TV Power Supply Unit

[Markus1]

It is a general question on Power Supply Units for TV (LCD and Plasma).
It seems to be very difficult to find any details, and certainly any circuit diagrams, related to the TV PSU's. Majority of the Service Manuals do not include information on the Power Supply boards either. I suppose the manufacturers expect them to be replaced rather then repaired. Would anybody know of a website or discussion forum (similar to this one, although I am not sure if there is any one better) where at least some information on that subject could be found?
Your help would much appreciated.

[selldoor]

Re: TV Power Supply Unit

Are you referring to built in power supply boards or external adapters?

[Markus1]

Re: TV Power Supply Unit

Hi Selldoor,
I am referring to internal power supply boards the TV unit.

[selldoor]

Re: TV Power Supply Unit

Ok Then what you say is correct. There are often schematics and service manuals available for tvs but they exclude the power supply. I believe this is because they sometimes change
the power supply in the same set and also they use different revisions. In addition the power supply manufacturer may not allow them to publish it. You can sometimes get information direct from the website of the board manufacturer.
If you are looking for information on a particular set it would be better to ask in the TV troubleshooting section posting pictures of your boards.

[Markus1]

Re: TV Power Supply Unit

Thanks Selldoor, I will try with the picture of the power supply board. I understand from your response that those boards are actually made by someone else, not necessarily the TV manufacturer (LG, Samsung, etc).
In my case it is LG 32LX2D-AA.APAULL TV model and the power board is just marked PSU 6871TPT303B. It also has a bar code number. How can I find the board manufacturer from that?

[selldoor]

Re: TV Power Supply Unit

Well I just googled 6871tpt and got loads of hits looks like an LG board has it not got the lg "face" logo

[Markus1]

Re: TV Power Supply Unit

Yes, Google shows a lot of hits but they are pretty much all to sell (or repair) the board. I have checked out many of them, none is presenting any schematic details.
It appears that 6871TPT303B is just an LG part number for power supply board.
I was hoping that someone from the forum community could help me with or point me to a source for a circuit diagram.

[budm]

Re: TV Power Supply Unit

There is the name of the company by the AC inlet and also on the bottom side of the board: H & E Co.Ltd., model # BT-EFL32230W-A
http://www.hnelec.co.kr/
http://hnelecco.en.ec21.com/company_info.jsp

http://www.shopjimmy.com/media/catal...303b-top_1.jpg

http://www.shopjimmy.com/media/catal...b-bottom_1.jpg

What is it not doing any way?

[Ahakim]

Re: TV Power Supply Unit

I think the main cause is that manufacturers don't reveal their "secrets" as easily...specially of newer brands (The laws of Market competition!!). I might be wrong though...
How do you explain that PSU circuit diagrams of older (such as 2002 or 2008 versions of LCD/PLASMA TVs) can easily be found?
"FORUMS" is the way to share, help and learn...

[Markus1]

Re: TV Power Supply Unit

There is the name of the company by the AC inlet and also on the bottom side of the board: H & E Co.Ltd., model # BT-EFL32230W-A

Thank you budm for details on the board manufacturer. I will search further along these lines.

What is it not doing any way?

TV is dead. No 5VST. I suspect a problem with the so called Power IC (Viper 22A) on the primary side of the Stand-by transformer.
Just wanted to find a schematic diagram to follow up the circuitry. No luck with that as yet.
Any suggestion? Again it is LG32LX2D-AA.APAULL with PSU 6871TPT303B

[budm]

Re: TV Power Supply Unit

You can get the spec sheet of the Viper, then look at the typical application circuit, it will be really close to what you have on the board due to most manufacturers will just copy the application note of the diagram of the IC manufacturers and used them in their products which save them from re-engineering the circuit that was already proven and provided by the IC makers.
Most of the the power supply that will not start up is due to bad startup cap (22~56uF 50V is the typical value), the cap that is connected to the VCC (Vdd) pin of the SMPS IC.
What you can do for us is to check the resistance (TV not plugged in of course) between the GND pin against all other pins, make sure to report the reading if they are in OHM, K OHM, or M OHM.

[selldoor]

Re: TV Power Supply Unit

Can we assume you have a service manual and its just that it doesnt include the
schematic for the power supply. Even so they usually have some fault tracing test points
mentioned?
Failing that if you post good clear pictures front and back of your boards ( clear enough to read the numbers on the board) we may be able to help.
Post them using manage attachments jpgs up to 3mb usually work ok.

[Markus1]

Re: TV Power Supply Unit

Thanks budm and selldoor.
Attached are two pictures of the board (front and back) and one file of the circuit diagram (finally found it). Please note: the board # is 6871TPT303B but the diagram is for 6871TPT303A. However, it is probably close enough.
The Viper22A is designated as IC103.
Now the status update:
1. Resistance readings (each pin to GND) are:
pin 1 (Source) 0 ohm
pin 2 (Source) 0 ohm
pin 3 (FB) 1.2 kohm
pin 4 (Vdd) 30 kohm
pin 5-8 (Drain) 6 Mohm
So it seems to be reasonable.

