Vizio VF550M screen flicker (to black) then eventual shutdown

[germaldish]

Hello all,

I recently found this forum and I look forward to drawing on your expertise in fixing this problem I am having! Recently my neighbor gave us his broken TV because it was flickering and he had already upgraded to a new one. He said if I could fix it it's mine. Now we are upgrading from another freebie: a 20" CRT monstrosity (I think it weighs at least as much as this 55"), so this is a pretty major upgrade for us if it works.

A little background on me: I like to tinker and fix things; I've been building and fixing my own computers since I was a teenager and I have also designed and built a number of circuits both for work and for interest/hobby. I have a decent Tektronix scope and a good DMM as well as some basic soldering stuff (although lately I've really wanted an upgrade in this area). I have a bunch of components laying around from past projects including lots of caps, resistors, diodes, etc, and I'm not afraid to use them.

I already had the TV apart once... I thought I fixed it simply by the old "take it all apart, don't find anything wrong, put it all back together" method. Indeed for 6 hours on Saturday I was enjoying flicker-free TV on a 55" that is way too big for my tiny apartment but still fun.

I did take some pictures of all the boards but they were pretty crappy since I was mainly taking them to remember which screws go where when putting everything back together. When I take it apart again (maybe tonight?), I'll post better pictures of every board front and back. I didn't see any bulging or burst caps the first time, and I didn't notice that fussing with the cables while it was running helped the issue either.

THE PROBLEM IN DETAIL:
1. When I turn on the TV, everything looks great.
2. After a few seconds/a few minutes/a few hours depending on the day and the alignment of the moon and stars, the backlight will begin to flicker and quickly oscillate between completely dark and completely on. The frequency of the oscillation is probably about 5-10Hz.
3. If I mess around with the menu sometimes it stops the flickering. But eventually it comes back, regardless.
4. The input doesn't matter. It does the same thing with no input at all (from the blue screen), although sometimes switching from one input to another seems to temporarily stop the flicker for a few seconds.
5. On Saturday when it first started to happen again after 6 hours of no problems, I went into the menu and turned the backlight down to "zero" (which I assume limits current to the invertor boards by about 50% since there was still a viewable picture?), and it continued to work like that for a couple more hours.
6. Then it got worse again and I was back to square one. When the flickering gets bad enough, it seems to trigger some kind of shutdown to the invertor boards (I guess), and the picture goes black but the "Vizio" logo on the front of the TV stays white rather than amber, until I push the power button to turn it off again (then it will go to amber).
7. If I turn it back on, I pretty much get the same result every time.
8. I should also mention that when it's flickering, the sound goes out along with the picture (so you can really "hear" the flickering). And there seems to be a clicking noise coming from the top of the power supply board. I haven't investigated this noise in detail yet, so I can't really say more than that quite yet.

WHAT I HAVE DONE SO FAR
1. I disassembled the TV, removed all the boards to check for burned components or failed capacitors and found nothing obvious.
2. While doing this I accidentally dropped the metal cover from the T-con board onto/across the two heat sinks on the power supply board while it was on... so yeah I thought I'd try to use my PS as a welder it it only worked momentarily!
3. I resoldered a new T8A fuse to the power supply board and checked all voltage outputs with only the power supply board and the main board plugged in (for the turn-on signal). All voltages checked out within a few percent of the rated voltages, although I was thrown off for a minute because the 24VI stuff doesn't reference the chassis as ground. Took a minute to figure out it had a different refrence, but then it was also ok at 24V.
4. I then started plugging in additional boards one at a time... T-con first, then master invertor, then slave invertor. Until the invertor boards were plugged in, there were no issues with power supply cycling/turning off/clicking.
5. With those boards plugged in, there were issues...
6. I have not checked out anything on the scope yet. I'm new to TV repair and I'm not sure exactly what to look for. I read on the forum a bit about testing transformers by looking at their ringing/damping output from a square wave generator input. I do have access to one of those, but I'd rather not go that far if I don't have to...

CONCLUSIONS TO DRAW?
1. I think it's most likely a problem with either the backlight invertor cards or the part of the power supply that supplies current to those cards... or both.
2. I suppose it could also be some kind of internal short in the LCD or a failing backlight lamp, in which case this is probably all a waste of time...

MY BUDGET
I'm willing to spend exactly $100 on this project if needed. To speed the diagnosis along, I have already ordered a supposedly working power supply board ($50), a supposedly working remote control ($10), and two supposedly working invertor boards ($12/each). I already spent $5 on a 10 pack of inline 8A fuses for the power supply board to replace the one I blew on accident. If the boards actually don't end up working I intend to ship them back for a refund. I'm also willing to go to the component level, although since some of the caps on the power board are buried under the heat sinks, I'm not sure I really have a power enough soldering iron to access those ones... and may not have time to take the machine gun approach to every cap and diode on all the boards. But definitely the ones on the invertor boards I could try changing out too.

