TLX-04240B Backlights Attempt to Fire Up, Over and Over

[pillit83]

Hello! First time poster, though I frequently lurk around trying to learn things. I have this garbage (in all senses of the word) Polaroid TLX-04240B LCD TV which I would like to get working, with your help.


The issue:
When I turn on the TV the backlights fire up for a split second, then a faint *Click* can be heard and the backlights shut right off (2 seconds to black). However, a second later the backlights attempt to fire back up, *click*, and then shut off again. This process repeats, with the same rhythm, forever or until I press the power button and shut it down.


I swapped out the large and very swollen 220uF 450WV capacitor but the issue persisted. Then I saw one of budm's photos showing what bad ceramic capacitors might look like (dark, tinted color), so I replaced the three 10pF 6kV blue ceramic capacitors next to the inverters, to no avail.

After close inspection in a dark room I noticed the *Click* that precedes the black screen is actually a spark inside one of the small cylindrical white-and-red components next to the inverters. (what are these by the way?)

I'm wondering WHY this TV attempts to re-fire the backlights immediately after failing over and over? Shouldn't it go into some kind of protection or shut-down mode?

Anyway, I'd really like to get this TV working and back together, so any suggestions would be very much appreciated!

Thanks in advance for your help!
Here are some photos of the TV and boards:

[pillit83]

Re: TLX-04240B Backlights Attempt to Fire Up, Over and Over

Also here are the voltages on CON4 and CON3 on the bottom of the power board:

CON4: (TV OFF)------(TV ON)
GND: 0.00v---------------0.00v
EPWM: N/A---------------N/A
BK_ON: 0.00v------------4.54v
FDATA2: N/A-------------N/A
FDATA1: N/A-------------N/A
APWM: 0.48V------------3.08v~3.20v

CON3:
+Vo: 0.13v--------------24.9v
oGND: 0.00v--------------0.00v
GND: 0.00v---------------0.00v
GND: 0.00v---------------0.00v
GND: 0.00v---------------0.00v
+12V: 0.00v-------------12.1v
+5V: N/A------------------N/A
5VSB: 5.09v--------------5.00v
STB: 0.05-----------------4.70v

[budm]

Re: TLX-04240B Backlights Attempt to Fire Up, Over and Over

That white-red cylinder with the two leads are the gas discharge spark gap that will fire when the high voltage exceed the break down voltage of the spark gap rating.
It sounds like there is open circuit some where which the high voltage is feeding into which will allow the voltage to get very high. When the lamps fired up (as high as 2500V) and start conducting, then the high voltage will drop down (to around 1000V or so) into lower running voltage and the current is limited by those blue ballast caps. So look real close at all the lamp connectors and solder joints on the inverter board.
You should check the resistance of all those blue caps, it should show infinite resistance.

[pillit83]

Re: TLX-04240B Backlights Attempt to Fire Up, Over and Over

(speak of the budm and and budm appears!)
Thanks for the response! I have checked each ballast cap on both inverter boards. They all show infinite resistance. (when you say "inverter boards" you're referring to the 2 long boards on either side of the TV, correct? Even though the actual backlight inverter transformers are on the same board as the power supply?)

I also unplugged, sprayed contact cleaner, and re-seated all 20 of the lamp connectors after looking closely for shorts and bad joints.

When I disconnect the 2 side boards from Power Supply/Inverter Board (via the large white connector) and turn the TV on, the inverters still produce the repeated voltage spikes and sparks in gas discharge tubes.. Is this normal? I guess what I'm asking is, does this narrow down a possible short circuit to the Power Supply/Inverter board, or will it do this normally if it's not connected to the lamps?