Vestel 17PW20 intermittent backlight

[shikuru]

I have a strange problem with a Vestel chassis (Luxor LCD 328). When there is a lot of white in the picture, the backlight will start a very quick flickering. It will recover itself once the picture changes but reappear every time there is a certain amount of white space in the picture.

When the picture drops it will flicker on/off very quickly. It looks as if the whole backlight gets shut off.

Do you have any pointers? Could the PSU be the culprit? Thanks

[tom66]

Re: Vestel 17PW20 intermittent backlight

There are two caps, 15n/630V, near the power FETs and transformer; these can go bad and cause the symptoms mentioned. Strange though, does the backlight brightness perhaps change with scene brightness?

[shikuru]

Re: Vestel 17PW20 intermittent backlight

Thanks tom66. Will check. In the meantime, some additional perceptions:
The big mains capacitor carries a mere 225 Volts. The 24V line carries 23 volts, no more. It goes down to 22.8 after several minutes in operation.

Edit: Nope, the backlight will shut off entirely. Also, with an open case, I cannot seem to reproduce the fault. Perhaps it's bad solder lines. I cannot find 15n capacitors. Do you mean C878 and C877? They are shown as 12n in the schematics? Or perhaps C834 and C835 which are shown as 33n in the schematics?

[shikuru]

Re: Vestel 17PW20 intermittent backlight

Tom66: OK it seems you were spot on. I heated C878 and C877 with a hairdryer and off it went. Then, a short while after it cooled down, all is fine again. I do not detect a voltage drop at the 24V line during the flickering, but I suppose this is because my Voltmeter isn't quick enough to detect it.

Now, with the replacement of those caps, would you recommend to replace any other components while I have the TV open?

Thanks again mate!!!

[tom66]

Re: Vestel 17PW20 intermittent backlight

Tom66: OK it seems you were spot on. I heated C878 and C877 with a hairdryer and off it went. Then, a short while after it cooled down, all is fine again. I do not detect a voltage drop at the 24V line during the flickering, but I suppose this is because my Voltmeter isn't quick enough to detect it.

Now, with the replacement of those caps, would you recommend to replace any other components while I have the TV open?

Thanks again mate!!!

Ah, the earlier revisions used the 12n caps. Newer ones use 15n. Even newer ones use just one 27n/1000V, and leave the other empty.

The failure mode of these caps is to go open circuit. I've never had luck with a hair dryer, so I'm surprised that worked, but good for you.

A repair kit for this PSU is available on eBay.

[tom66]

Re: Vestel 17PW20 intermittent backlight

Thanks tom66. Will check. In the meantime, some additional perceptions:
The big mains capacitor carries a mere 225 Volts. The 24V line carries 23 volts, no more. It goes down to 22.8 after several minutes in operation.

Those rails are too low. According to the service manual, with PFC on the main cap should be 390V. The 24V rail should be 24.5V. The caps may be intermittent.

[shikuru]

Re: Vestel 17PW20 intermittent backlight

The failure mode of these caps is to go open circuit. I've never had luck with a hair dryer, so I'm surprised that worked, but good for you.

I was surprised as well. I used an old C64 to get a white background image to trigger the fault, together with the hairdrier.

A repair kit for this PSU is available on eBay.

Damn, I didn't read your post until after ordering with my component seller of choice. But I suppose the total price is about the same.

For general purposes, here are the components and values contained in the repair kits for the 17PW20.1:

IC829 - 78L05 (if on board)
Q809 - BC337 (if on board)
C877 - 15nF 1600V
C878 - 15nF 1600V
C817 - 220uF 25V
C892 - 100uF 50V
C801 - 33uF 63V
C901 - 3,3uF 50V
R817 - 330 kOhm
R818 - 330 kOhm
R819 - 330 KOhm

Cheers!

[shikuru]

Re: Vestel 17PW20 intermittent backlight

So I've replaced the aforementioned components today. All tested fine in my multimeter, except the 220nF electrolytic capacitor C817. I did not replace the 78L05 voltage regulator as I did not have a spare.

Tom66, should I replace any additional components considering my particular fault? Can I let the 78L05 be, or must it be exchanged. It's pretty close to the broken capacitor on the board...

Thanks

Oh one more thing: There's an error up there. C901 should be 4,7uF 50V, not 3,3 uF.

[shikuru]

Re: Vestel 17PW20 intermittent backlight

Some perceptions after the replacements: The inverter gives a loud buzz, so does the smaller of the transformers on the 17pw20 (the one near the mains input). Everything appears to be working.

What is really worrying me though is that the 2.5 supposed Volts are actually more like 0.5 Volts. I am measuring them on the connectors.

[tom66]

Re: Vestel 17PW20 intermittent backlight

So I've replaced the aforementioned components today. All tested fine in my multimeter, except the 220nF electrolytic capacitor C817. I did not replace the 78L05 voltage regulator as I did not have a spare.

Tom66, should I replace any additional components considering my particular fault? Can I let the 78L05 be, or must it be exchanged. It's pretty close to the broken capacitor on the board...

Thanks

Oh one more thing: There's an error up there. C901 should be 4,7uF 50V, not 3,3 uF.

The 78L05 only affects standby. If the unit appears dead or is intermittently stuck in standby, replace it.

[tom66]

Re: Vestel 17PW20 intermittent backlight

Some perceptions after the replacements: The inverter gives a loud buzz, so does the smaller of the transformers on the 17pw20 (the one near the mains input). Everything appears to be working.

What is really worrying me though is that the 2.5 supposed Volts are actually more like 0.5 Volts. I am measuring them on the connectors.

