Vestel 17MB18 Chassis

[JEWilson]

Hi

Been some time
Have been passed a Dual DLCD2001 20 inch widescreen LCD TV for repair.
This experiences loss of backlight but has video and sound.
However, the odd thing is when I change channel or use AV, the backlight
will momentary come back on and go off again
Everything else with this set appears to be ok otherwise.

Anyway, suspected bad caps and found the usual suspects with
bad ESR. These were around the PSU and these have all been replaced
with Pana FM. Additionally, I found both of the only lytic caps on
the standalone inverter pcb to be ok ESR wise but read under 1uF where they when rated at 30V 180uF.
These were replaced with Rubycon YXB 35V 470uF.

The problem persists.
My view is the inverter may be ok as opposed to complete loss of backlight
as when a channel is selected from the remote, the channel signal will
appear with the backlight on. Note, I do not have this connected to
any terrestrial or sat. TV source but have used it with a scart lead and
a DVD player.
Same issue, get video and then the backlight goes out.
If I then change a channel, will get the selected channel then loss of backlight again.

I am beginning to suspect this is a logic problem.
So.. to look into this, I got the service data for the chassis.
All the rails for analogue and digital appear ok and nominal.

The backlight is turned on by a signal (pin60) from a Pixelworks PW1306
as well as receiving a PWM signal from same (pin 260).
I can't seem to get a datasheet for this part (anywhere) and y, I
have sent an eMail to PixelWorks.
These are fed into the non-inverting inputs of a dual voltage comparator
LM373. The 12V supply (separately derived) for the inverter is present and
correct.
The outputs from the LM373 are also fed to the inverter as well as being
fed back to the inverting inputs respectively.
The BLKON signal is 3.2 or so V
The PWM signal is 15.2mV

I am unsure if this inverter board is at fault. I got another but have
from fleabay but have the same issue. This could also be faulty.

The inverter is a Sampo LVE0321 Rev .01
I can't get service data for this either.

This is a 6 CCFL model.
I have checked the fuses - these are ok
I have checked the resistance of the CCFL connectors on the
inverter pcb. These are all 1.9kR. Assume these are ok.

Next steps, get CCFL tube or tubes to eliminate whether (not) the
tubes or tube connections are at fault (I doubt it as a change of
channel brings the backlight back on again) in the panel.

Any suggestions?

[PlainBill]

Re: Vestel 17MB18 Chassis

You are making good progress on your own. I only spotted one erroneous assumption. You are experiencing 'Two seconds to black'. It occurs when the inverter controller senses a fault condition such as over voltage, over current, etc. When the fault is sensed, the inverter controller shuts down the drive to the inverter transformers. They stay shut down until the power to the controller is cycled (off, then on) OR the BL_ON signal is cycled. IF the video processor cycles the BL_ON signal the inverter will restart and you will see a picture until the fault occurs again.

I'd suspect a bad CCFL, or connection to a CCFL triggering an 'open lamp' fault.

PlainBill

[cadiman]

Re: Vestel 17MB18 Chassis

I would agree since the fault is attempting to reset. I have done this manually by removing the 3volt signal feed (some units) to check the inverter turn on circuit.

The fun part is a faulty inverter will give the same symptoms as a bad ccfl.

I have had tvs with a bad inverter caused by bad ccfls. (only takes a couple or three) or in your case one.

Easy pickens method says it is easier to replace the inverter than to take apart the panel

I have since made up a test inverter for this purpose.

[JEWilson]

Re: Vestel 17MB18 Chassis

Thanks for the info

Have advised client, it would be best to prepare an
insurance report as unit is beyond economical repair.
I don't want to waste money stripping this unit to
replace lamps. It has damage to the LCD panel
(a scratch) and would realise little (if any) resale
value.

This is a really cheap TV. Client should be able
to get replaced with contents insurance.

May use parts for the purpose of testing CCFLs etc.

Thanks again

[tom66]

Re: Vestel 17MB18 Chassis

Whilst it could be a backlight problem, wouldn't rule out the power supply.

These Vestel units have major problems with two polyester capacitors near the main transformer bulging (on the power supply.) 15n or 12n/630V each. Like electrolytics fail but caused by other factors and a rare sight, only seen on cheap supplies. These can cause intermittent/flickering backlight, poor sound or stuck in standby symptoms. This only applies for 17PW20, 17PW15 and similar supplies.