Coby TFTV3227 Takes Time to "Warm Up"?

[Maxstang]

Hey Guys.

I have a Coby LCD TV that starts fine, but has no picture (backlight) for the first 10 minutes or so. Eventually it "warms up" and displays beautifully. I figured it must be dying caps (although the TV is only a year old) but looking at the power supply, they all look good. I don't see any cold solder etc. Anybody have any ideas where to check first? Here's the power board.

[tom66]

Re: Coby TFTV3227 Takes Time to "Warm Up"?

Do you have a digital multimeter available?

[Maxstang]

Re: Coby TFTV3227 Takes Time to "Warm Up"?

Yep got a multi. Thanks Tom. I'll rig it back together so I can test some voltages. Dang Coby TV is REALLY hard to work on as everything is mounted to the plastic back of the case component side DOWN.

What should I be looking for?

[tom66]

Re: Coby TFTV3227 Takes Time to "Warm Up"?

Just check the outputs from the power board. If they're out of whack (usually labelled, but common voltages are 5V and 12V) then it is probably a power board issue.

[budm]

Re: Coby TFTV3227 Takes Time to "Warm Up"?

Check the DC voltage right at the two legs of that large 450vdc cap laying side way, it should have 370~390VDC when you activate the power switch. Be careful not to touch anything with your hands.

[Maxstang]

Re: Coby TFTV3227 Takes Time to "Warm Up"?

Sorry for the wait. Too many soccer games...

Well this is interesting. I got it all back together, and it started right up...repeatedly. I can't get it to display the "warm up" behavior again. I'm thinking cold solder joints now. I must have moved something when I disassembled it. I'll go over the board and see if I see any cracks etc.

I did test the voltages for future reference:

Large cap reads 169V at standby, 392 when TV is on. Voltage is steady.

This board has five main lines:

The 12V lines both read 12.2-12.28V. The readings wandered constantly. I guess it's unregulated?

The two 5V lines were 5.06V, very steady.

The standby voltage was 5V.

Interestingly, with this board, only the standby voltage is on when the TV is on standby. The 12V and 5V lines are at 0V.

[budm]

Re: Coby TFTV3227 Takes Time to "Warm Up"?

That is correct, when TV is off, the 12v/24v are turn off to save power, only the standby power is on to run the main board. You can read basic guide under my signature.

[Maxstang]

Re: Coby TFTV3227 Takes Time to "Warm Up"?

Thanks Bud. I have a samsung where some of other lines are still active at idle (13V). Perhaps that's what wrong with it!!!

Here are some photos of solder joints on the power board. I'm surprised how many look like cold joints. I sucked and resoldered basically all the major components on the board. The worst looking ones were in the backlight headers and transformer. I've circled a few.

CFL Headers




One of the Mosfets



I'm watching it right now. Seems to be working. I'm gonna let it sit on standby and occasionally turn it on in the next 24 hours as a burn in.

[Maxstang]

Re: Coby TFTV3227 Takes Time to "Warm Up"?

Well it appears I spoke too soon. I'm getting video artifact and loss of signal now. I'll post a short video when I get time later today.

[Maxstang]

Re: Coby TFTV3227 Takes Time to "Warm Up"?

Hmm.. I'm trying to get a video of the problem, but it's very intermittent. I get colored lines across the video, both vertical and horizontal in many different widths and colors as well as video stutter and complete loss of video, but not the backlight. Looks to me like I have a problem with one of my 5V lines... I need to get it to do it consistently and measure the voltage outputs when it's happening. Right now it looks great...

[Maxstang]

Re: Coby TFTV3227 Takes Time to "Warm Up"?

Here it is. I can get it to behave this way by leaving it off but on standby for a while. When I turn in back on, it displays this behavior for a few minutes and then the picture stabilizes and its fine after that. I'll take it back apart and test the voltages on start up and post them.

http://s122.photobucket.com/user/max...61520.mp4.html

[Maxstang]

Re: Coby TFTV3227 Takes Time to "Warm Up"?

I should have mentioned that the backlight doesn't flicker. Just the video.

[Maxstang]

Re: Coby TFTV3227 Takes Time to "Warm Up"?

OK. Pulled it apart and tested the power supply during the flicker. As I noticed earlier the 12V lines both wander from 12.24V to 12.29 and when they do, I get flicker and artifacts on the screen. The voltage swings settle down after a few minutes and this corresponds to when the picture clears up. The 5V lines are rock solid. I also checked the voltage on the big cap for fluctuations, but didn't see any.

SO....are the 12V lines fluttering because the regulations sucks, or because the main board is drawing power erratically? Anyone?

[truep]

Re: Coby TFTV3227 Takes Time to "Warm Up"?

Where are you located? I sold a Sanyo to a guy a couple of years ago that displayed similar symptoms. I tested it at my house, and it worked fine, even after being outside on a 100 degree day or inside at 75.
I took it back to him and it started flickering. It turned out his ac power would drop to 94V whenever his AC kicked in!!

[budm]

Re: Coby TFTV3227 Takes Time to "Warm Up"?

"12.24V to 12.29" that is nothing to be concern, you may have to look at it with the scope to really see what the variation is on the waveform. That 12V may be used on the T-CON board. 12V is usually for the Audio circuit, the 12V may be fed to the T-CON regulated power supplies on the T-CON. I would try cooling and use the hair dryer for heating the T-CON board to see what happen.

[Maxstang]

Re: Coby TFTV3227 Takes Time to "Warm Up"?

Good suggestions!

Thanks Bud. I'll give that a try. This TV has the worlds smallest TCON board. It's only 2x2. I'm wondering if it actually uses the 12V at all. Here's a pic.

[budm]

Re: Coby TFTV3227 Takes Time to "Warm Up"?

Holy Cow!

[tom66]

Re: Coby TFTV3227 Takes Time to "Warm Up"?

I've seen similar boards. One was marked "AU OPTRONICS TEST BD"; I'm guessing it wasn't intended for production, but it worked, so it ended up in the final product.

It's just translating some of the LVDS signals. The tab drivers do most of the work.

[budm]

Re: Coby TFTV3227 Takes Time to "Warm Up"?

There may be another long board on the LCD panel assembly for the Gamma voltage ref generator and buffer and some switching PS. 32" TV with one Inverter transformer!

[Maxstang]

Re: Coby TFTV3227 Takes Time to "Warm Up"?

There may be another long board on the LCD panel assembly for the Gamma voltage ref generator and buffer and some switching PS. 32" TV with one Inverter transformer!

There's another small board visible inside the LCD display, but I don't think it's accessible. The LCD is in a plastic case! There doesn't seem to be a 12V path to it anyway as the ribbon cable from the T-CON is the only thing that connects to it. I'll try the freeze spray on the T-CON and on the main board and see if I can make it do its thing...