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Old 12-12-2010, 11:29 AM   #1
stevepw
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Default Viewsonic 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
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Last edited by stevepw; 12-12-2010 at 11:49 AM..
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Old 12-12-2010, 11:50 AM   #2
jetadm123
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Default Re: VX 724 black screen green light flashing

Try checking these transistors for shorts. What's the part number?
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Old 12-12-2010, 12:02 PM   #3
stevepw
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Default 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
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Old 12-12-2010, 12:26 PM   #4
stevepw
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Default 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
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Old 12-12-2010, 01:02 PM   #5
retiredcaps
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Default Re: VX 724 black screen green light flashing

Quote:
Originally Posted by stevepw View Post
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 ...

http://www.badcaps.net/forum/showpos...2&postcount=10
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Old 12-12-2010, 12:56 PM   #6
Rtech
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Default 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.
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Old 12-12-2010, 01:37 PM   #7
stevepw
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Default Re: VX 724 black screen green light flashing

Quote:
Originally Posted by Rtech View Post
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
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Old 12-12-2010, 01:46 PM   #8
retiredcaps
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Default Re: VX 724 black screen green light flashing

Quote:
Originally Posted by stevepw View Post
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.
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Old 12-12-2010, 02:19 PM   #9
stevepw
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Default Re: VX 724 black screen green light flashing

Quote:
Originally Posted by retiredcaps View Post
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
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Old 12-12-2010, 03:25 PM   #10
retiredcaps
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Default Re: VX 724 black screen green light flashing

Quote:
Originally Posted by stevepw View Post
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

http://www.badcaps.net/forum/showpos...1&postcount=14
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Old 12-13-2010, 01:47 AM   #11
Rtech
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Default 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.
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Old 12-13-2010, 12:31 PM   #12
stevepw
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Default 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?
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Old 12-13-2010, 01:18 PM   #13
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Default Re: VX 724 black screen green light flashing

Quote:
Originally Posted by stevepw View Post
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.

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Old 12-14-2010, 02:24 AM   #14
Rtech
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Default Re: VX 724 black screen green light flashing

Whenever i see one of these,the faults are caps,being the lytics or the blue/brown cap near to the tfmr,the output transisitors on the bottom of the board and the tfmr,or a combination of these.In each case it is ONLY the side near to the Heatsink and associated Schottkys that do fail,and so I think the overall problem is simply caused by the heat generated in that area.
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Old 12-14-2010, 11:53 AM   #15
stevepw
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Default Re: VX 724 black screen green light flashing

I have just removed the two brown radial polypropylene caps near the transformers
Rtech you mentioned these on another thread thyre H154 J , with my meter set on 2UF
i get 0.141 on the good side and 0 on the one nearest burnt out transformer
I have somne transistors ordered from RS , shoukd be in in next 2 days
It may be worth checking the shottkey diode MBR101100CT, as this has been highlighted as an issue on threads does this need to be removed from board to do it properly,
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Old 12-14-2010, 12:00 PM   #16
Rtech
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Default Re: VX 724 black screen green light flashing

You can check the schottkys in situ...outside pins are common to each other.and simple resistance measurements between the centre and the extremes should soon tell you if there is a problem...they normally go short cct when they fail.Tfmr posted today,if you still have probs possibly with caps after you have put the tfmr and transistors in then let us know.
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Old 12-14-2010, 12:27 PM   #17
stevepw
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Default Re: VX 724 black screen green light flashing

Many thanks Rtech
looks ok, outsides common , from centre pin to either outside pins gives 198ohms and swopping leads gives open reading both ways.
for the brown H154 J there are some with 150nF is this the same as 0.15UF both rated at 250V these are blue too , does it matter , seebelow
http://uk.rs-online.com/web/search/s...duct&R=6689961
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Old 12-14-2010, 03:24 PM   #18
Rtech
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Default Re: VX 724 black screen green light flashing

Your cap values are fine BUT the tolerance has to be + or - 5%(usually code J)as you have noted.
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Old 12-21-2010, 12:30 PM   #19
stevepw
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Default Re: VX 724 black screen green light flashing

HI All

just an update , got some new caps to day to replace a duff H154 put them in and WOW a working screen again, been on for an hour now looks good with no funny smells..

to get it working I replaced

5 domed caps on the power board
one burnt transformer (many thanks rtech, I would have struggled without this )
The 2 small transisitors underneath above
both of the H154 caps one of which had gone

Thanks for the diagnosis, tips, help ive enjoyed the learning and repairing so much I bought another duff one of ebay to play with!
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Old 12-21-2010, 01:25 PM   #20
Rtech
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Default Re: VX 724 black screen green light flashing

Well done, glad to be of help......I hope the one you bought was NOT the one I was bidding on.....lol
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