Technics SU-VX800 amplifier

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

  1. MrChips

    Moderator

    Oct 2, 2009
    16,303
    5,011
    Measure voltages at:

    Left Channel
    R417, R419, R421, R423, D409, D411

    Right Channel
    R418, R420, R422, R424, D410, D412.
     
  2. Marius83

    Thread Starter Member

    Dec 28, 2017
    146
    3
    @BR-549 Thanks for the tip, i like electronics, but i'm just an amateur asking for help here, i don't know if i got the skills to use a scope :) but maybe..

    Thank you for explaning things thoroughly Mr.Chips :)

    My readings at these components are a bit diffuse to me.. on all the resistors it variys a lot, they seems to hold my reading voltage, then drop to zero, and up again, again and again.
    The readings are taken on the leg where the resistor is, not the side were the leg goes over the resistor and down to PCB, if you know what i mean.
    Also my readings on the D's, the voltage can suddenly drop to 0, then come back again.
    Readings are the same on both legs on the D's.

    R417: -63,7 Volt
    R419: -65,1 Volt
    R421: -27 Volt max ... variy up and down
    R423: -25 Volt max .. variy up and down
    D409: -63 Volt
    D411: -64,1 Volt

    R418: -64,4 Volt
    R420: -65,5 Volt
    R422: -64,4 Volt
    R424: -65,5 Volt
    D410: -63,5 Volt
    D412: -65,6 Volt
     
  3. MrChips

    Moderator

    Oct 2, 2009
    16,303
    5,011
    You should immediately recognize that we have a problem on the positive supply rail.
    I am going to assume that the negative values you listed are correct and not an error on your part in making the measurements.

    I have listed the expected voltages rounded to ignore the decimal fraction and to avoid confusion. I am aware that in Europe the comma is used to indicate the decimal place. At this point, we are not too concerned with accuracy in the actual numbers, just as long as we are close enough.
    Your latest measurements are different from what you posted in post #17.
    Hence, something has changed since then. Possibly R505 is damaged and reads as open circuit.
    With the power OFF and unplugged, measure the resistance across R505. If you find that it is open circuit, we can temporarily jumper across this resistor R505 until replacements arrive. R505 is 6.8Ω ¼W and is not critical for this stage of tests.

    (The next time you have to order more components it is probably a wise idea to make a list of all the possible components that will likely or definitely need replacing.)

    As for the varying voltages, this is where testing with an oscilloscope becomes superior, even indispensable.
    Whether you get an oscilloscope or not will be up to you, your future endeavors in electronics, and your budget. In the current market, you can get a good new digital oscilloscope in the US$300 - $400 range. You can also get a suitable used analog scope in the US$50-$100 range.

    I would personally recommend the used option as a starting experience for your situation. Since this repair example is in audio equipment, you don't need the high frequency bandwidth and even 1MHz would do. Most inexpensive used analog scopes would have a max bandwidth up to 20MHz - 60MHz range which is perfectly adequate for analog test and repair.
     
  4. Marius83

    Thread Starter Member

    Dec 28, 2017
    146
    3
    Okey, my measurements are way out..:confused:
    Well, i have used an oscilloscope to check sensors on the motor in cars, so i understand it's needed when the voltage variys..
    Maybe i will get one, i will have to check for one used as you said!
    And i also agree and buying lots of components next time when we find out what i need/would need.

    For starters, you are right, R505 is open circuit.. to check my tester, i tested R506 which is right below and it read 7Ω.
    So R505 is busted.

    According to my readings yesterday, i had to check again!:oops:
    And i got 0 volt on every point you told me to measure, i got frustrated, but then i found the reason, explained below.

    Signal PCB which stands vertically down in the main PCB was loose at the rear end, (where you plug in the audio cables).
    When i measured yesterday, i put a finger on that board to stabilize my point probe to measure the points, and when i did that, i push the outer
    audio connector in contact with GND at the back cover.
    Then i get the negative measurement on every point, if i don't touch it, i measure 0 volts on every point.
    So i put in the screws to fasten it, and the screw at the GND "phono" point.

