Sony Bravia LCD KDL-32L4000

[el3ctroded]

Re: Sony Bravia LCD KDL-32L4000

You may have one of a couple of problems: CCFL tube gone bad. PSU/Inverter gone bad.

Most likely it's the PSU. You didn't mention if it's generating high voltage for the CCFLs...

You need a cap ESR tester to test the caps in the PSU. They often are NOT visibly bad but reguardless have failed.
You should also test all transistors for shorts.
Many PSUs have a switch that when switched will cause it to ignore the turn-on signal lines such that it will simply turn on. This may help you diagnose the problem.

[Kevin O'Henly]

Re: Sony Bravia LCD KDL-32L4000

el3ctroded & Fred,

Agreed. If one or more ccfl's fail then the inverter will fail as it is supposed to do.

Would it not be great if we could test the PSU (board-g) and CPU (board-a) without the panel inverter connected ? This would rule-out 1/2 to 3/4 of the LCD faults in one simple test.

So the question is; With a properly functioning Bravia LCD can the PSU cable to the inverter board be removed and still have the PSU come out of standby with board-a still connected?

I think so but I do not have a working set to prove it.

This is how the inverter/ccfl rule-out test may work?

"PANEL_STATUS" (board-a, cn2, pin 1) appears to be an input to the board-a CPU from the panel inverter. My guess is that this signal is the inverter-OK signal and is probably active low. This signal should float above ground if the board-a CN2 connector is disconnected.

So, with board-g, CN203 disconnected from the inverter board and board-a, CN2 disconnected from the inverter board then I suspect that the PSU (board-g) would come out of standby when the set is powered on.

If this is so then the panel CCFL inverter can easily be eliminated as a suspect.

Is someone in a position to check this? Fred?

[el3ctroded]

Re: Sony Bravia LCD KDL-32L4000

Fred: If you decide you don't want to work on it, please don't trash it, instead PM me.

Kevin: You can test the CCFLs without a working psu or inverter. There's a tester out there for that. I haven't bought one yet as I think you could make one fairly easily from a junked small PC Monitor, bypassing all control circuits. Here's the link to the one for sale: http://store.lcdparts.net/LST01_p_2699.html

Don't assume the inverter control signal line is active low... It very well could be active high. It may pulse for a second at initial plug-in or when the power button is pressed, so with a scope you could get a better idea of what's going on. You could maybe see it happen with a fast multimeter too.

[Kevin O'Henly]

Re: Sony Bravia LCD KDL-32L4000

Cool link. THX.
The point about signal dynamics is also useful.

Is there a general method for fixing a set?

If so then it may go something like this?

~ PSU stand-by power available? NO - then fix/replace PSU
YES
power on the set

~PSU comes out of stand-by? NO - is "PS-ON" signal high?
YES
No picture? NO - check ribbon cables and inverter/CCFL, spend
some time with the good people at Badcaps.

...something like that.

What I suggested was a step to isolate the fault.
I can trick the inverter to illuminate all the working ccfl tubes when one or more are bad. That would isolate the bad ccfl.

What I was interested in was a SIMPLE method of isolating the fault in or out of the panel/ccfl-inverter.

ie. pull two cables, power on the set and if PSU is out of standby then it's a panel/ccfl-inverter issue; something like that.

Worth a try or no?

[el3ctroded]

Re: Sony Bravia LCD KDL-32L4000

There's no reason you can't do what you're asking, and you've got the basic flow right, except:
Having standby voltage doesn't mean the PSU is good. It just means that rail is working.
At the end, you say "No Picture". Unless your inverter is working and you forced it to turn on there won't be a picture. Also, that No Picture could also be a voltage rail on the PSU not working, though it likely won't stay on if a rail has failed.

If you can't get the PSU to kick on, there could be 2 things wrong: 1) PSU bad and self-sensing a fault. 2) Main board not kicking the PSU on.

If it's a plasma, the same theory applies. Disconnect the Ysus and Zsus boards and then force the PSU to kick in and test all rails.

[Kevin O'Henly]

Re: Sony Bravia LCD KDL-32L4000

Cool,
Is it fair to say that until the PSU has been eliminated as a fault then the PSU is suspect; let's say, "Guilty until proven innocent?"?
Is it also fair to say that if a PSU has the stand-by rail up and "PS-ON" is active (high or low as the case may be for that particular design?) then a properly working PSU will come out of standby with all the rails “on”; ie. powered-up?
If all of this is fair to say then the next issue to tackle is PSU “loading”.
It has been my experience that a properly working modern design of SMPS does not need a load to come out of stand-by with all the rails “up”. My practical experience is very limited and so I am asking.
Only with the proper load though can we eliminate the PSU as the fault. “Acquitted”, we might say.
If this is the case then it may be fair to say that the PSU can be tested and repaired separately with no load. The basic PSU function can then be verified and then the PSU can be returned to a “loaded” state to complete the PSU testing, generally speaking?
The CPU (board-a processor in this case) will sense the state of the PSU, maybe the inverter state and device-on request from the user and set “PS-ON” to the active state ( in this case high) if all is well.
The inverter will shut itself off as will the PSU if internal fault conditions exist.
Are we on the same page?