Any thoughts on this before I get in any deeper? I will add images for all the individual boards as soon as I have it apart again.

UDPATE: I added a service manual for this TV to the attachments. It's not terribly useful from what I could see. No board level schematics or anything just a few diagnosis flow charts for a few basic issues.

[Agent24]

Re: Vizio VF550M screen flicker (to black) then eventual shutdown

If a new inverter gives the same problem, then you likely have a backlight issue with bad cable, bad lamp, or arcing somewhere.

This would possibly require disassembly of the panel to replace\repair which is tricky to do without breaking it.

[germaldish]

Re: Vizio VF550M screen flicker (to black) then eventual shutdown

From what I read on other posts, the only thing that makes me think it may not be related only to the backlight is that when the backlight cuts out, it takes the sound with it. Maybe that's normal, I really don't know. Maybe it draws too much current from the power supply board and somehow that also shuts down the sound. But if it is a bad lamp, can you even buy new ones? I know most people have said at that point you may as well throw out the TV... I'm willing to be adventurous here if the cost for a new lamp is low.

[Agent24]

Re: Vizio VF550M screen flicker (to black) then eventual shutdown

Depending on the fault, it could well be shutting the main supply down too. Do you just lose sound or sound and everything else?

Yes, lamps are typically quite easy to buy. It's installing them that's the tricky part. But first you should verify if that is the problem, but since you have ordered new inverters and a power supply, it should be relatively easy to find which is the problem by swapping them around.

[germaldish]

Re: Vizio VF550M screen flicker (to black) then eventual shutdown

I lose sound in the same moments that I lose picture during flickering. Then, with enough flickering, the whole thing goes into a shutdown state and doesn't come back on until I cycle the power.

[Agent24]

Re: Vizio VF550M screen flicker (to black) then eventual shutdown

That is rather odd. Perhaps it is the power supply itself. Either way, post back once you have tested with the new inverters and power supply and the results.

[germaldish]

Re: Vizio VF550M screen flicker (to black) then eventual shutdown

Okay, here are some pictures of all the boards. I didn't notice anything terribly out of the ordinary... but maybe some better trained eyes can see... The new power supply and invertor boards are not yet in, so I haven't really tested anything else other than listening during the flicker a little bit. There was quite a hum coming from the invertor boards, and the large transformer in the middle of the power supply board (near the top) sounded like it was arcing inside... it had a sort of high frequency spark sound; higher frequency then the actual flickering...

[germaldish]

Re: Vizio VF550M screen flicker (to black) then eventual shutdown

So I'm thinking (and hoping) more and more that this is going to be a power supply issue... but in case it's not, I was looking into alternative solutions, i.e. if some of the CCFL bulbs are toast or partly toast. I am putting this information here for information/documentation only. I called this place in Nebraska ((402) 330-2222), http://www.ccflwarehouse.com/lacclaforlcd.html in hopes that they could help out with a few of my questions. They were very helpful in guiding me into how I might select the correct CCFL replacement, but also very skeptical about whether it would be worth the cost. The CCFLs are at least $20 each, and a set of this size probably has at least 24 CCFLs (I haven't removed the panel yet to see). At any rate, I think the power supply is coming in the mail today, so I can rule that out first.

[Agent24]

Re: Vizio VF550M screen flicker (to black) then eventual shutdown

If it was the CCFLs, it's unlikely for all of them to die at once.

[germaldish]

Re: Vizio VF550M screen flicker (to black) then eventual shutdown

I got my new power supply board in the mail today. It looks like that was indeed the culprit in this case. It appears to me that the problem was in the main transformer... on the dying one you could hear arcing coming from the transformer. Not sure if that was the only problem with the board, but that's the one that seems most likely to me.

They jury is still out on whether this fix is truly done, but it has been working great for the last few hours, event with 100% backlight.

Agent24, thanks for you comments!

[Agent24]

Re: Vizio VF550M screen flicker (to black) then eventual shutdown

Good to see you appear to have the problem solved

One thing it could be is cold solder joints on the transformer pins where it connects to the PCB. I would have a look at the joints on the transformer you heard the noise from. Post a macro shot of them if you wish. If you're interested and they look bad, you could try reflowing them and seeing if it fixes the problem.

Otherwise, If there was actually arcing *in* the transformer it would probably due to insulation breakdown, either from age, crap quality insulation, or external damage. It may be possible the PSU was faulty and overdriving the transformer with a higher voltage, but I think this is very unlikely.

Suffice to say in such a case a new PSU would probably be the best you could do anyway - repairing the transformer would likely require expensive rewinding, and good luck finding a replacement one!