Philips LCD 32PFL6305 inverter sometimes hard to start

[PowerAmpFreak]

Hello!

I have a Philips LCD model 32PFL3605 which the inverter block sometimes refuse to start. Or better described; the inverter gives a shot but right away shuts itself off. It seems to have nothing to do with cold/warm starts. More like bad contact somewhere, which makes this hard to cure.
I've checked the pcb in a microscope without seeing anything abnormal.
Something tells me this is design related and there might be a service bulletin out somwhere to cure this with some modification.
It can start 50 times in a row. Next time it won't. It may be hard to start one time or 25 times. Bla bla bla
Anyone has an idea or familiar with this?

EDIT: Got the model number wrong in the subject line. Correct model is 32PFL3605

[ben7]

Re: Philips LCD 32PFL6305 inverter sometimes hard to start

The ccfl(s) could be going bad. Did you check for bad capacitors?

-Ben

[PowerAmpFreak]

Re: Philips LCD 32PFL6305 inverter sometimes hard to start

Checking bad caps first thing! Though it doesn't behave like a classic cap failure this one obviously.
More like an over sensitive protection kicking in.

[PowerAmpFreak]

Re: Philips LCD 32PFL6305 inverter sometimes hard to start

This is driving me insane very soon. I begin to believe the devil himself has moved in to this Philips, really. I made a very close inspection of the powersupply board. The PFC, and the SMPS are working properly at all times. What happens is, when set is turned on - the inverter tries to ignite the lamps, and stops. The inverter is using a controller IC called UBA2071 ()
This can happen 50 times in a row, or 1 time. If the set start - it will be on without problems. It appears the initial ignition of the lamps cause the problem. Then it can work properly more than 50 times.
If the lamps were bad, it shouldn't work so many times in a row, right?
It doesn't matter if heating the board with a hot air gun, it's not logical in any way.
I still suspect there is over sensitivity in either the overvoltage or overcurrent protection. I would really need your help guys!

[ben7]

Re: Philips LCD 32PFL6305 inverter sometimes hard to start

Whats the voltage the inverters run off of? If it is low, the inverters will draw excess current to keep the power output the same.

Also, if the circuit is poorly designed, it could be oversensitive about the aging ccfls.

What brands of capacitors are inside? Have you checked all of the ones on the inverter board(s)?

[PowerAmpFreak]

Re: Philips LCD 32PFL6305 inverter sometimes hard to start

The inverters run directly from the PFC at about 390VDC primary referenced. The inverter transformer then provide the galvanic isolation. This is a not a so common design but very efficient in terms of power dissipation. The inverter is part of the powersupply board.
I have a feeling about oversensitive protection, but what happens with aging ccfls? Could these false trig the overcurrent protection?
The caps was first thing to check and they are even replaced.

[ben7]

Re: Philips LCD 32PFL6305 inverter sometimes hard to start

The inverters run directly from the PFC at about 390VDC primary referenced. The inverter transformer then provide the galvanic isolation. This is a not a so common design but very efficient in terms of power dissipation. The inverter is part of the powersupply board.
I have a feeling about oversensitive protection, but what happens with aging ccfls? Could these false trig the overcurrent protection?
The caps was first thing to check and they are even replaced.

Wow, didn't expect them to be powered off that high a voltage!

As the bulbs age, it becomes harder to strike an arc inside; so more current is needed to output a higher voltage.

Does the inverter board have a different circuit layout than in the datasheet?
If so, we will need to either find the schematic for the board, or take some pictures of the board and trace out some of the circuits.

[PowerAmpFreak]

Re: Philips LCD 32PFL6305 inverter sometimes hard to start

I have the schematics and I must say they followed the application datasheet very similair. The controller IC UBA2071 does have a lot of internal protection features, this is combined with some external circuitry which sense the lamp voltage and so on, probably as a form av extra safety. I disconnected the external protection and it didn't help. So it appears the IC's internal protection kicks in. So this boils probably down to either overvoltage or overcurrent protection.

