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Viewsonic N3235W Black Screen - Need some help
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OldIBMer



Joined: 21 Aug 2009
Posts: 27
Location: USA

PostPosted: Fri Aug 21, 2009 5:51 am    Post subject: Viewsonic N3235W Black Screen - Need some help Reply with quote

Hello, This is my first post here. Hoping maybe someone can put me on the right track. Have a 22 month old Viewsonic LCD TV N3235W which has no history of problems. With no warning the screen went black, sound OK. I have some electronic troubleshooting experience, but no real knowledge of LCD's and no test equipment except a VOM. Also no TV repair help anywhere around here (and the TV is out of warranty). So, here is what I have done so far:
1. Carefully inspected power supply PCB for signs of heat, bulging caps, or anything else out of the ordinary...looks OK.
2. Metered 24V DC on the connector going to the inverter PCB.
3. Inspected inverter PCB...looks OK.
4. Metered each of the (7) inverter transformers primary & secondary windings; all are the same (on my meter x100 range = ~650 ohms.)
5. Looked for a fuse or fuses on the inverter PCB. Only thing there are four components etched as F1-F4, three are marked with a "P" and one with a "N" and none of them meter the same. All act like a diode, and all are different (measured each both ways: inf.-1k, inf.-6k, 5k-inf., and inf.-600k on the one marked "N".) Since I am metering them while still in their circuits, I know results would be unpredictable unless they were normal fuses.

I notice that the normal diagnosis in this kind of case seems to be a bad inverter board, but is there something else that I can check to confirm this?
The p/n of the inverter PCB is CPT 320WB02C DAC-24T042 AF.

Any help would be appreciated. I know I am in over my head here, but I don't like the thought of sending the TV to the landfill. Sad


Last edited by OldIBMer on Thu Sep 10, 2009 10:37 pm; edited 2 times in total
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OldIBMer



Joined: 21 Aug 2009
Posts: 27
Location: USA

PostPosted: Sat Aug 22, 2009 3:56 am    Post subject: Re: Viewsonic N3245W Black Screen - Need some help Reply with quote

Well, I'm still plugging away on this. I think I have about run out of things that I am able to do. Have been searching for threads/posts that contain info that may help. Whatever I know about this problem has come from some of the great posts I've found here.

Took the shield off the inverter PCB and made sure that 24V was on the (4) VIN pins. Also, about 4.6V at the on/off pin and 1.7V at the PDIM pin.

Still black screen, sound OK. Tapped on some of the inverter board components under power...nothing happened.

How big a gamble do you think it would be to order a new inverter board?
Is there something else that I can do to test? Sure would be nice to have a troubleshooting flowchart which, with only a VOM, could at least point to the bad sub-assembly.

Just as a little aside, I do love this stuff. My electronics training was mostly on vacuum tubes...had one class near the end of my training on those "newfangled transistors"!! Worked on lots of airborne electronics back when miniature vacuum tubes were the norm. Spent 30 years in the computer business, but seldom did any component level troubleshooting. I'm starting to think maybe I need to go back to school since we are now surrounded by so many electronic "gadgets" that like to break! Rolling Eyes

Anyway, any help would be appreciated. Very Happy
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vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Sat Aug 22, 2009 6:21 am    Post subject: Reply with quote

If it is an inverter issue, you should still be able to see a dark image or better yet shine a flashlight at the screen. All the inverter does is to provide just a backlight so you can see the image.
Not quite sure of the part number or the cost but, if you do a search you ought to be able to find it. Not much involved in replacing it.

As far as fixing it, it may in fact be something simple but other than checking for the obvious, really not a whole lot that can be done. Shorted transistors or caps are common failure but often hard to find replacements even if you do spot them. Most if not all manufacturers, don't even provide any type of print for that.
You spoke about the good old days when you could fix things. Unfortunately, today's consumer electronics is very much disposable. While it is never late to go back to school to learn new things, Consumer electronics is probably the last thing you'd want to get in to. I spent over 26 years in it. Everything from having my own business to work for a big Corporation.
If I had to do it all over again, it would definitely NOT be consumer electronics.
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OldIBMer



Joined: 21 Aug 2009
Posts: 27
Location: USA

PostPosted: Sat Aug 22, 2009 3:44 pm    Post subject: Reply with quote

Vtech, Thanks for your reply. I can see a faint image when shining a flashlight at the screen. So, if:
1. Power supply PCB is OK since I have 24V, 4.6V on/off, and 1.7V PDIM.
2. Logic PCB is OK since I have faint image.

Then, is there anything else I can check to see if the problem is outside of the inverter PCB? I did not know that the sole function of the inverter PCB is to provide the backlight. Do the (7) outputs of the inverter board simply fire a CCFL array?

If it is time to order an inverter PCB (CPT 320WB02C), I found one on Discount-Merchant.com for $99.99. I think their parts come out of returned equipment. Does that look like a good source or would you recommend another? Thanks for your help on this.