2. Voltage readings on each pin are:
pin 1 (Source) 0 V
pin 2 (Source) 0 V
pin 3 (FB) 0 V
pin 4 (Vdd) 11.5 V
pin 5-8 (Drain) 345 V

More info and more questions:
* My board (6871TPT303B) has some modifications done with an additional cap and 2 additional diodes added at the back (you can see them in the picture). They are around pin 4 of IC103 and also around Q102.
* I am not sure where the startup capacitor is. Is it C126 or C127 as per circuit diagram? Please indicate the path for startup voltage.
* How does Vdd (11.5 V) arrives on pin 4? I thought that it would have come through R150 and D150 (as per diagram) but R150 and D150 do not exist, the path is open there.
* In the end still no 5VST.

Hope this helps. Would like to hear more from you.

BTW Sorry, I am yet to learn how to mark items on the drawing with an arrow or text.

[selldoor]

Re: TV Power Supply Unit

Hi Well budm is the circuit man - I will go for the simple stuff In the picture D301 appears to have a leg lifted. This seems to be in the 5v sby circuit so first thing would be to confirm that it has been put back in?

Ok Perhaps just change both C 126 and C127 ( already changed -what brand series is that) with good quality low esr capacitors panasonic FR or equivalent.

Do you know who did the modifications - check the glue is not conductive

[Agent24]

Re: TV Power Supply Unit

Ok Then what you say is correct. There are often schematics and service manuals available for tvs but they exclude the power supply. I believe this is because they sometimes change
the power supply in the same set and also they use different revisions. In addition the power supply manufacturer may not allow them to publish it. You can sometimes get information direct from the website of the board manufacturer.
If you are looking for information on a particular set it would be better to ask in the TV troubleshooting section posting pictures of your boards.

I think it's because the TV manufacturer (especially cheap models) doesn't usually make the PSU, they buy one from someone else, so perhaps they don't have the schematic anyway.

[selldoor]

Re: TV Power Supply Unit

Sorry too late to add via edit

* How does Vdd (11.5 V) arrives on pin 4? I thought that it would have come through R150 and D150 (as per diagram) but R150 and D150 do not exist, the path is open there.

Feed to Vdd pin looks to be via R126 and D110 - If you notice on the schematic D150 and R150 have asterisks which suggests a modification but we dont have that information

BTW Sorry, I am yet to learn how to mark items on the drawing with an arrow or text.

There are a few programs you can use - I just use Paint - if you have that
(usually load with windows ) Then you can use line draw to draw boxes
or make arrows and there is an icon with a capital A - you can use that for text.

[stj]

Re: TV Power Supply Unit

i think startup cap c127 has leaked.

[budm]

Re: TV Power Supply Unit

At 11.5V on VDD pin 9the startup cap is charged up through the internal current source, that is below the startup voltage requirement which is around 14.5VDC.
Page 6/20 of the spec sheet I provided: It is good to read the description on how the IC works.
Power supply of the control circuits. Also provides a charging current during start up
thanks to a high voltage current source connected to the drain. For this purpose, an
hysteresis comparator monitors the VDD voltage and provides two thresholds:
- VDDon: Voltage value (typically 14.5V) at which the device starts switching and turns
off the start up current source.
- VDDoff: Voltage value (typically 8V) at which the device stops switching and turns on
the start up current source.
Page 10/20:
This device includes a high voltage start up current source connected on the drain of the
device. As soon as a voltage is applied on the input of the converter, this start up current
source is activated as long as VDD is lower than VDDon. When reaching VDDon, the start up
current source is switched OFF and the device begins to operate by turning on and off its
main power MOSFET. As the FB pin does not receive any current from the optocoupler, the
device operates at full current capacity and the output voltage rises until reaching the
regulation point where the secondary loop begins to send a current in the optocoupler. At
this point, the converter enters a regulated operation where the FB pin receives the amount
of current needed to deliver the right power on secondary side.

So the startup cap connected to the VDD pin may have leakage resistance or the internal current source inside the IC is bad.

[budm]

Re: TV Power Supply Unit

BTW, did you lift up the Anode side of that rectifier Diode next to C301 for the STBY supply for a reason?

[Markus1]

Re: TV Power Supply Unit

In the picture D301 appears to have a leg lifted.

Well spotted. The picture was taken just after lifting D301 for checking. It is now back in its place.

Perhaps just change both C126 and C127 (already changed -what brand series is that) with good quality low esr capacitors panasonic FR or equivalent.

It may be a good move to change both caps. C127 has been allraedy replaced. Not sure what brand it is, 105 C rated though.

Do you know who did the modifications - check the glue is not conductive

The modification seems to be "global". I have seen a picture of the same board earlier on this forum. It had those added components on the back as well.

Feed to Vdd pin looks to be via R126 and D110

Not sure here. This feed would have to come from the transformer T102, but the T102 is not operating yet, because IC103 is not delivering switching pulses.

Thanks selldoor for head-up on how to use line draw/arrows on the pictures. Will do it next time..