    I thought i was doing something wrong with pushing the board with my finger, but after putting in the screws, my measurements are the same as yesterday, but they are now stable, and R421 and R423 are -60 Volt's as the rest.

    So my readings are correct..

    Some pictures of my test probe, i have a cheap DDM, but i hope it'll do the job.
    Also picture of a measure taken at a resistor so you can see if i'm doing it right..?


    [​IMG]

    [​IMG]
     
  5. MrChips

    Moderator

    Oct 2, 2009
    16,303
    5,011
    I forgot to mention that when you jumper or replace R505 don't be surprised to see components going up in smoke again.

    PS. no need to edit your most recent post. Just let me know what is your next step.
    Oops, we were cross posting and you beat me to it.
     
  6. Plamen

    Member

    Mar 29, 2015
    63
    5

    Petkan:
    As a precaution to unnecessary damage during testing, you can power the amp through a filament bulb (in series).
    Take a mains cord, a bulb socket and mains outlet. Connect the Neutral through (join all 3). Connect the Live from the plug to the central pin of the bulb socket. Connect the other bulb socket pin to the mains outlet live terminal.
    What you have now is an universal current limiter. Keep it for future needs. In normal use the bulb should light briefly on account of "inrush current and then stay dark as the amp is not supposed to consume much in steady state when idling. It the worst case the current will be limited to less than 0.5A (100W/230V). You can add a switch bypassing the bulb to be able to eliminate the bulb when necessary.

     
    R.E. likes this.
  7. Marius83

    Thread Starter Member

    Dec 28, 2017
    146
    3

    Hehe, i just thought i would explain, at first i thought the readinga were faulty because of me, but they were not.

    Well, my next step would be what you think i should do.. :)
    I have got hold of two 6,8 ohm 0,25w resistors, so i can try to replace R505?

    What about the "current limiter" @Plamen mentions?
    If i use a such and nothing smokes.. i have to measure every component..?
     
  8. MrChips

    Moderator

    Oct 2, 2009
    16,303
    5,011
    The light bulb in series would work in some cases but not all. The light bulb would effectively reduce the supply voltage and would continue reducing the supply voltage as the current draw increases.

    Something to be aware of, when a resistor or any component blows, it is not necessarily that component at fault. When a resistor overheats, it is because another component such as a transistor that is allowing too much current. Hence we have to look at the bias voltage at the transistor and also the condition of that transistor.

    My analysis tells me that you have one or more bad transistors in both channels. As I said, you can go and replace all the transistors and hope that that covers the bad one.

    The alternative is to remove only the transistors that are likely to cause the overcurrent and then test the voltages in the circuit. Then we test the transistors that have been removed.

    Let us compile a list of blown components so far:

    R414 (repeatedly)
    C425 (repeatedly)
    Q420

    R509
    Q507
    R448 (flamed)
    R505

    Components replaced:
    Q510
    Q508
    Q518
    Q516

    R510
    R514
    R448
    R414

    C425 (repeatedly)

    Q651
    Q653
    Q652
    Q654

    R509 (twice)
    R513 (repeatedly)

    Components now removed:
    Q507
    Q509

    R509
    Q501

    Q505
    Q515
    Q517

    C425

    You will notice a pattern in the component designations.
    Numbers ending in an ODD digit is in the LEFT channel.
    Numbers ending in a EVEN digit is in the RIGHT channel.

    Numbers starting with 4 is in the driver section.
    Numbers starting with 5 is in the power section.

    I point this out because we have four separate sections that will potentially cause problems and we want to stop blowing more components in each section.

    Edit: Added C425 to the removed list.
     
  9. MrChips

    Moderator

    Oct 2, 2009
    16,303
    5,011
    To prevent blowing R414, remove Q406 and Q408. Replace R414.

