Telefunken d48u300n4cw with sound and picture but no backlight

[vigorcent]

Hey there everyone,
so I was trying to repair my grandpas TV, but I came to my limits. I only have basic knowledge and
I'm still a novice, but it actually was kind of fun and I learned a lot this far.
As this forum helped me already, I finally decided to make my own thread and I hope someone of you can help me here.

So first, the models are:
TV: Telefunken d48u300n4cw
PSU: Vestel 17ips20-r9 23253538
Mainboard: Vestel 17mb100

I don't really know which number on the mb is relevant for further specification, so I will attach
some pictures with all the numbers.

What is the problem:
I do have sound and I can also see the menu while using a flashlight, but there is no backlight at all.
In the beginning when I turned on the TV there was a very short flash, but after a while this flash disappeared.

What I did:
First I checked for bad caps and as far as my amateur eye can see, they all seemed to be ok. Then I opened the panel and checked every LED. There was one single LED not lighting up, so I thought
that I have found the culprit. After replacing the strip however, nothing has changed. So I bought a
replacement PSU and replaced it (from here on, I will just call them "old" and "new" PSU). Now the initial flash when starting the TV is back, but still the same problem, so I guess it could be the main board?
It was very hard to find the schematic for my PSU model. The best I could find was this from another thread:

https://www.badcaps.net/forum/attach...5&d=1603209027

At the very last page, it says for the CN4
1) +24V_AUDIO
3) ST_BY
5) BL_ON-OFF
7) PWM_DIM
9) +5V_STBY
11) +12V_STBY_MB
2) 2D-3D OPS.
4) GRND
6) GRND
8) GRND
10) +12V_STBY_MB
12) +12V_STBY_MB

Here is what I measured (first value: mb not connected, second value: mb connected and TV on, third value: mb connected and TV off):
1) 24.4V ... 24.8V ... 0V
3) 0V ... 0V ... 2V
5) 0V ... 3.2V ... 0.2V
7) 2V ... 2.6V ... 0V
9) 0V ... 0V ... 0V
11) 11.9V ... 11.8V ...11.9V
2) -1.7V ... 0V ... 0V
4) GRND ... GRND ... GRND
6) GRND ... GRND ... GRND
8) GRND ... GRND ... GRND
10) 11.9V ... 11.8V ...11.9V
12) 11.9V ... 11.8V ...11.9V

Also, there are two "empty" pins on the main board between pin 9 and 11, as well as 10 and 12 which show 11.8V (turned on) and 11.9V (turned off).
There is one thing to mention: Whenever I measure pin 9 (+5V_STBY) on the new PSU when the TV is on, my multimeter shows 0.5V, and it quickly drops to 0V.
The schematics I posted above show that this pin leads to Q212 but when checking the PCB it seems to not be connected to anything on this PSU model.
So my guess is that the main board is actually managing this (and possibly the backlight as well?).

Then I also measured the pins leading to the LEDs (see attached pictures).
When I turn on the TV with the new PSU and the flash occurs, the voltage peaks at about 140V for the red and brown cable and 60V for the orange and black cable.
They then slowly decrease to
orange: 8.1V
red: 78.2V
black: 7.8V
brown 78.2V
within about a minute and then steadily stay there. When I turn the TV off, orange and black drop to 0V and 1V in about 30 seconds. Red and brown slowly drop to 0V in about 5 minutes, while black rises to 1.7V.
The old PSU does not peak to 140 and 60V (because there is also no flash) but immediately stay at about 79.2V for red and brown, 8.8V for orange and 3.9V for black.

All measurements for the LED pins as well as CN4 were taken by placing my black lead on the chassis and red lead on the pins.

My questions right now are:
1) Even if the PSU is not responsible for the TV having no backlight, there must be something wrong with my old PSU, right? I want to know this,
because otherwise I can send the new one back within 14 days and save some money.
2) I do get 3.2V for the BL_ON-OFF pin and 2.6V for PWM_DIM pin, but how do I find out what is needed here?
3) I don't get any voltage at the +5V_STBY, but as far as I understand, the main board is responsible for this. Am I right with this assumption?
4) What would be the next step? Any ideas?

