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Dynex DX-L42-10A Intermittent Issues

Discussion in 'Troubleshooting and Repair' started by leggot, Sep 21, 2014.

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  1. leggot

    leggot

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    Mar 28, 2012
    Hi,

    I have a Dynex TV DX-L42-10A that has intermittent issues - both turning on and staying on. To get it to turn on, it requires the power button to be pressed in rapid succession until the stand by light turns blue. (red light indicates standby mode, and blue light indicates being on) However, this is not a reliable method of turning the unit on - often it requires being unplugged for a period of time which varies from several hours to a few minutes.

    Once the unit is on, the screen will go blank after a period of time. Typically this time is short (around 5 minutes) or longer (2 hours). The blue light stays on after this amount of time and cannot be turned off via the power button, rather must be unplugged from the outlet.

    I have disassembled the unit and inspected the power board and the main circuit boards to ensure that they do not have cracked solder and that the capacitors are not bulging. I also tested the power coming out of the power board, which varies according to the state of the TV. I am not sure if this is the normal functionality, or that the power board is faulty. In the standby mode, the board only outputs 5V. When the TV is on, it outputs 5V, 12V and 24V, however when the screen goes black, but is still on it only outputs 5V. This has led me to believe that the powerboard is at fault for this, not the main board because when the screen goes black, the blue light is still on, but the powerboard no longer outputs 12v and 24v. However, I may be wrong because there are several signal connections from the mainboard to the powerboard.

    I have also tested the button for possible faults and it seems to be functioning normally. This led me to test capacitors on the powerboard. Unfortunately, my multimeter does not have a capacitance testing mode, so I used ohms. All the capacitors had either high, climbing or infinite resistance, but one had a steady 0.06 ohms. I am not sure if this is normal, or if it is causing the fault. Please advise and if necessary I can provide more pictures of the boards.

    I have been unsuccessful in finding a circuit diagram, but here is a link to the powerboard: http://www.tristatemodule.com/p-140...-supply-6kt0032012-569kt02200-dx-l42-10a.aspx

    Here is a link to the main board's circuit diagram (scroll to the bottom): https://www.dropbox.com/s/cjiz3kz2d...isense-Service-Manual-DX-L40-10A (2).pdf?dl=0
     
    Last edited: Sep 21, 2014
  2. davenn

    davenn Moderator

    13,866
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    Sep 5, 2009
    hi there
    welcome to EP :)

    I'm assuming its a TV or some other domestic device....
    the link for the service manual requires a login, which I am not willing to do
    Can you upload the service manual to somewhere that doesn't require a login :)

    cheers
    Dave
     
  3. leggot

    leggot

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    Mar 28, 2012
    Hi,

    Thanks for replying so quickly. I updated the link to a version that doesn't require a login. Please let me know if you still have issues. And yes, it is a TV.
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    OK, it's a Chinese LCD TV. See http://www.cnet.com/products/dynex-dx-l42-10a/specs/

    The service manual is available on elektrotanya.com but it's 100 MB.

    The 5V rail is provided by an always-ON power converter using T501. The main board sends a control signal called POWER to the power supply, to control the main power converter, operating through T502, which provides the 12V, 24V and 32V supply rails used by the main heavy loads in the set.

    You can tell whether the power board is operating properly by measuring voltages on connector X503. Use pins 2, 6, 7, 10 or 12 as your 0V reference for your multimeter. Pin 11 should always have 5V on it, while the set is plugged in. This powers a small portion of the rest of the set which is able to detect and respond to the commands from the remote control. This part of the set generates a voltage on the POWER signal on pin 13 of that connector. If that pin is positive (at least +2V), the rest of the power supply should power up. If there is no voltage on that pin, then the other part of the set is not trying to turn the power supply ON.

    This signal seems to be controlled by pin 256 of NS01 which is a large microcontroller on the main board.

    It could be that the set is shutting down because a fault is being detected in one of the circuits powered from the main rails from the power supply. The backlight driver board is a common failure point. If it detects a failure while it's powering up, and reports this to NS01, NS01 may decide to shut down the power supply. So there are many possible causes. But you can eliminate the power supply board itself from suspicion by checking those voltages.
     
  5. leggot

    leggot

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    Mar 28, 2012
    Hi.