    To prevent blowing R448, remove Q424 and Q426. Replace R448.

    Replace R505.

    Apply power and keep fingers crossed.

    PS. Never leave the unit unattended with power ON.
     
  10. Marius83

    Thread Starter Member

    Dec 28, 2017
    146
    3
    Thank you for further analysis Mr.Chips, your list over components replaced,blown and removed are correct.

    I have now done what you wrote.

    Removed Q406, Q408 and replaced R414
    Removed Q424, Q426 and replaced R448
    Replaced R505

    Power on, fingers crossed.

    R448 went up in flames instantly. Nothing else.
    R448 is 560Ω , right? Just to verify im doing it right.

    hmm
    [​IMG]


    I noticed Q424 was damaged, it sits right next to C425, possibly it has cracked because of the "explosion" of the capacitor?:rolleyes:
    [​IMG]


    The "service hatch" in the bottom of the chassi was not big enough to get to the components on the side
    [​IMG]
     
  11. MrChips

    Moderator

    Oct 2, 2009
    16,303
    5,011
    Check the solder joints under Q428 and Q430 for any shorts, especially between base and collector pins.
    (E marks the emitter pin. The other two pins are collector and base.)
    If everything looks good, Remove Q428 and Q430.
    I am very certain these two are bad. I will show you later how to test these with your DMM.

    Replace R448, 560Ω (assuming you still have spares, if not, you can omit for now).

    Power up and check for smoke.:(
     
  12. Marius83

    Thread Starter Member

    Dec 28, 2017
    146
    3
    I got spares :)
    I also have new transistors for Q428 and Q430 if needed.

    The soldering under Q428 and Q430 was perfect, no chance for a short.

    Removed Q428 and Q430
    Replaced R448

    Power on..
    No smoke!

    I noticed a bad soldering on one of the small PCB on the front transformator, which actually sparked when i turned it on.
    I gues it has nothing with our problems to do. Just mentioned it.
     
  13. MrChips

    Moderator

    Oct 2, 2009
    16,303
    5,011
    Ok. Time to take a breather after all that hard work.

    I am curious to discover why your photos are so sharp and so well illuminated.
    What camera are you using?
    Why do you have such wide depth of field, i.e. near and far objects are all in focus?
    What are you using for lighting?
    Are you an experienced photographer?

    Now some comments about your DMM probes.

    Meter Probe.jpg

    The long exposed tip on the red probe is an accident waiting to happen. I would insulate most of the metal part and just leave the tip exposed. Use a piece of heat shrink or the outer insulation from a piece of hookup cable and slip this over the tip.

    Meter Probe GND.jpg

    Your black grounding clip on the negative probe looks somewhat agape. To make sure that you always get a consistent ground, I suggest that you solder a length of insulated hookup wire to jumper J401, long enough so that it is not likely to create an accidental short. Then clip the black clip on to the end of this hookup wire.

    Technics SU-VX800 J401.jpg
     
  14. MrChips

    Moderator

    Oct 2, 2009
    16,303
    5,011
    Let's go back and do those voltage measurements:

    R417
    R419
    R421
    R423
    D409
    D411

    R418
    R420
    R422
    R424
    D410
    D412

    I will add:
    R413
    R415

    R414
    R416
     
  15. Marius83

    Thread Starter Member

    Dec 28, 2017
    146
    3
    Glad you like my photos, i'm not a pro :) i just have a good lamp which i move around and point to where i want it.. the photos are taken by my Iphone SE camera(12megapixel). It's a mix of a little zoom,(sometimes) and just to let the camera take it's time to focus on the right spots, and i almost never use the flash. Sometimes i can take many photos to get one good enough.

    I took a picture of the bad factory soldering which sparked, just to show you, i have now fixed it.
    I have also fixed my test gear after your recommendations. See picture in the bottom. :)

    Here are my readings, all resistors read the same at either leg.
    The D's have different readings, the first i list on them, is at the leg nearest the heat sink, if it matters...
    Edit: in post #43 you wrote i should excpect 8 Volts.. so it's no correct anyway...

