VX 724 black screen green light flashing

[retiredcaps]

Re: VX 724 black screen green light flashing

1. If you are in the USA, use Digikey's USPS First Class Mail shipping option. Something ordered today would arrive Thursday and shipping would be in the order of $2.75.

The (eh) and $8 s/h says to me he is in Canada.

[PlainBill]

Re: VX 724 black screen green light flashing

Ok, there's still something wrong... (eh)

I notice that i get really different readings on the 2 inverter sides.

On the good one :

Q8/Q9 Readings

From ground plate (middle) to left pin : .73kohm
to right pin : .75kohm

Bad side..

From ground plate (middle) to left pin : .26kohm
to right pin : 0.22kohm

So it's really different on both sides..

On the transformer secondaries, i see .44k on both transformers (in circuit).

Several points:

1. If you are in the USA, use Digikey's USPS First Class Mail shipping option. Something ordered today would arrive Thursday and shipping would be in the order of $2.75.

2. How badly do you need this monitor? I've got the same monitor waiting for me to get to it. The transistors short as soon as power is turned on; I'm going to use a variable power supply and a scope to see if I can figure out what is going wrong. But that won't be this month.

3. The Belinea 101910 monitor is identical to this one; the service manual has schematics.

PlainBill

[cheebster]

Re: VX 724 black screen green light flashing

Ok, there's still something wrong... (eh)

I notice that i get really different readings on the 2 inverter sides.

On the good one :

Q8/Q9 Readings

From ground plate (middle) to left pin : .73kohm
to right pin : .75kohm

Bad side..

From ground plate (middle) to left pin : .26kohm
to right pin : 0.22kohm

So it's really different on both sides..

On the transformer secondaries, i see .44k on both transformers (in circuit).

[cheebster]

Re: VX 724 black screen green light flashing

I've replaced the transistors with some i had on another inverter, they were DK QM.

With them installed, same result. The original ones were shorted !

I've also tried applying 12V and GND to the controller board with a bench top PSU, i get the steady green light and the controller board seems to work.

I'm missing something here, but not so sure what.

I've placed an order for 2 transistors a few minutes ago on digikey, just wondering if i should cancel the order (ain't sure if the ones ive put it are a good replacement ?)

Kinda bad to pay 8$ shipping for 1.50$ worth of parts when i'm not even sure i need them hehe

Out of ideas here :/

[PlainBill]

Re: VX 724 black screen green light flashing

The usual problem with this monitor is the transistors for the Royer oscillator - DK Q*, AKA 2SC4672 short. Before you get too carried away, make sure they aren't shorted. You can also turn on the inverter by applying 3.3 volts to the BL_ON pin on the connector to the signal card.

If the transistors are not shorted, and the power supply is stable when the inverter is turned on, yes it is possible that the signal card is bad. I'd say it is not a common problem, but it would be foolish to assume it NEVER happens.

PlainBill

[cheebster]

Re: VX 724 black screen green light flashing

One thing i've tried and noticed, is that when i put the PSU by itself, just plug the ac cord to it, it fires up fine and gives 12.55V on the ouput, and it doesnt struggle to fire up. Does that mean something is bad in the controller board ?

[cheebster]

Re: VX 724 black screen green light flashing

Rtech, i'm unsure how to properly measure the secondaries. I got 4 pins on this secondary side, 2 pairs. I got to measure between the 2 pins on each side ?

The capacitors i got near the transformers are red, marked H154J, they both look fine (no shorts.. but i sold my esr meter, never used it, so i can't test further)

:|

[Rtech]

Re: VX 724 black screen green light flashing

Its the secondaries that you have to measure..that the two pins on the ends nearest to the lamp connectors,they must be very very close to the same reading.Also the other problem with these, is the Blue or Brown Capacitors near to the transformers...if one of them is 'bulging' compared to the other, then that is the problem.

[cheebster]

Re: VX 724 black screen green light flashing

Ok. I've removed both transformers now and they have all the same readings. A wild guess would be that both are good...

I'm unsure where to look and what to test now, but i do know there's still something to fix since it doesn't fire up..

I don't see much continuity in the transformer, only shorts between pins

1-6, 2-4, 3-5. I see 0.7 ohms there.. Do i need to test it some other way ? I've read the tutorial but i'm not so sure i'm testing it the good way.

[Rtech]

Re: VX 724 black screen green light flashing

The transistors 'DK', can also have codes with it of QA,QB,QO,QP,QF etc, and these are production codes, the original part was 2SC4672.
As far as the transformer is concerned,then you need to be 100% sure it is the same,eg the secondary resistance being the same as the original, but you really need a schematic or tech details of each to be certain.

[cheebster]

Re: VX 724 black screen green light flashing

Resurrecting an old thread here ! Got my hands (was on the side of the road) on a vx724. Flashing green light..

I recapped it, no joy. I got some other inverters i could scavenge parts of here, but i was wondering... are those transformers all the same (if they're the same size, with 4 inputs, 6 outputs) ?

I was wondering if i could use them. I got a board with 2 transformers that say: EST0029A.. The ones in the viewsonic are STJ00029

Looks pretty similar to me, but just don't want to kill the board here so any advice would be really appreciated !

Another Q about it.. I had on the same donor board, some transistors marked as DK QM, and the original ones on the viewsonic were DK QN.

Can those be used as well, of they're different ?

Thanks !!!

[Rtech]

Re: VX 724 black screen green light flashing

Well done.The problem comes when its not caps,thats when the fun starts.I suppose a lot of us on here ,get a lot of satisfaction, by turning a non-working electronic/electrical item,into a fully functional item once more.

[stevepw]

Re: VX 724 black screen green light flashing

HI Rtech, If is was sorry, couldnt let it go
Got my VX912 off Ebay, same board as the VX724, but screen is 2" bigger, it had the 2 seconds to black flashing , simple fix, swapped about 4 domed caps over and Bobs your Uncle

YEEE now got a 2 screen set up

[Rtech]

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

[stevepw]

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!

[Rtech]

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.

[stevepw]

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

[Rtech]

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.

[stevepw]

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,

[Rtech]

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.