    Thanks for replying so quickly. I checked pin 11 on X503 and it returns 5V regardless of the state of the TV. There also appears to be 3V on pin 13 (PON), but there is no 12v, 24v or 32v from their respective pins.

    I did not find the service manual on elekrotanya.com, but I did find the schematics for the the powerboard: https://www.dropbox.com/s/a6jo3o2qtip2w6w/Dynex_oem_6kt0032010_sch.pdf?dl=0

    Also, I cannot find documentation referring to X503 and NS01, but the numbers are printed on the board.

    Thanks for your help.
     
  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Right. That schematic looks like it matches the one in the service manual.

    You said there "appears to be" 3V on pin 13. Are you sure? That's the pin that controls whether the other power rails are enabled or not. If there's 3V on it, and the other rails are not present, that indicates a problem in the power supply. But if there's not 3V on that pin, the problem is somewhere else.

    If there's definitely 3V on pin 13 and the other rails are not running, the fault is probably in the power supply. A good place to start is the overvoltage crowbar circuit.

    epoint 270389 power supply crowbar.png

    The overvoltage crowbar circuit detects excessive voltage on the outputs of the main power supply section (not the 5V part) and shuts down the main power converter, to protect the rest of the set from possible damage due to overvoltage.

    The two red arrows at the top of the diagram are the output voltages. They feed through individual zener diodes D516 and D518, and through R547 into the gate of V505, which is an SCR. If the voltage on either output rail is too high, the corresponding zener diode will conduct and this will trigger the SCR.

    When the SCR is triggered, it drags its anode voltage down close to 0V. This removes the voltage from the anode of the LED inside optocoupler N505 (also circled). N505 enables the main power converter in the power supply. So when the SCR fires, the main power converter shuts down. The SCR remains latched because of current flowing through R540. The only way to unlatch it is to unplug the set, which will cause the 5V rail to disappear.

    It's quite possible the fault has nothing to do with the overvoltage circuit, but the symptoms sound suspiciously like it could be, and anyway, you have to start somewhere :)


    Working on a power supply is potentially very dangerous. Even when it's unplugged there are high voltages present on the capacitors, which can kill you. Really they can. So you have to be very careful when working on the power supply. The best way is to unplug the set, discharge the capacitors (see http://www.google.com/search?q=safely+discharge+a+capacitor), clip your multimeter test leads where you need them (or use alligator clip leads, and make sure nothing can move around and touch anything), then power it up and read the measurement.

    If you're not confident you can work safely, say so. We can give you more specific advice and point you to tutorials that will help.


    So the first place to start is to measure voltage across the SCR, V505. Find the track that connects V505, optocoupler N505, and resistor R540, and connect the positive lead of your multimeter to it. Connect the negative lead of your multimeter to the 0V rail on pin 3, 6, 7, 10 or 12 of connector X508, or something else connected to the same rail.

    You should measure about 5V when the set is in standby, and about 1V when it's running. If the voltage is less than about 0.8V then the SCR is triggered. In that case you can try adding a 1k resistor in the position I've marked. This should have been included in the original design. It prevents the SCR from being triggered by slight leakage currents through the zener diodes.
     
  7. leggot

    leggot

    30
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    Mar 28, 2012
    Hi,

    Thanks for the reply. I should have been more clear when I stated that there is 3V on pin 13, as this is the case when the TV is on, when the TV is on and the screen is blank, or when the TV will not turn on. If this pin is ~0V, then the TV will turn on.

    However, I measured the voltage across V505 and it also varies from state to state. When the TV is on standby and before it turns on, it is at about 5V. If it decides to turn on, it drops to about 0V then rises to 3.24V shortly before turning off. The voltage is 3.24V when the TV will not turn on and when the screen is blank.

    In this case, should I add the 1K resistor?

    I hope this helps and thanks for the help.
     
  8. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    OK, sorry, I made a mistake with the voltages I listed in post #6. The 3.24V that you measured is the right voltage to make the power supply start up. So the problem is not in the crowbar circuit. So there's no need to add the 1k resistor.

    When you have around 3.24V on the junction of N505 and R540, the main power supply should run. So the next thing to check would be the power supply to the control IC for the main part of the power supply. This is a 16-pin IC called N502. The pins are pin 2 (positive supply) and pin 4 (0V).