    R417: +64
    R419: -64
    R421: +64
    R423: -64
    D409: -55 / -47
    D411: -64 / -55

    R418: +64
    R420: -64
    R422: +64
    R424: -64
    D410: -56 / -47
    D412: -64 / -56

    R413: -56
    R415: -56

    R414: -56
    R416: -56


    [​IMG]

    [​IMG]
     
    Last edited: Jan 10, 2018
  16. MrChips

    Moderator

    Oct 2, 2009
    16,303
    5,011
    More voltage measurements:

    LEFT CHANNEL
    R401
    R403

    R409
    D401

    R411
    D403

    R425

    RIGHT CHANNEL
    R402
    R404

    R410
    D402
    R412
    D404

    R426
     
  17. Marius83

    Thread Starter Member

    Dec 28, 2017
    146
    3
    Measurements:

    When i started measurements on left channel, it was first at 13 Volts, it was rising slowly all the time, and when i got to the last, it was about 17 Volts.
    All the resistors measures the same on either leg.(except 425,426)

    LEFT CHANNEL:
    R401: -17 Vollt
    R403: -17 Volt

    R409: -17 Volt
    D401: -17 Volt

    R411: -17 Volt
    D403: -17 Volt

    R425: -63 Volt (Leg nearest heatsink) +63 Volt on the other leg

    RIGHT CHANNEL:
    R402: +35 Volt
    R404: +35 Volt

    R410: +36 Volt
    D402: +36 Volt

    R412: +36 Volt
    D404: +36 Volt

    R426: -64 Volt (Leg nearest heat sink) +64 Volt on the other leg



    I ordered 10 mini grabbers from ebay which came in the mail today.
    Really small ones :)

    [​IMG]
     
    Last edited: Jan 12, 2018
  18. MrChips

    Moderator

    Oct 2, 2009
    16,303
    5,011
    Technics SU-VX800 CN302A.jpg

    Connector CN302A is shown circled in green above.

    I am going to assume the measurements you made in the last post #57 were made with this 4-pin connector disconnected.

    Leave the connector disconnected.

    Measure voltage at each of the four jumpers:
    J222
    J223
    J224
    J225

    Now, with jumpers, hook-up wires, whatever is small and convenient, connect all J222, J223, J224, J225 together and connect them to J401.
    You might try inserting four wires into the white connector, twist the ends together and connect to J401.

    Repeat the voltage measurements:

    LEFT CHANNEL
    R401
    R403

    R409
    D401

    R411
    D403

    R425

    R413
    R415
    D409
    D411

    RIGHT CHANNEL
    R402
    R404

    R410
    D402
    R412
    D404

    R426

    R414
    R416
    D410
    D412

    btw, nice grabber clips. Great for hooking on to leads. Make sure that you do not short adjacent components while hooking up to leads while the power is ON. It is easier to make all the measurements with the power ON. If there is any danger of creating a short, turn the power OFF between measurements if you must.
     
  19. Marius83

    Thread Starter Member

    Dec 28, 2017
    146
    3
    Yes the measurements where done with CN302A disconnected.

    I just measured the jumpers, so you get those readings before i connect them togheter.. :rolleyes:

    J222: -13 Volt
    J223: -13 Volt
    J224: +36 Volt
    J225: +29 Volt

    Ok to proceed? :)
     
    Last edited: Jan 14, 2018
  20. MrChips

    Moderator

    Oct 2, 2009
    16,303
    5,011
    I am just confirming what I suspected.
    J223 and J225 are labeled on the board as GND. Your measurements indicate that they are not connected to GND with CN302A disconnected.

    Leave CN302A disconnected.

    Connect
    J222
    J223
    J224
    J225
    all together and connect to J401 (which we can be sure is our GND).

    Now proceed with the voltage measurements. I am expecting to see new readings from what were previously reported.
     
Loading...