TCL LCD TV Model L32M11HD - no standby

[Markus1]

I am seeking some help from the experts here, please.
There is no 3.3V standby voltage and so the set appears dead.
I found the same model with the same symptoms on this forum under https://www.badcaps.net/forum/showthread.php?t=50363 .
budm tried to help but the tread ended quickly without any final conclusion.

Schematic of the power supply board (attached below) is from the above mentioned tread. It says there that it is for this specific model. However, the IC1 which drives the standby transformer TS2 is Viper12A on the schematic, while the IC1 on my board is FSQ510 (7 pins chip). Other components though seem to agree.
I did check CB4, DB7 and ZB6 as suggested by budm in that earlier tread, they all appear OK.
There are no signs of overheating, no swollen caps, and diodes in the area check fine.
Two resistors RB30 and RB31 on the supply line to the transformer TS2 are shorted with a fine piece of wire (looks like a modification on the board).
There is another tread, http://ww.badcaps.net/forum/showthre...t=18338&page=2, this one deals with FSQ510 in Philips TV with no standby. Also ended unresolved.

Here are my measurements (all plugs, except 240V mains, disconnected from the board):
On Main Filter Cap (C1, C2) 337V
On Filter Cap CB1 (for stand by supply) 337V
On IC1 (FSQ510)
pin 1 (GND)
pin2 (GND)
pin3 (Vfb) fluctuates between 2.5V and 3.04V
pin4 (Sync) nil
pin5 (Vcc) fluctuates between 7.55V and 7.69V
pin6 no pin there
pin7 (D) 336V
pin8 (Vstr) fluctuates between 297V and 304V
On Connector P2 no voltage on any of the 12 pins.

Discrepancy between the schematic showing IC1 as Viper12A and the board showing FSQ510 confuses me. Would they be compatible? Would this be a possible problem?
Not sure where to go next? Any help would be appreciated.

[piernov]

Re: TCL LCD TV Model L32M11HD - no standby

For sure IC1 is wrong on the schematics, though the rest may be ok.
I'm not an expert, but I think IC1's VCC (pin 5) should probably be higher for it to start switching. Datasheet says 8.7V, so I there is probably an issue there.
Check resistance to ground and caps (especially CB4 on the schematics) connected to VCC pin. If everything looks ok, IC1 may be bad.

[R_J]

Re: TCL LCD TV Model L32M11HD - no standby

Have you tried disconecting the plug that goes to the main board and see if the 3.3 volts comes up on the power supply?

[Markus1]

Re: TCL LCD TV Model L32M11HD - no standby

Thank you piernov and R-J for quick response.

I did check the components in Vcc line (pin 5), look OK.
And yes, the measurements were made with all other inter-board plugs disconnected.
These slightly fluctuating voltages on Vfb (pin 3), Vcc (pin 5) and Vstr (pin8) concern me though.
Should I go with the IC1 replacement or is there a problem somewhere else?

[R_J]

Re: TCL LCD TV Model L32M11HD - no standby

Usually a fluctuating voltage means the chip is trying to turn on and stops. this pulsing will cause give a small a/c voltage out of the TS2 (pin 4-5), rectified by DB87 and charging CB4 a bit but not enough. You could try either replacing CB4 or solder another 10uf or 22uf cap accross it to see if it makes a difference
Have you checked DB10 secondary diode and do you get any voltage at all on its cathode?
and yes the chip could be bad

[budm]

Re: TCL LCD TV Model L32M11HD - no standby

I do not remember what happen to that thread.
So at this point there on fluctuation DC on the 3.3VDC output on the cold side at all?
The rectifier for that 3.3V is OK?

[dskall]

Re: TCL LCD TV Model L32M11HD - no standby

So doesn't vcc need to get to 8.7v on FSQ510 to start up? Seems I saw an exact post just like this with same voltage on vcc

[budm]

Re: TCL LCD TV Model L32M11HD - no standby

Yes it does, and if he is getting any DCV on the cold side at all then it indicates that the IC is trying to run but it may be in shutdown/restart mode due to fault some where.
Once it is running the VCC can drop down below 8.7V and it will keep on running as long as it does not drop down to low Voltage shutdown point.
That is why he needs to see if there is any thing at all on the output side and no bad rectifier or bad caps either.

[R_J]

Re: TCL LCD TV Model L32M11HD - no standby

On page 3 of the pdf it says:
Pin 8 Vstr, This pin pin is connected directly, or through a resistor, to the high-voltage DC link. At startup, the internal high-voltage current source supplies internal bias and charges the external capacitor connected to the Vcc pin. Once Vcc reaches 8.7 V, the internal current source is disabled.
If the circuit is working correctly at the rated frequency the transformer will supply the voltage via DB7/RB5 to charge CB4 (to a higher voltage) and keep the ic running.
But for some reason this supply is not running, for example a short on the secondary or maybe damaged ZB1 or ZB6

[dskall]

Re: TCL LCD TV Model L32M11HD - no standby

Is that why the vcc is 7.6v because it is oscillating between 6.7v and 8.7v low voltage shutdown and start up voltage?

[Markus1]

Re: TCL LCD TV Model L32M11HD - no standby

You could try either replacing CB4 or solder another 10uf or 22uf cap accross it to see if it makes a difference
Have you checked DB10 secondary diode and do you get any voltage at all on its cathode?

Soldering additional 10uF cap across CB4 did not make any difference, voltage still slightly fluctuates.
Secondary diode DB10 tests good and there is no voltage readings on its cathode.

I can't read anything on the cold side outputs (connector P2). Neither rectifier diode nor caps indicate any problems.

Also checked RB5, DB7 and DB8 - all seem Ok. Resistor RB6 however, measures about 9k while the schematic indicates 27k. I am guessing it is not that critical.

So, still don't know, no clear faults in the components, unless something fails under load.