    This part of the circuit is live when the power supply is running. Make your connections with the set unplugged and the big capacitor (C507) discharged. Make sure none of your connections can touch anything else, especially yourself!

    So connect your multimeter positive lead to pin 2 of N502, or to the end of R508 that connects to it, and connect your multimeter negative lead to pin 4 of N502 or any other component that's connected to the live 0V rail.

    I assume you only have one multimeter, so you won't be able to measure the N505/R540 voltage at the same time. But you're familiar with the set's behaviour and what conditions cause the 3.24V on N505/R540. That condition, that causes 3.24V on N505/R540, should cause the supply voltage to appear on N502 and indicate on your multimeter.

    I don't know the exact voltage you should expect to see, but I imagine it will be between about 12V and about 40V. So can you measure that for me. Remember to be careful with your positioning of all the connections so nothing can move and touch onto anything!
     
  9. leggot

    leggot

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    Mar 28, 2012
    I tested the 2 pin on N502. Oddly, all the voltages appear negative when connected to the ground. The results are as follows - when the TV is on, when it won't turn on and screen is blank its about -45.3V. When the TV turns on it changes from -61.2V to -45.3V.

    This means that when V505 is 3.24V, N502(2) is -45.3V. When V505 changes from 0V to 3V, N502(2) changes from -61.2V to -45.3V, when the TV actually turns on.

    Thanks for the help!
     
  10. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Did you connect the black lead of the multimeter to pin 4 of N502 (or something that is connected to it)? That voltage should definitely be positive!
     
  11. leggot

    leggot

    30
    0
    Mar 28, 2012
    Sorry, I did not connect the black end to pin4. After retesting, the voltage is about 23volts when in standby mode. I will post the voltage when the TV turns on.
     
  12. leggot

    leggot

    30
    0
    Mar 28, 2012
    Interestingly, the set now refuses to turn on. Typically letting it sit over night would yield successful results, however it has been about 16 hours with no avail. Pin 13 still shows 3volts and V505 also shows 3.24volts.

    Thanks and please advise what I should do next.
     
  13. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    OK. 3V on pin 13 means the rest of the set is telling the power supply to start up. And 3.24V on V505 means the crowbar isn't activating, so the set should start up. This should cause the supply voltage to appear between pins 2 and 4 of N502.

    So if you have that 23V between pins 2 and 4 of N502, the rest of the set should be starting up.

    So let's concentrate on N502. This is on the half-live side of the circuit, and the black lead of the multimeter must be connected to the half-live ground rail, which is on pin 4 of N502. You have to set up the measurements in advance, while the set is unplugged and after discharging the main electrolytic capacitor. Are you confident you can do this safely? You haven't said anything about this yet.

    Start by measuring the DC voltages on pins 1, 2 and 9.

    Also can you investigate R502. It is probably a fusible resistor, and the value isn't marked on the schematic. Can you read the colour bands?
     
  14. leggot

    leggot

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    0
    Mar 28, 2012
    Thanks for responding. I am confident that I can do the set ups safely, as the pins are relatively exposed and visible on the board. It is also positioned in such away that I can test it while it is still mounted on the TV, without taking the board off.

    I believe that you are referring to the pins on N502 and the voltages are as follows (this is when the unit refuses to turn on) - N502(1) = 2V, N502(2) = 19V and N502(9) = 0V. These were taken between the specified pin and N502(4) = GND. For some reason the voltage between N502(2) and N502(4) changed from 23V to 19V since my last test.

    R502 is Blue - Gold - Green - Black - Brown [​IMG]

    Thanks
     
    Last edited: Sep 23, 2014
  15. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    OK. That 0V reading on pin 9 is wrong. But I didn't explain how pins are numbered. I've marked up the pin numbers on your picture:

    1zx4i00 marked up.jpg

    Can you confirm that there's 0V on pin 9?

    Also you said that the pin 1 voltage is 2V. Can you measure it again on a lower voltage range so we get more digits? If your meter has a 20V range, it should display two digits below the decimal point - for example, 2.13V. And measure it for 10~20 seconds, to see whether it's steady or not.
    It's the other way - brown black green. That's 1 MΩ (one megohm). This resistor sets the voltage on pin 1, which should be about 2V as you measured. It's probably worth checking that resistor though. You'll need to lift one end out of circuit, then measure across the component with your meter on resistance range - the 2 MΩ range, if it has a manual range switch.

