Rca led52b45rq

[RDC55]

RCA LED52B45RQ no video, no backlight, status light illuminated red and does not change when power is depressed. Power Supply shows a constant on state (On/Off 4.6V) when plugged in and voltages at the connector appear normal, but PFC voltage is only 165 volts suggesting no boost.

Pin----------Name----------Plugged In Measured
12----------12V-------------12V
11----------12V-------------12V
10----------GND--------------0
09----------GND--------------0
08----------GND--------------0
07----------5V---------------5V
06----------5V---------------5V
05----------5Vsb-------------5V
04----------ON/OFF----------4.6V
03----------5V----------------5V
02----------5V----------------5V
01----------GND--------------0


So it seems strange that the voltages are there, but the PFC is not working?

Checked Power Supply on bench (unloaded) and when On/Off is tied to 5Vsb through 1K resistor 12V and 5V appear on connector, but boost is 165V instead of the expected 395V.

Verified all components in the area of U101 (FAN7930C), verified VCC present on U101 pin 8 and it has 5V referenced to AC ground.

Power MOSFET and Diode check OK

No pulses (a bit of noise) on output pin 7 of U101

Any thoughts - order and replace U1

Thanks

[dick_barton]

Re: Rca led52b45rq

The Vcc voltage needs to be a lot higher to start the PFC (FAN7930C) to operate. See attachment.
Data sheet says

Once the supply voltage of the PFC
block exceeds 12V, internal operation is enabled until
the voltage drops to 8.5V.

[RDC55]

Re: RCA LED52B45RQ

The Vcc voltage needs to be a lot higher to start the PFC (FAN7930C) to operate. See attachment.
Data sheet says

Yea I missed that, and I am fairly clueless on the SMPS's. I just checked again and VCC is 6.5 VDC with respect to the negative lead on the large cap, which I wanted to mention just to be sure this is the correct return.

I tried to trace some of that out to understand how VCC is made but kudos to you guys that can make sense of these circuits when trying to follow the etch and maybe make a drawing since a schematic is not an option - that pin 8 seems like it goes everywhere when you look at the components that look like they could affect that voltage.

So, any helpful hints on how VCC might be made?

Would I be looking for something like the standby transformer rectified output on the AC side making that VCC, and then that would be dependent upon a seperate PWM driver doing its thing while also being able to enable / disable the VCC voltage from conditions on the DC side? But, since I have standby voltage doesn't that mean it should be working and making VCC, or maybe I am just missing enable / disable signal?

And just a note there are two PWM devices both LD7750 - data sheet attached.

[R_J]

Re: Rca led52b45rq

The standby circuit usually supplies the vcc voltage, it is then switched and regulated by Q108, This transistor will have a zener diode (ZD102) in its base which sets the voltage (15~18v) The supply will be on the collector and the output will be on the emitter. I would also replace those small electrolytics EC104, EC102 & EC103 in the standby circuit. EC102 is the filter for vcc for the regulator/switch Q108 input, EC104 is on the output.
Also check that R132 in the pfc mosfet source is ok.

[RDC55]

Re: Rca led52b45rq

The standby circuit usually supplies the vcc voltage, it is then switched and regulated by Q108, This transistor will have a zener diode (ZD102) in its base which sets the voltage (15~18v) The supply will be on the collector and the output will be on the emitter. I would also replace those small electrolytics EC104, EC102 & EC103 in the standby circuit. EC102 is the filter for vcc for the regulator/switch Q108 input, EC104 is on the output.
Also check that R132 in the pfc mosfet source is ok.

Thanks for that input, I pulled EC102, 103, and 104 and measured each one as I pulled it thinking I should be able to identify if one or all were bad. I only used a simple DMM in cap mode and they all tested close to their advertised value. Consequently, I was surprised when I installed new ones and powered it up and VCC was 14V :-)

Note I had been near there the night before and when I was checking the output (anode) of D105 I was only getting 6.8V, so perhaps EC102 was the real culprit. Q108, ZD102, and R132 all appear to be OK in circuit testing.

So, that is definately progress, but I guess there are still issues. The PFC voltage unloaded is 360V and when installed in the TV it drops to 315V. I have read other threads on here that suggested PFC voltage should be close to 390V.

And some other info I should have added earlier I had already replaced EC4, 5, and 8 earlier for obvious bulging, so perhaps this board is just full of.... BADCAPS!

[dick_barton]

Re: Rca led52b45rq

It's the ESR (Electrical Series Resistance) of capacitors that tend to fail by increasing. It's well worh investing in a cheap component tester as advertised on Ebay which can test ESR and capacitance along with other components like diode, leds, Mosfets, tansistors etc.

[RDC55]

Re: Rca led52b45rq

Anyone ever see a connector like this before - the wired connector has 18 pins of double rows, and the Main board has 20 pins of double rows. The photo is how the set was received but the connector can be moved up one or two pins as there is no keying.

So how would you know what is correct?

[dick_barton]

Re: Rca led52b45rq

The two reds wires are probably connected to the positive voltage so you can check if you have 12V on them.

And yes there are quite a few sets like this

[RDC55]

Re: Rca led52b45rq

So I finally got some video on this set and was pleased to see the panel is OK. However, I wanted to enquire a bit more about the PFC voltage that when unloaded is 360V and when installed in the TV it drops to 315V. I have read other threads on here that suggested PFC voltage should be close to 390V.

Any thoughts on those numbers and if low, what would be likely issues that I need to check?

Just to add the board has been completely recapped as far as electrolytic capacitors. The FAN7930C feedback resistors into pin 1 (INV) check OK, but this Power Supply seems to sum that with an output of the bridge rectifier, which is a bit confusing. Measured voltage at INV was 2.48 with unloaded condition and PFC at 360.