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Author Topic: any tv engineers here please  (Read 3307 times)

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cam.in.head

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any tv engineers here please
« on: 08 March 2018, 14:42:22 »

for anyone who knows the panasonic tx-p50xt50 and similar styles they suffer from the u203 chip causing a loss of power. i oredered a fan6755u to replace it and found the exact same chip already in there.a fairchild one.   replaced it anyway and put set back together. all fired up fine and worked all evening without issue.next morning tv was dead.i always turn mains off at night and dont leave anything on standby.
ordered another fan chip just incase the other was faulty and will fit it when it comes.
questions i have are
what does this chip actually control and can it be bypassed ? .
can you bypass the power button (is it just a flip fliop ? )
can i run an additional 5v stby to this circuit?
anything else to check/ replace ?
many thanks.
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Marks DTM Calib

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Re: any tv engineers here please
« Reply #1 on: 08 March 2018, 16:30:16 »

Its a PWM power supply controller, no way to bypass that.

I would be checking the state of the electrolytic caps, particularly the one associated with VDD on pin 6
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cam.in.head

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Re: any tv engineers here please
« Reply #2 on: 08 March 2018, 16:53:40 »

do you know the caps in question and il just replace them to be sure.
as far as ive read this was an issue from new with these and similar sets ? was there ever a cure
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redelitev6

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Re: any tv engineers here please
« Reply #3 on: 08 March 2018, 18:11:30 »

have you checked all the valves ?
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Bigron

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Re: any tv engineers here please
« Reply #4 on: 08 March 2018, 21:34:00 »

Fuse 19, may I intrude into the OP's topic and ask you a semi-related question, please?
In a previous life I did work as a TV engineer, but in the days of CRT televisions, not the newer flat screen LCD/LED models.
My question relates to a Toshiba Regza 32LV665DB, which very often refuses to wake up from standby unless it is switched off at the mains, wait for a minute and switch back on again; only then will it wake up.
I have looked on Toshiba's website and they claim to know all about the fault, but can offer no cure.
Do you know differently?

Ron.
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cam.in.head

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Re: any tv engineers here please
« Reply #5 on: 08 March 2018, 23:05:01 »

this sounds like a capacitor priblem. are there any bulging tops on any of them?
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Marks DTM Calib

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Re: any tv engineers here please
« Reply #6 on: 09 March 2018, 07:58:41 »

Fuse 19, may I intrude into the OP's topic and ask you a semi-related question, please?
In a previous life I did work as a TV engineer, but in the days of CRT televisions, not the newer flat screen LCD/LED models.
My question relates to a Toshiba Regza 32LV665DB, which very often refuses to wake up from standby unless it is switched off at the mains, wait for a minute and switch back on again; only then will it wake up.
I have looked on Toshiba's website and they claim to know all about the fault, but can offer no cure.
Do you know differently?

Ron.

Toshiba don't make many TVs (if any), chances are its a Vestel with a Tosh badge, have a little search as there are known electrolytic capacitor issues on Vestel chassis
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cam.in.head

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Re: any tv engineers here please
« Reply #7 on: 09 March 2018, 09:49:58 »

hi fuse 19. the cap relating to pin 6 looks intact and soldered well.the resistor next to it also tests ok.i dont have an esr meter or any other way of trsting it for value.
 i assume it gets a voltage to pin 7 constant when plugged in .does the vdd go live when the on off button is pressed or when you come out of standby.? i ordered a 6755u chip and will fit it when it comes just incase the other was faulty ? so even though there is no way to remove it ,is there anyway to keep it powered as i beleive this fault only occurs when mains is disconnected..
seperate 5v supply /small rechargeable battery ?
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Marks DTM Calib

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Re: any tv engineers here please
« Reply #8 on: 09 March 2018, 11:59:52 »

hi fuse 19. the cap relating to pin 6 looks intact and soldered well.the resistor next to it also tests ok.i dont have an esr meter or any other way of trsting it for value.
 i assume it gets a voltage to pin 7 constant when plugged in .does the vdd go live when the on off button is pressed or when you come out of standby.? i ordered a 6755u chip and will fit it when it comes just incase the other was faulty ? so even though there is no way to remove it ,is there anyway to keep it powered as i beleive this fault only occurs when mains is disconnected..
seperate 5v supply /small rechargeable battery ?

I would just replace it, its the external smoothing cap for the main chip supply line.