[ben7]

Re: Philips LCD 32PFL6305 inverter sometimes hard to start

I have the schematics and I must say they followed the application datasheet very similair. The controller IC UBA2071 does have a lot of internal protection features, this is combined with some external circuitry which sense the lamp voltage and so on, probably as a form av extra safety. I disconnected the external protection and it didn't help. So it appears the IC's internal protection kicks in. So this boils probably down to either overvoltage or overcurrent protection.

Hm, the mosfets are connected directly to the chip, I wonder if one of them has become leaky, tripping the protection.

If you disconnect the voltage or current sensing, of course the chip won't work! It will think there is a problem and it will not light the bulbs.

Did you measure the voltage at the chip? it should be getting about 12v

OOO, it says in data sheet that it has ignition failure protection, maybe there is a way to change that so it will not be so sensitive. There is a capacitor called CT, its used to set the amount of time before the chip will shutdown if there is a fault. Of course we don't want to make it very large (danger of blowing things up when there is a fault) though!

Can you post your schematic so we can direct you to check certain components?

[PowerAmpFreak]

Re: Philips LCD 32PFL6305 inverter sometimes hard to start

You misunderstood me. What I was saying was I disconnected the external protection circuit - and it still behaves the same way, can work 50 times, then not ...
The chip has an bidirectional port called NONFAULT which when is pulled low by external circuitry goes into shutdown mode. This is also pulled low by the chip itself during a fault condition. (after the time set by CT)
So it must be the internal protection, since it all what is left.
I will try to increase that value of CT a bit.
Schematics attached.

[ben7]

Re: Philips LCD 32PFL6305 inverter sometimes hard to start

okokok,

So, how about the HV caps on the secondary? They might be internally arcing sometimes, causing the thing to shut down.

[PowerAmpFreak]

Re: Philips LCD 32PFL6305 inverter sometimes hard to start

The HV caps are not replaced, but it's sure worth trying!
I would expect to see some problems during operation aswell, in that case though..

[ben7]

Re: Philips LCD 32PFL6305 inverter sometimes hard to start

The HV caps are not replaced, but it's sure worth trying!
I would expect to see some problems during operation aswell, in that case though..

You would, but actually the highest voltage they see is when starting the arc in the bulbs!

Also, any of the (HV???) diodes could be internally arcing too, these kinds of things are hard to troubleshoot unless you hear the arcing or see it.

[PowerAmpFreak]

Re: Philips LCD 32PFL6305 inverter sometimes hard to start

I now added 100nF in parallell with CT, which would give some higher tolerance to protection modes. Numerous startups, not been able to see the failure again.
Hopefully this small modification will help the thing to start up.
Because of the nature of the problem I cannot yet say if this has cured it or not. Hopefully......

[PowerAmpFreak]

Re: Philips LCD 32PFL6305 inverter sometimes hard to start

Gahh! Nope - the beast did not turn on the 30th time of startup....
The 31th time it DID start up. Completely weird, something is really on the edge here..

[RON_CET]

Re: Philips LCD 32PFL6305 inverter sometimes hard to start

maybe this might help with your issues see attached

[PowerAmpFreak]

Re: Philips LCD 32PFL6305 inverter sometimes hard to start

Great Ron! This is exactly the kind of bulletin I was looking for. I will rework the psu according to instructions. Will let you know the result! Again Thanks!

[budm]

Re: Philips LCD 32PFL6305 inverter sometimes hard to start

Boy Ron, you sure do have a lot of great service info, do you mind if you can upload them to TOM's server?
https://www.badcaps.net/forum/showthread.php?t=21188

[RON_CET]

Re: Philips LCD 32PFL6305 inverter sometimes hard to start

your welcome PowerAmpFreak and budm i'l be glad to upload that information to Tom's server will do that tonight from home

[budm]

Re: Philips LCD 32PFL6305 inverter sometimes hard to start

Thanks Ron, that will help out so many people around the world that come to this forum. Glad to have you here.