This is quite normal. You can fix it if you like with some super glue - dab it on L819 (the unshielded coil) and allow a few seconds to dry. They do that when the inductors are manufactured, but these TVs run hot so the glue breaks after a year or so and makes the inductor very audible.

[shikuru]

Re: Vestel 17PW20 intermittent backlight

Thanks about the hint with the coil. A friend has a 78L05 at hand so I am replacing it just to be sure.

Do I need to worry about the 2.5V line being at 0.5V?

[tom66]

Re: Vestel 17PW20 intermittent backlight

Thanks about the hint with the coil. A friend has a 78L05 at hand so I am replacing it just to be sure.

Do I need to worry about the 2.5V line being at 0.5V?

I know of no 2.5V line! () If it works, it works... Sorry to say that but I have no idea. I don't think it is used by most Vestel TVs as they use a 1.8V CPU (forget the name, it's a Micronas part IIRC.)

[shikuru]

Re: Vestel 17PW20 intermittent backlight

I know of no 2.5V line! () If it works, it works...

So that might just be the reason I did not measure 2.5V!!!

I am really discouraged at the moment. The problem did not go away despite my fixing the bad cap. I think for any additional steps I would need an Oscilloscope, which I do not have The original issue still appears. After warming up, the backlight flashes on/off, when there is a lot of white in the picture.

Could this be inverter-related? Does the main logic regulate backlight brightness according to how much white there is in the picture?

[tom66]

Re: Vestel 17PW20 intermittent backlight

So that might just be the reason I did not measure 2.5V!!!

I am really discouraged at the moment. The problem did not go away despite my fixing the bad cap. I think for any additional steps I would need an Oscilloscope, which I do not have The original issue still appears. After warming up, the backlight flashes on/off, when there is a lot of white in the picture.

Could this be inverter-related? Does the main logic regulate backlight brightness according to how much white there is in the picture?

Was it the 15nF/630V caps you replaced? Some other people have also had luck with replacing the big 150uF/450V cap with a 220uF/450V cap (the 220 cap was in the original design, but Vestel reduced it to reduce the cost of the TVs by about 50p.)

If you find it difficult to fix, I -think- the 17PW15-9 PSU is compatible with the 17PW20 (I fitted a 17PW15 for 17PW20 in a Goodmans with no problems), and reasonably cheap (about £20 on eBay.)

[shikuru]

Re: Vestel 17PW20 intermittent backlight

Was it the 15nF/630V caps you replaced?

Yes, but they seemed to be working. I tested with cap meter. So I went and put an entire reliability kit. Only the 220uF was broken (C817). After fitting the reliability kit, the output voltage did increase from 22.3 to 24.3V though.

Some other people have also had luck with replacing the big 150uF/450V cap with a 220uF/450V cap (the 220 cap was in the original design, but Vestel reduced it to reduce the cost of the TVs by about 50p.)

I might try it. I do measure about 339V across its pins though. That should be about right.

I am thinking it might also be the inverter. It does make a loud buzzing noise. And upon thinking about it: When using the hairdryer, hot air also got to the inverter. So what I would really like to know is whether the PSU tells the inverter to shut down, the 24V drop, or if it's only the inverter that is doing it. The flickering also gives out a loud clicking noise, like a relay constantly switching on/off. I wonder what's causing it.

No way to find out without a scope, though, I suppose. The flickering is way to quick for my multimeter to show voltage drop.

[tom66]

Re: Vestel 17PW20 intermittent backlight

I might try it. I do measure about 339V across its pins though. That should be about right.

It should be 390V if the PFC is working. Can you post a pic of the freestanding LM317T on the board? Sometimes this can go bad.

I am thinking it might also be the inverter. It does make a loud buzzing noise. And upon thinking about it: When using the hairdryer, hot air also got to the inverter. So what I would really like to know is whether the PSU tells the inverter to shut down, the 24V drop, or if it's only the inverter that is doing it. The flickering also gives out a loud clicking noise, like a relay constantly switching on/off. I wonder what's causing it.

Could be a cold solder joint.

[shikuru]

Re: Vestel 17PW20 intermittent backlight

It should be 390V if the PFC is working.

I re-checked. It definitely has only 333.9V (@150uF just as you said). It also discharges fully in less than a minute after power is gone.

Can you post a pic of the freestanding LM317T on the board?

Will do, didn't have the schematics at hand today.

Could be a cold solder joint.

Let's hope not. Those are hella hard to detect. It may well be though, since I can make the problem appear/vanish with a hairdryer.

Another question: Shouldn't there be 5V in standby? I get 0.5V on some lines (just as I mentioned) but never 5V. Same when running.

PS. I really enjoy this. Thanks for all the hints and pointers!!!

[tom66]

Re: Vestel 17PW20 intermittent backlight

I re-checked. It definitely has only 333.9V (@150uF just as you said). It also discharges fully in less than a minute after power is gone.Will do, didn't have the schematics at hand today.
Let's hope not. Those are hella hard to detect. It may well be though, since I can make the problem appear/vanish with a hairdryer.

These have common problems with cold joints around the PFC voltage divider resistors and sometimes with the LM317.

Another question: Shouldn't there be 5V in standby? I get 0.5V on some lines (just as I mentioned) but never 5V. Same when running.

PS. I really enjoy this. Thanks for all the hints and pointers!!!

The standby should be present on the single red wire on the left or right, can't quite remember, but if it's not there I couldn't tell you why! Could be the 7805.

[shikuru]

Re: Vestel 17PW20 intermittent backlight

Should not R817-R819 give 620kOhm each, even when measured in circuit (and not overload on my multimeter)?