VX 724 black screen green light flashing

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  • PlainBill
    replied
    Re: VX 724 black screen green light flashing

    Originally posted by stevepw
    Rtech and Plainbill

    I was wondering what bit if any started the chain of events leading to failure to work,

    transformer getting hot making transistor fail or vice versa

    electrolytics going domed and causing above to fail

    or the other caps... brown , blue , yellow even

    i read on another thread asjemb? who seemed to replace everything just to be sure

    if the transistorsand transformer are replaced would another duff cap just make them fail again?
    Without detailed analysis by an expert, it would be impossible to say. A Royer oscillator uses the pulse generated by one transistor to turn the other off. The frequency is limited by the film capacitor. If the capacitor goes open the frequency and the current drawn will rise until the transistors short. I one of the pins on the transformer primary opens (cracked solder joint) the transistor will stay on, overheat, and short. There is a (zener?) diode in the circuit that has been known to short, resulting (I believe) in shorted transistors. I've got a Viewsonic monitor sitting on the shelf right now that will short the transistors in seconds. The transformer passes ring tests, the capacitor checks out, the diode isn't shorted. My next plan is to use a current limited variable power supply to gently power it up and see if I can identify the faulty component.

    PlainBill

    Leave a comment:


  • stevepw
    replied
    Re: VX 724 black screen green light flashing

    Rtech and Plainbill

    I was wondering what bit if any started the chain of events leading to failure to work,

    transformer getting hot making transistor fail or vice versa

    electrolytics going domed and causing above to fail

    or the other caps... brown , blue , yellow even

    i read on another thread asjemb? who seemed to replace everything just to be sure

    if the transistorsand transformer are replaced would another duff cap just make them fail again?

    Leave a comment:


  • stevepw
    replied
    Re: VX 724 black screen green light flashing

    Originally posted by retiredcaps
    I forgot to mention that since you have 2 transformers, you can measure both and see if there are any differences.

    Try the test below with both transformers unsoldered. It won't catch a shorted turn, but maybe the transformer is good and you may not have to order a replacement?

    See #4 (transformer) at

    https://www.badcaps.net/forum/showpo...1&postcount=14
    hi
    Yes tested both transformers
    T1 next to shorted transistors all readings between 0.7-1.6ohms
    T2 shows
    1-2 746ohms
    1-3 746ohms
    1-4 746ohms
    1-5 746ohms
    1-6 0
    2-3 0
    2-4 0
    2-5 0
    2-6 556ohms
    3-4 0
    3-5 0
    3-6 559ohms
    4-5 0
    4-6 559ohms
    5-6 559 ohms

    guess T1 is a goner!

    Leave a comment:


  • Rtech
    replied
    Re: VX 724 black screen green light flashing

    Have also had cases where the Brown cap(as in Bill's note above)has failed,but similar to an electrolytic, it has swollen a great deal.Have also seen similar PSU/Inverter PCBs in models VX710,910 and 924,and I suspect some others as well.As the poster is in the UK,eqivalents for the transistors are available via RS,and I may be able to help him with the Tfmr,so hopefully all the help people have given may result in another 'saved' Monitor.

    Leave a comment:


  • PlainBill
    replied
    Re: VX 724 black screen green light flashing

    Good work troubleshooting. Again, this is the same Royer oscillator circuit we see on Benq, (Dell, HP), etc monitors. Also, I've seen cases where the actual failure was caused by the timing capacitor (the brown cap near the transformer).

    PlainBill

    Leave a comment:


  • retiredcaps
    replied
    Re: VX 724 black screen green light flashing

    Originally posted by stevepw
    SPI 8T J00029
    0523 1 FC
    I forgot to mention that since you have 2 transformers, you can measure both and see if there are any differences.

    Try the test below with both transformers unsoldered. It won't catch a shorted turn, but maybe the transformer is good and you may not have to order a replacement?

    See #4 (transformer) at

    https://www.badcaps.net/forum/showpo...1&postcount=14

    Leave a comment:


  • stevepw
    replied
    Re: VX 724 black screen green light flashing

    Originally posted by retiredcaps
    This might be a match

    http://cgi.ebay.com/CCFL-transformer...item563715b969

    If yes, you might be able to find it cheaper.