What happens is that the HV feed charges this capacitor via an internal current source, once the capacitor is charged (VDD on voltage is reached), the PWM oscillator starts switching and the current source turns off. The capacitor then remains charged thanks to a winding in the switching transformer.


Other key items are the high voltage current limit resistor to pin 4 which will have a pair of rectifier diodes from near the mains bridge rectifier.

Similarly, around the VDD and GND pins there are a few more diodes.

If you want absolute belt and braces then you change the cap(s) and diodes plus measure the current limit resistor (which I would guess will be 100's of k ohms)
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ronnyd

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Re: any tv engineers here please
« Reply #9 on: 09 March 2018, 13:31:30 »

Sorry to go off topic, but an aside to Bigrons post, i,ve got a Toshiba Regza which has chassis rattle on a certain frequency. As Ron says, Toshiba are aware of all these faults but seem to to sod all about a solution. Can,t complain too much as it was a freebie from SWMBOs sister. :D
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Bigron

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Re: any tv engineers here please
« Reply #10 on: 09 March 2018, 13:49:12 »

Chassis rattle? You mean a physical vibration? That's a new one on me, but thank you for confirming Toshiba's lack of interest/knowledge/customer care!
I'm reluctant to believe that any electrolytic capacitors are to blame as the TV is less than 2 years old and that would be short-lived even for Japanse components?

Ron.
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Marks DTM Calib

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Re: any tv engineers here please
« Reply #11 on: 09 March 2018, 15:32:22 »

Chassis rattle? You mean a physical vibration? That's a new one on me, but thank you for confirming Toshiba's lack of interest/knowledge/customer care!
I'm reluctant to believe that any electrolytic capacitors are to blame as the TV is less than 2 years old and that would be short-lived even for Japanse components?

Ron.

As said, it is quite possibly not actually a Toshiba but simply ANother brand (Hisense or Vestel) who pay Toshiba for a license to use the badge.....

I would be considering the caps and diodes over an IC every time.

You might get lucky with the IC and get a sweet spot on tolerance but, they generally either work or they don't.  :y
« Last Edit: 09 March 2018, 15:34:02 by Fuse 19 »
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cam.in.head

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Re: any tv engineers here please
« Reply #12 on: 09 March 2018, 19:36:36 »

hi fuse 19. the cap relating to pin 6 looks intact and soldered well.the resistor next to it also tests ok.i dont have an esr meter or any other way of trsting it for value.
 i assume it gets a voltage to pin 7 constant when plugged in .does the vdd go live when the on off button is pressed or when you come out of standby.? i ordered a 6755u chip and will fit it when it comes just incase the other was faulty ? so even though there is no way to remove it ,is there anyway to keep it powered as i beleive this fault only occurs when mains is disconnected..
seperate 5v supply /small rechargeable battery ?

thankyou for that excellent description and for taking the time to explain it all.
I would just replace it, its the external smoothing cap for the main chip supply line.

What happens is that the HV feed charges this capacitor via an internal current source, once the capacitor is charged (VDD on voltage is reached), the PWM oscillator starts switching and the current source turns off. The capacitor then remains charged thanks to a winding in the switching transformer.


Other key items are the high voltage current limit resistor to pin 4 which will have a pair of rectifier diodes from near the mains bridge rectifier.

Similarly, around the VDD and GND pins there are a few more diodes.

If you want absolute belt and braces then you change the cap(s) and diodes plus measure the current limit resistor (which I would guess will be 100's of k ohms)
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Kevin Wood

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Re: any tv engineers here please
« Reply #13 on: 09 March 2018, 22:08:37 »

That cap is particularly critical in some designs since it needs to power the inverter chip for a few cycles until the inverter starts feeding power back to the chip from the secondary of the transformer. If it degrades a little in value or develops leakage, suddenly it can't sustain the inverter long enough and the whole thing just sits there repeatedly failing to start up. Once it's running, it's fine, and often the inverter keeps running in standby mode, so it can work for years with a dead cap until mains is removed. I would just change it for a decent branded 105C cap.
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TheBoy

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Re: any tv engineers here please
« Reply #14 on: 10 March 2018, 08:40:21 »

but thank you for confirming Toshiba's lack of interest/knowledge/customer care!
As has been stated, Toshiba no longer make TVs, and haven't for years. Its all rebadged stuff.