    One end of R502 connects to the 200V side of the electrolytic. Make sure you unplug the set and discharge it!
     
  16. leggot

    leggot

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    Mar 28, 2012
    Thanks for responding. I remeasured the voltages on both pin 1 and pin 9. The voltage on pin 1 fluctuated from 2.03 to 2.04volts for about 10 seconds. The voltage on pin 9 was 10.51volts.

    Tomorrow, I will desolder the resistor and measure the resistance across it as well as ensure that the electrolytic has no charge.
     
  17. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    OK, that's fine. There's no need to measure R502.

    I've found a Powerpoint-type document on the Sanken SSC9500 (N502) but I can't find a full data sheet for it. There's also an error in the schematic which is causing a little confusion.

    I think it might be worthwhile simply replacing N502 on suspicion. It's available from several Chinese suppliers on eBay: http://www.ebay.com/itm/SSC9500-Man...012?pt=LH_DefaultDomain_0&hash=item2ecdfe1b3c
     
  18. leggot

    leggot

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    Mar 28, 2012
    Alright, I am working on ordering SSC9500. I plan on borrowing a multimeter to check the capacitors to make sure they are all ok. Is there anything else that I should check while I wait?
     
  19. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Yes you could try a few things.

    You could replace C513 (2.2 µF) on suspicion. I don't think it's likely to be a problem but it's an easy change.

    Check D503 and D506 with one lead lifted out of circuit. Set the multimeter to the highest resistance range and connect the red lead to the cathode (the end with the stripe). The meter should still show overload. This is not a conclusive test though.

    Most of the resistors should measure OK in circuit. Any resistor that doesn't, post the reference and the value you measured.

    You can check the circuit that supplies the operating power to N502. You said that the voltage on N502 was 23V, then later you measured 19V. Can you connect your multimeter permanently onto that voltage, then try some tests on the 4-pin optocoupler N505. Try shorting pins 1 and 2. These are on the non-live side of N505. You can use a flat blade screwdriver for this. Don't worry, it won't damage anything. When you do this, the voltage should drop to 0V.

    Then try shorting N505 pins 3 and 4 together with the screwdriver. These pins are on the live side. The meter should show the full supply voltage.

    Let me know the result of those tests. Be really careful not to touch anything while you're working in there with the power on!
     
  20. leggot

    leggot

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    Mar 28, 2012
    As I was poking around on the board, I found a capacitor that was oddly off from the measurement listed on the data sheet. The data sheet lists C524 as 68n/630v and as measured it is 0.22uf which is confirmed by googling the model number. I am not sure, but this does seem quite odd. It does give rise to the idea that the circuit board doesn't exactly match the diagram. There were several components that when removed measured higher than the datasheet.

    I measured and broke C213 and it came to about 2.51uf, so I suppose I would have to replace it anyhow. Other than being a 2.2uf axial capacitor, is there any other requirement that I would need to find a replacement?

    Both D503 or D506 have overload when removed from the circuit and tested.

    I have not been able to test shorting out N505 due to the fact that I need to replace C513.

    There are several resistors that have different values than what is printed on them, but mainly by small variation:(they were all attached to the board when I measured them, this may account for some of the variation)
    ID --------color code -----(printed value) --- measured value
    R512 - Brown Gray Black(18ohms) 2ohms
    R501 - Red Red Green(2M ohms) 0.666M ohms
    R507 - Brown Black Orange (10k) 8.3k
    R513 - Brown Black Orange (10k) 8.3k
    R542 - Orange Orange Red (3.3k) starts at 400k and increases/decreases
    R545 - Orange Orange Red (3.3k) starts at 400k and increases/decreases
    R532 - Orange Orange Orange (33k) - fluctuates around 2k
    R536 - Red Violet Orange (27k) - fluctuates around 2k
    R543 - Red Violet Red (2.7K) 2.2k
    R539 - Orange Orange Orange (33k) 2.2k
    R512 - Brown Grey Black (18) 2ohms
    R534 - Brown Red Black (12) 10k and increasing
    R535 - Yellow violet orange (47k) 27k

    Thank You
     
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