open the other
    D410: 947 one way and open the other
    D412: This measures 084 both ways !o_O
     
  12. MrChips

    Moderator

    Oct 2, 2009
    16,282
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    Yes, Q406 gets killed along with R414 when this happens.

    See if you can test Q412 and Q416 while they are still in-circuit.
    If we get uncertain results we might have to pull them and measure out of circuit.

    Looks like D412 is bad. We will have to replace both D410 and D412 at the same time.
    Part # MA4082MTA 8.2V zener diode

    Substitutions are:
    NTE5016A 8.2V 500mW
    1N5237
     
  13. Marius83

    Thread Starter Member

    Dec 28, 2017
    141
    3
    I stopped by the local service shop and asked if he had the right type of diode.

    He got these Motorola 8,2 V zener diode, but they are a bit bigger then the ones in the amp.
    They are marked with C8 and V2, he didn't know the exact wattage but he said sure i could use them...
    I have googled the numbers and i believe they "are" 1300 mW ...

    What do you think MrChips? Could we use these?


    [​IMG]
     
  14. MrChips

    Moderator

    Oct 2, 2009
    16,282
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    Ok. Since you have them on hand we can try it and see if it will work. But before we do anything, please check Q412.
     
  15. Marius83

    Thread Starter Member

    Dec 28, 2017
    141
    3
    I removed both Q412 and Q416 and measured out of circuit.


    Q412:
    Red to B:
    Black to E: 842 mV
    Black to C: 824 mV
    Other measurments are open circuit. So this is ok.

    Q416 has short in every measurement.
     
  16. MrChips

    Moderator

    Oct 2, 2009
    16,282
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    Leave out Q406 and Q408 for now.
    Install Q412.
    Install new Q416.
    Instead of installing the D412 replacement, just solder-tack it on the solder-side of the the PCB. Watch the direction of the diode very carefully. The cathode (black band) of D412 goes towards J124. (The black band of D410 goes away from J124. We are not going to replace D410 at this stage.)
    Install C410.

    Power ON and hope for the best.
    Switch the DMM back to voltage measurement.
    Measure voltages at
    R414
    R416
    D410
    D412 (on back of PCB)
    R428
    R430
     
  17. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    1,703
    MrChips,

    May I recommend the measurement of the Q416 emitter to ground as well as Q414. The shorting C-E of Q416 is the primary cause of R414 dissipating more than 0.25 W. A Q414 C-E short will test R416's ability to dissipate more than 0.25W. If the readings are similar, I would eliminate the loading as a cause of the C-E short.
     
    Last edited: Feb 14, 2018
  18. MrChips

    Moderator

    Oct 2, 2009
    16,282
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    Thanks for the tip. As far as I can tell, Q406, D412, and Q416 all shorting would cause R414 to blow. I cannot tell what was the cause and what was the after-effect.

    I'm also concerned that it took out dual opamp IC402 (M5218AP) in the process. We have to contemplate having to replace IC402 before going any farther.
     
  19. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    I agree. Individually Q406 C-E short or Q416 C-E short can cause R414 to smoke. If only D412 were shorted, R414 and R416 both would smoke.

    I'll get back to reviewing the preceding and post circuitry and get back later.
     
  20. MrChips

    Moderator

    Oct 2, 2009
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    @JoeJester - I'm afraid that when Q416 shorted, it put -60V on IC402 (M5218AP). I think we should go ahead and replace it anyway.

    Take a look at the M5218 Datasheet.
    Possible replacements are:

    RC4558
    RC5532
    LM4562
    MC1458
    MC1558

    Any suggestions?
     
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