Any help is much appreciated, and sorry for the long text. I hope I didn't write too much unnecessary stuff.
As I said, I just want to repair my grandpas TV if I can and learn as much as possible along the way.

[dick_barton]

Re: Telefunken d48u300n4cw with sound and picture but no backlight

The mainboard service manual 17MB100 is available from


Test the voltages on U45 (TPS54528) pins 1 to pin 8 as shown on page 66 of the manual.

[vigorcent]

Re: Telefunken d48u300n4cw with sound and picture but no backlight

First of all, thanks for helping me!
So I just tested the voltages and they show:
1) 11.5V
2) 0.8V
3) 5.4V
4) 5.4V
5) 0V
6) 5.1V
7) 10.3V
8) 11.8V

Also, thanks for posting the link to the manual.

[dick_barton]

Re: Telefunken d48u300n4cw with sound and picture but no backlight

From your readings given above, Pin 6 shows 5.1V which is the 5V_STBY voltage that should be fed back to the power supply on Pin 9 of CN4 (if the power supply schematic is correct).

Are the two white smd components fuses? (Page 65)

[vigorcent]

Re: Telefunken d48u300n4cw with sound and picture but no backlight

Yes, they are fuses and they seem to be ok (0 Ohm on both of them). I posted a screenshot where they are located in the schematic on page 66 (FS1 and FS2).

I'm not sure if the PSU schematic I posted above is consistent with my 23253538 model. Because pin 9 seems not to be connected to anything on the PSU. It only leads to this J57 bridge (see pictures), which is not connected.

[dick_barton]

Re: Telefunken d48u300n4cw with sound and picture but no backlight

It looks like the power supply does not provide the 5V_STBY but this is generated by the mainboard by U45.
What component is soldered next to J57 ?

[vigorcent]

Re: Telefunken d48u300n4cw with sound and picture but no backlight

That's exactly where Q212 would be. I actually tried to find this location the other day.

[dick_barton]

Re: Telefunken d48u300n4cw with sound and picture but no backlight

Reading through the thread again I think I have gone down the wrong path.

You say you have sound and a picture but no backlight. What voltage do you have at the led connector when you switch on. It should peak very quickly and then drop just as quickly.

Since the backlight (BL_ON-OFF) voltage is present, it is starting to look as though there's a problem still with a led(s) or the connections between the led strips.

[vigorcent]

Re: Telefunken d48u300n4cw with sound and picture but no backlight

No problem, I still appreciate that you're helping me.

So I do have two PSU right now. When using the old one, there is no backlight flash anymore, so the voltage doesn't peak like you said and only stays roughly at 80V for the red and brown wires and at around 8V for orange and 4V for black.

When using the new PSU, there is a flash and the voltage peaks at this value -> and then slowly drop to this voltage (takes about 20 seconds if I remember correctly):

orange: 60V -> 8.1V
red: 140V -> 78.2V
black: 60V -> 7.8V
brown 140V -> 78.2V

Considering that I just bought the second PSU from a replacement store (and considering that they didn't sell me a faulty one), my guess is that the error is either on the main board or LEDs. But I checked all the LEDs before -> found one that didn't light up -> replaced the strip -> nothing changed.

Maybe I should check the LED strips as a whole, since I only checked them individually before?

[vigorcent]

Re: Telefunken d48u300n4cw with sound and picture but no backlight

Since the backlight (BL_ON-OFF) voltage is present, it is starting to look as though there's a problem still with a led(s) or the connections between the led strips.

Ok, I will check them again later and let you know. How do I check the connection exactly? Just looking for continuity at the connectors between the strips?

[dick_barton]

Re: Telefunken d48u300n4cw with sound and picture but no backlight

When using the new PSU, there is a flash and the voltage peaks at this value -> and then slowly drop to this voltage (takes about 20 seconds if I remember correctly):

orange: 60V -> 8.1V
red: 140V -> 78.2V
black: 60V -> 7.8V
brown 140V -> 78.2V

The 140V -> 78V is exactly what happens when there is an open circuit either in one of the connectors between the led strips or when faulty led(s) are present.