    Thanks it looks like it could work
    there are some other numbers on the side SPI 8T J00029
    0523 1 FC

    i will have a look at some Uk suppliers

    Leave a comment:


  • retiredcaps
    replied
    Re: VX 724 black screen green light flashing

    Originally posted by stevepw
    a new transformer it has EPC19 stamped on it , would this be the part number hopefully
    This might be a match

    http://cgi.ebay.com/CCFL-transformer...item563715b969

    If yes, you might be able to find it cheaper.

    Leave a comment:


  • stevepw
    replied
    Re: VX 724 black screen green light flashing

    Originally posted by Rtech
    Looking at the Board from the top,and having had something to do with these boards,I suspect the top right transformer, has failed,if you unsolder it you will probably find it is burnt on the underside.

    Hi Rtech

    thanks for that,having a closer look with eyeglass the underside of the transformer is burnt just where the Q6 transistor is, Q6 also seems to be pitted on top.

    so transistors Q6 and Q7 need replacing and a new transformer it has EPC19 stamped on it , would this be the part number hopefully

    thanks retiredcaps ill have a look for the transistors

    Leave a comment:


  • retiredcaps
    replied
    Re: VX 724 black screen green light flashing

    Originally posted by stevepw
    testing from Left to right with pin 1 at top
    MMset at 200ohm MM set at 2k ohm
    PINS No1 No2 No3 No4
    1-2 37.8 38.3 0.500 0.500
    1-3 36.7 38.3 0.563 0.564
    2-3 1.1 1.4 0.559 0.559
    The above readings suggest that transistors No1 and No2 are shorted. PlainBill identifies them as Rohm transistors in this post below ...

    https://www.badcaps.net/forum/showpo...2&postcount=10

    Leave a comment:


  • Rtech
    replied
    Re: VX 724 black screen green light flashing

    Looking at the Board from the top,and having had something to do with these boards,I suspect the top right transformer, has failed,if you unsolder it you will probably find it is burnt on the underside.

    Leave a comment:


  • stevepw
    replied
    Re: VX 724 black screen green light flashing

    hi jetadm guessing its just the three small connectors, i changed the multimeter setting for3and4 as they appeared open on 200ohm

    testing from Left to right with pin 1 at top
    MMset at 200ohm MM set at 2k ohm
    PINS No1 No2 No3 No4
    1-2 37.8 38.3 0.500 0.500
    1-3 36.7 38.3 0.563 0.564
    2-3 1.1 1.4 0.559 0.559

    Leave a comment:


  • stevepw
    replied
    Re: VX 724 black screen green light flashing

    HI
    The part number of the board is AS02B012D75
    all 4 resisitors have DK QR on them
    to test for shorts am i just checking the 3 small connections on the right of each transistor or the fat one on the left too

    Leave a comment:


  • jetadm123
    replied
    Re: VX 724 black screen green light flashing

    Try checking these transistors for shorts. What's the part number?
    Attached Files

    Leave a comment:


  • stevepw
    started a topic VX 724 black screen green light flashing

    VX 724 black screen green light flashing

    HI Guys

    My Viewsonic VX724 monitor died a few weeks ago, following a few hours where it smelt of burning, it now gives the black screen, green on/off button flashes. (im not an electronics bod but i know which end of the soldering iron to hold!)

    I have replaced 5 domed caps on the power board, the green board caps look ok. The button still flashes green.

    following previous threads and a bit more troubleshooting, there is 12.3v on the connector that joins the 2 boards with power lead connected.

    There are two small connectors at top left and top right of power board ( not sure what these do)

    unplug both top right connectors screen flashes
    unplug both top left connectors screen does not flash

    when i connected up the mains just now there was a bit burning smell and smoke coming from power board although i couldnt see which bit as i turned it off quick.

    hope there is enough clues here for you to help me

    any help gratefully recieved

    steve
    Attached Files
    Last edited by stevepw; 12-12-2010, 12:49 PM.

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