Gerald
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vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Sat Aug 22, 2009 4:01 pm    Post subject: Reply with quote

Gerald,
The fact that you can see the image, it is definitely pointing to the inverter and you are correct it is mainly used for firing the backlight. What I am not sure is where to get or how reliable the discount merchant is? Perhaps minnie or one of the regulars could chime in before you order that.
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jts1957



Joined: 24 Nov 2008
Posts: 2472
Location: Far, Far Away

PostPosted: Sat Aug 22, 2009 8:51 pm    Post subject: Reply with quote

Ordered a power supply from D-M. It failed to even produce stand-by 5 volts. They had it picked up, and sent a second. That one worked!
They "claim" the check all boards ... I don't think so. I think they are just "pulled from non-working sets," cleaned up, sold & hope for the best. Exclamation
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PostPosted: Sat Aug 22, 2009 9:46 pm    Post subject: Reply with quote

jts1957 wrote:
Ordered a power supply from D-M. It failed to even produce stand-by 5 volts. They had it picked up, and sent a second. That one worked!
They "claim" the check all boards ... I don't think so. I think they are just "pulled from non-working sets," cleaned up, sold & hope for the best. Exclamation


Thanks for the input. I found another site that seems to have a lot of Viewsonic parts (plus other stuff...even appliance parts).

ShopJimmy.com Pretty good prices. Looks OK, I may order from there.
Question
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OldIBMer



Joined: 21 Aug 2009
Posts: 27
Location: USA

PostPosted: Thu Aug 27, 2009 9:32 pm    Post subject: Re: Viewsonic N3245W Black Screen - Need some help Reply with quote

Today, I received a replacement inverter PCB that I ordered from ShopJimmy.com. Smooth transaction, reasonable shipping cost, and fast...ordered Sat. night, 8/22, and received (by UPS) on 8/27. Cost $69.99 plus 9.78 shipping.

Installed the board and the TV came back to life. Another electronic casualty plucked from the jaws of the landfill!! (at least for now)

So, now I have a dead inverter on my hands. Wish I was smart enough to fix it. I thought some of comparing some voltage levels between the old and new (only have a VOM; no scope), but thought better of it...might end up with two dead boards!

As I mentioned before, there are four components at sites etched with F1-F4. They each carry a single letter identifier: P, P, P, N. They "appear" to meter with an ohmeter like a diode, but they are still in their circuits.
Is it likely that these are being used as fuses?
Is there a way I can test these with only a VOM?
I know this is a tough question to answer without a schematic, but I don't know where one of those might be found.

OldIBMer wrote:
... So, here is what I have done so far:
1. Carefully inspected power supply PCB for signs of heat, bulging caps, or anything else out of the ordinary...looks OK.
2. Metered 24V DC on the connector going to the inverter PCB.
3. Inspected inverter PCB...looks OK.
4. Metered each of the (7) inverter transformers primary & secondary windings; all are the same (on my meter x100 range = ~650 ohms.)
5. Looked for a fuse or fuses on the inverter PCB. Only thing there are four components etched as F1-F4, three are marked with a "P" and one with a "N" and none of them meter the same. All act like a diode, and all are different (measured each both ways: inf.-1k, inf.-6k, 5k-inf., and inf.-600k on the one marked "N".) Since I am metering them while still in their circuits, I know results would be unpredictable unless they were normal fuses...

Sad


All in all, a happy conclusion and I am thankful for the help that the folks on this forum have provided! Very Happy
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vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Fri Aug 28, 2009 2:10 am    Post subject: Reply with quote

Can you post a picture of what you are referring to?
I would guess you'd probably have a cap or transistor & /or driver issue. Problem being that a lot of times even if you find the device(s), it may take longer to find a replacement.

Your idea of comparing voltages was not a bad idea but still inconclusive and as you mentioned, possibility of two bad boards.
Now if you have the time & determined to find the culprit(s), one good approach is to attempt a resistance check/comparison with a known good board.

ie; You'd choose a ground plane as a reference point on both boards and select random identical points(start around major components) and compare resistance readings. Chances are pretty good that you would come across a drastic difference at some point between the two boards. Then it would be a matter of tracing it to the faulty device....used to do this on many instances and always had good luck in finding the elusive culprit.

Of course it can't be too complicated of a circuit to get accurate readings & an inverter is definitely not complex.
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J/TECH
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PostPosted: Thu Sep 10, 2009 6:41 pm    Post subject: Re: Viewsonic N3245W Black Screen - Need some help Reply with quote

I would like your old inverter board; have one with bad transformer on; can't find supply of board or transformer. Open to offers?
Installed the board and the TV came back to life. Another electronic casualty plucked from the jaws of the landfill!! (at least for now)

So, now I have a dead inverter on my hands. Wish I was smart enough to fix it. I thought some of comparing some voltage levels between the old and new (only have a VOM; no scope), but thought better of it...might end up with two dead boards!

As I mentioned before, there are four components at sites etched with F1-F4. They each carry a single letter identifier: P, P, P, N. They "appear" to meter with an ohmeter like a diode, but they are still in their circuits.
Is it likely that these are being used as fuses?
Is there a way I can test these with only a VOM?
I know this is a tough question to answer without a schematic, but I don't know where one of those might be found.

[quote="OldIBMer"]... So, here is what I have done so far:
1. Carefully inspected power supply PCB for signs of heat, bulging caps, or anything else out of the ordinary...looks OK.
2. Metered 24V DC on the connector going to the inverter PCB.
3. Inspected inverter PCB...looks OK.
4. Metered each of the (7) inverter transformers primary & secondary windings; all are the same (on my meter x100 range = ~650 ohms.)
5. Looked for a fuse or fuses on the inverter PCB. Only thing there are four components etched as F1-F4, three are marked with a "P" and one with a "N" and none of them meter the same. All act like a diode, and all are different (measured each both ways: inf.-1k, inf.-6k, 5k-inf., and inf.-600k on the one marked "N".) Since I am metering them while still in their circuits, I know results would be unpredictable unless they were normal fuses...

Sad[/quote]

All in all, a happy conclusion and I am thankful for the help that the folks on this forum have provided! Very Happy[/quote]
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