The fact that the voltage peaks would indicate that power supply is working correctly.

[vigorcent]

Re: Telefunken d48u300n4cw with sound and picture but no backlight

Ok, I just checked them again and everything seems fine. Since I'm by far no professional, I'll describe what I did and maybe you can tell me if I did something wrong.

First of all, there are two separate groups. Each group consists of 2x11 leds and 1x12 leds. Each strip is also separated into sub strips (left and right).
The wires are as follows:
First group: red = led+ / orange = led-
Second group: brow = led+ / black = led-

1. I checked continuity between each pin on the psu to the strip connector on the side of the panel. Everything seems fine.
2. Checked continuity between each strip according to their group.
3. Checked continuity between left and right sub strip.
4. Applied 33V to the strips with 11 leds and 36V to strips with 12V. Everytime the whole strip lid up (left and right). All leds seemed to be fine.

One thing I found out is that both led+ pins are connected with a bridge (S530, see picture below) but I think that's how it supposed to be.

[vigorcent]

Re: Telefunken d48u300n4cw with sound and picture but no backlight

So, as you said: BL_ON-OFF voltage is present, but how do I know how much voltage I actually need and if the 3.2V I get are ok? I can't really deduce it from the schematics.

[dick_barton]

Re: Telefunken d48u300n4cw with sound and picture but no backlight

If you do a search on the forum for the MP3394, the voltage given for EN(able) is in the order of 3.2V.

What dc voltage do you have between the two legs of C534 which should be around 380V so do not slip with the meter leads.

What dc voltage do you have on Pins 15 & 16 of MP3394

Have you checked the mosfets Q510 and Q511

Running out of ideas so if anyone else can offer assistance I am sure it would be appreciated.

[vigorcent]

Re: Telefunken d48u300n4cw with sound and picture but no backlight

I measured C534, as well as C104 and C105 the other day. They were at 388V (sorry forgot to mention that I already checked that).
I will check MP3394, Q510 and Q511 when I'm at home later. The thing is a that I bought this new psu just a few days ago. So considering that they didn't sell me a defective one, it should be alright.

So my best guess would be (like you said, too) that there is something wrong with the leds, but they all looked fine to me yesterday.

[dick_barton]

Re: Telefunken d48u300n4cw with sound and picture but no backlight

The problem with testing leds is that they appear to work perfectly when the current is well below their normal operating current. For example a led tester providing 30 to 40mA would fully light the led strip which unfortunately could fail when in the set running at 200mA or more.
That's not to say you definately have a faulty strip. Normally as I said earlier when you have a voltage at the led connector that peaks and then drops quickly is one indicator that the problem is within the led driver/ led strip area.

[vigorcent]

Re: Telefunken d48u300n4cw with sound and picture but no backlight

Quick update: Pin 15 and 16 of MP3394 are 11.6V and 5.9V
Sorry for the dumb question, but how exactly do I check q510 and q511 (picture below)?

[dick_barton]

Re: Telefunken d48u300n4cw with sound and picture but no backlight

So you have supply voltage to the led driver IC which is not dropping and you have the regulated output voltage from that IC which is 6V so it looks as though the problem is not with the IC. Maybe worth checking D528

If you can give the part number of Q510 and Q511 so as to find the pinout for them.

[vigorcent]

Re: Telefunken d48u300n4cw with sound and picture but no backlight

None of them seem to be present. I just tried to look for a schematic for my specific model again, but I can't find anything.

[dick_barton]

Re: Telefunken d48u300n4cw with sound and picture but no backlight

Is there a burnt component in the red box?

It looks like the MP3394 drives the leds directly which is why the components are missing.

Also you say

But I checked all the LEDs before -> found one that didn't light up -> replaced the strip -> nothing changed.

Are you certain the replacement strip was for you model?
Did you replace one strip or all of them?