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[cam.in.head]

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.

[Marks DTM Calib]

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

[cam.in.head]

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

[redelitev6]

have you checked all the valves ?

[Bigron]

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.

[cam.in.head]

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

[Marks DTM Calib]

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

[cam.in.head]

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 ?

[Marks DTM Calib]

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)

[ronnyd]

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.

[Bigron]

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.

[Marks DTM Calib]

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. 

[cam.in.head]

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)

[Kevin Wood]

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.

[TheBoy]

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