But even if it was a Tosh product, as they are going titsup, I'm guessing customer service is not too high on their priorities ;D
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Re: any tv engineers here please
« Reply #15 on: 10 March 2018, 08:48:32 »

I thought our shitty Grundig badged PoS in the kitchen was suffering a cap problem, when most times it was turned on, as soon as it turned on the panel's backlight, it would shut down and repeat, until eventually it would start.

Not really being a fan of the thing anyway (and CBA issues), I was all for replacing it...   ...until I found I couldn't get one small enough to fit. Which is why we ended up with this crappy thing 7 or 8 years ago.

*SIGH*, so last weekend I HAD to have a look.

Imagine my delight when I pull the TV out, and see its an external PSU...   ...and even better, there is a bit of corrosion built up on the power plug.  Quick clean up, jobs a good'un.


Its still a PoS, takes 30s for the DTV tuner to boot, piss poor view angle. But a working PoS ;D
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cam.in.head

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Re: any tv engineers here please
« Reply #16 on: 10 March 2018, 14:19:27 »

hi just one more thing(columbo has left the building)
i have just picked up a txp50x50 for parts (broken screen ) and the power supply in this is almost identical albeit a few layout changes and some components.
this power supply works fine on the bench so i could use it to fix my other dependant on bits or would it actually work in the txp50xt50 ?
or ! would it work if i swapped it as a unit with the main control board too .the rest of the panels have the same part numbers and i beleive the x version just didnt have smart tv ? whereas the xt is smart
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cam.in.head

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Re: any tv engineers here please
« Reply #17 on: 13 March 2018, 17:41:06 »

spoke to a couple of tv shops near me who say the x board will be fine in the xt so will fit it and see. now on with the dreaded tnpa4782 board from a pg ..thanks for help and advice.
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cam.in.head

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Re: any tv engineers here please
« Reply #18 on: 18 March 2018, 17:15:57 »

4782 board has many many shorts as i was expecting. Replaced all found transistors and a couple of diodes that were shorted.missed something thou as the board popped upon powering and one transistor shorted again.there was a very small flash but cannot see where it came from as it was too quick.three of my local shops wont touch the board and have advised looking for another but they are hard to come by and seem expensive when they do.
Anyone got one .its a tnpa4782ab from a tx-p50g10b.i may just replace the two buffer boards too to be sure but they are cheap enough and available.
So now that the other tv with the darfon b159-201 board is up and running im gonna have a look again at the old board and see if i can get it going as a spare. Ive replaced the 6755 chip twice but its still not firing up.no 5v stby.
Silly novice question. Does the vdd voltage get generated inside the chip and then sustained by the transformer or is it fed externally from over the other side of the board from the transistor next to the optocoipler. There is 5v over there going into one side of this transistor ref main cap negative. .
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Re: any tv engineers here please
« Reply #19 on: 18 March 2018, 20:03:41 »

4782 board has many many shorts as i was expecting. Replaced all found transistors and a couple of diodes that were shorted.missed something thou as the board popped upon powering and one transistor shorted again.there was a very small flash but cannot see where it came from as it was too quick.three of my local shops wont touch the board and have advised looking for another but they are hard to come by and seem expensive when they do.
Anyone got one .its a tnpa4782ab from a tx-p50g10b.i may just replace the two buffer boards too to be sure but they are cheap enough and available.
So now that the other tv with the darfon b159-201 board is up and running im gonna have a look again at the old board and see if i can get it going as a spare. Ive replaced the 6755 chip twice but its still not firing up.no 5v stby.
Silly novice question. Does the vdd voltage get generated inside the chip and then sustained by the transformer or is it fed externally from over the other side of the board from the transistor next to the optocoipler. There is 5v over there going into one side of this transistor ref main cap negative. .

Sounds like you need to make yourself a dim bulb tester. ;)
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cam.in.head

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Re: any tv engineers here please
« Reply #20 on: 19 March 2018, 17:42:37 »

yeah its ok through that via a 60w and a 100w but popped on straight through.there are so may items thst these bosrds that can cascade apparently that many shops dont want to touch it and just like to swap boards which i will probably end up doing too.
back to the other board thou.the fan 6755 chip isnt getting any voltage at all to pin 6 vdd but im not sure if its genersted internally or supplied elsewhere as a kick start ?
ive replaced the smoothing cap and checked the two diodes for shorts.the other pins seem to be getting a supply where required so is it a faulty chip or somewhere else on the board.
other tv working fine so its only a spare board.
anyone local to west yorks have a board ( or any other panasonic plasma for throwing !)
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