[el3ctroded]

Re: Sony Bravia LCD KDL-32L4000

Cool,
Is it fair to say that until the PSU has been eliminated as a fault then the PSU is suspect; let's say, "Guilty until proven innocent?"?
Is it also fair to say that if a PSU has the stand-by rail up and "PS-ON" is active (high or low as the case may be for that particular design?) then a properly working PSU will come out of standby with all the rails “on”; ie. powered-up?

Yes and Yes, for those I've worked on.

It has been my experience that a properly working modern design of SMPS does not need a load to come out of stand-by with all the rails “up”. My practical experience is very limited and so I am asking.

Correct. As for load testing: From what I've experienced thus far working on TVs (which isn't a lot) and amps (a bit more) it's rarely necessary for testing a SMPS.

The CPU (board-a processor in this case) will sense the state of the PSU, maybe the inverter state and device-on request from the user and set “PS-ON” to the active state ( in this case high) if all is well.
The inverter will shut itself off as will the PSU if internal fault conditions exist.
Are we on the same page?

Yes

[Kevin O'Henly]

Re: Sony Bravia LCD KDL-32L4000

Cool,

...on the same page then.

Still with the power supply board...

I know that with the smaller (15 in. to 22 in. ) LCD PSUs it does not seem to matter what value of capacitor you stick back in to the PSU circuit as long as the capacitance value is higher than the one you pulled AND the replacement capacitor has a voltage equal to or higher than the one you are replacing.

For example; I have replaced a bulging 820 micro-farad capacitor @ 25V with a 1000 micro-farad capacitor @ 35V; no problems. I have replaced a 630 micro-farad cap. with 1000 micro-farad cap.; no problem. Works like a charm.

Is this your experience also?

If the cap. is used for filtering or timing, of course this will not work, but if the cap. is just storing charge then the value does not seem to matter that much!

I have not dared to try this with higher current devices and I do not recommend this substitution for anyone that does not know what to expect.
ie. an exploding cap. is messy and loud and potentially dangerious.
The bigger they get the louder they boom.

[el3ctroded]

Re: Sony Bravia LCD KDL-32L4000

Using a higher voltage is always Ok so long as it fits and doesn't negatively affect ESR of the cap. Using a higher capacitance of up to 20% is almost always ok in the output of a PSU. The main concerns is that the ESR is low enough and capacitance high enough to provide sufficient filtering of any ripple current caused by the SMPS.

Filtering this ripple generates heat in the caps. So a larger physical size cap can be beneficial to dissipate that heat. In any area where caps fail due to bulging, I usually use larger size, lower ESR, and/or higher voltage caps than what was originally there IF they will fit. And generally I use Nichicon or Panasonic low ESR caps.

[Kevin O'Henly]

Re: Sony Bravia LCD KDL-32L4000

cool.
Very useful insite into the cap. substution issues.
THX, el3ctroded.

I have a home brew ESR using a scope and signal generator.
This "home brew" method requires good working caps to compare to the cap under test. This method works well and can often identify a bad cap incircuit, though it is baet to pull the cap out of the circuit for repeat testing.

...are we ready to plow into an actual defective PSU from the Sony Bravia LCD KDL-32L4000? I have standby Vdc and that's about it.

[el3ctroded]

Re: Sony Bravia LCD KDL-32L4000

Sure. Using a dmm, check for shorts on all transistors, especially the large ones.

Next, use diode test to check for all diodes to make sure they haven't failed. They will conduct both directions if they failed.

Carefully look over the board for hot spots, the board will be discolored. Check both sides.

[Kevin O'Henly]

Re: Sony Bravia LCD KDL-32L4000

Sorry for the long time to re-post on this thread.

I eventually fixed the power supply and since I did not have all the parts to fix this set I then sold the set for parts.

A big Thank You to all that contributed.

[Cryseth]

Re: Sony Bravia LCD KDL-32L4000

This thread might be a bit old but, since I'm pretty much confused and don't have the funds or skills, I'd like to know what you did to fix it? (If you remember that is...)