Samsung QN75Q60TAF Boot Loop fixed but no picture

[Nicktronic]

Re: Samsung QN75Q60TAF Boot Loop fixed but no picture

@Howardc64 - I tried checking all the caps on the buffer boards but no luck.

[nomoresonys]

Re: Samsung QN75Q60TAF Boot Loop fixed but no picture

Look at those caps reading short real closely with magnification maybe, see if you see anything that looks like a fine line or such, if you do then carefully remove it and recheck it and the pads where it goes on the board, see if you still have a short.

[Nicktronic]

Re: Samsung QN75Q60TAF Boot Loop fixed but no picture

I think the caps are actually good. I tested them originally in diode mode listening for a beep. Actual resistance is about 12 ohms. Maybe that's still bad, but there is no dead short.

[lotas]

Re: Samsung QN75Q60TAF Boot Loop fixed but no picture

12 ohm is already low resistance and it's bad.

[howardc64]

Re: Samsung QN75Q60TAF Boot Loop fixed but no picture

Here are some steps

Confirm test point shorts

In post #14, you traced masked off pins 35/36 to test points on the board. This is likely a shorted rail. Since you masked off these pins, no voltage is sent to these rails from main board and so TV continues to operate. But of course some power section is lost to the buffer board/panel

Put positive multimeter tip on the test point and negative on ground (TV metal chassis that boards mounts on (usually many GND pathways from buffer boards to chassis) or some of the open copper rectangles on the buffer boards which are usually also GND and read the resistance. Maybe you will see 12ohm again like post #23.

If short on buffer boards, which one?

If there is a short on the buffer board, then need to determine which one. First, can you post a pic of the entire back of the TV? Looks like you have 1x main board to buffer board flex cable. I'm guessing for a 75" TV, there are 4x buffer boards and 3x dark orange colored flex that join them. Your short could be on any of the 4 buffer boards as voltage rails goes to all of them.

To find out which buffer board has the short. First confirm the short on a particular buffer board, then remove the flex to other buffer boards/main and check the short again against the GND on the buffer board. If disappears, then short is on other boards. If remain, then its this one. Probably check all 4 buffer boards in this manner.

Which component is shorted on the buffer board?

This requires even more skill and experience as there are many components and usually multiple caps on every voltage rail. Pros inject voltage (need to estimate how much voltage and max current limit given the circuit) by bench power supply and use a thermal imager (and often a macro lens + video recording) to quickly which component heats up first. There are other ways... by touch, evaporating rate of isopropyl alcohol etc.

Lets confirm you have short on the buffer boards and which one first.

====

Probably can skip the main board for now, The R82 is an inductor and just a loop of wires so it will read continuous. Anyway, its next to TI TAS5749M which google says is an audio IC and your speaker connectors are just below that.

Here is a annotated pic of your main. The red rectangle is where all the voltages for the LCD panel comes from. Rectangular chip in the middle takes 12v from PSU and generates all the voltages for the LCD (vgh ~= 20v+, vgl ~= -10v, vcom ~<= 10v) so don't go probing around the tight pitch pins around this chip. The octagons are inductors and they will probably read the voltages I gave you above (+ probe on either side of inductor, - probe on TV chassis GND). vgh/vgl/vcom are used to turn each TFT (thin film transistor) at each subpixel (R,G,B) and turn the LCD polarizer to allow the backlight through. Panel buffer boards usually also get 3.3v to run various digital logic chips.



The solid red arrows shows traces going to the SOC under the heatsink (pixel data) and traces towards the rectangular section (where LCD power supply is)

The dash red arrow is probably where the pin 35/36 trace points continue.. making their way towards the LCD panel power supply section.

Anyway, probably focus on finding the short in buffer board now.

[Nicktronic]

Re: Samsung QN75Q60TAF Boot Loop fixed but no picture

Great information, thank you @howardc64.

When I'm doing the continuity checks from the test points to ground, do i do this with the LVDS cable disconnected?

[lotas]

Re: Samsung QN75Q60TAF Boot Loop fixed but no picture

Yes, you have to unplug the LVDS cable and unplug the U-flex between the brackets on one side.

[Diah]

Re: Samsung QN75Q60TAF Boot Loop fixed but no picture

LVDS rail P 35 & 36 they are not the main short circuit... they are VDD. so making them mean shutdown Panel / TV booted ..., the seeking after short should be LVDS disconnected.. DMM on Diode mode black probe to buffer boards GND and the red probe move it on all caps ESD D first.

[Nicktronic]

Re: Samsung QN75Q60TAF Boot Loop fixed but no picture

Checked every single capacitor on the buffer boards. No shorts that I can find. That means it's in the panel then, correct? Isolating the VDD (35/36) just removed the ability for the short to affect the VDD supply right? So beyond capacitors, what? Anything?

[howardc64]

Re: Samsung QN75Q60TAF Boot Loop fixed but no picture

Checked every single capacitor on the buffer boards. No shorts that I can find. That means it's in the panel then, correct? Isolating the VDD (35/36) just removed the ability for the short to affect the VDD supply right? So beyond capacitors, what? Anything?

Maybe check if any shorts from buffer boards with all cables connected without masking off 35/36? (Don't power on TV of course)

Masking changes circuit connectivity. Might be best to not have it while hunting for the short.

[Nicktronic]

Re: Samsung QN75Q60TAF Boot Loop fixed but no picture

I will check, thanks.

[Nicktronic]

Re: Samsung QN75Q60TAF Boot Loop fixed but no picture

No difference with all cables connected

[Diah]

Re: Samsung QN75Q60TAF Boot Loop fixed but no picture

No difference with all cables connected

keep the mask on 35 & 36.. turn on and measure the V on them at MB side

[howardc64]

Re: Samsung QN75Q60TAF Boot Loop fixed but no picture

What is the minimum connection configuration that will reproduce the 12ohm reading in post #23? Can you post a pic of this cap and any labels around it?

Ideally within a single buffer board but make sure reproduce it first.

4x buffer boards also can provide patterns for comparison. If the 12ohm short is reproducible in 1 buffer board, are there similar circuit elsewhere for comparison?

[howardc64]

Re: Samsung QN75Q60TAF Boot Loop fixed but no picture

keep the mask on 35 & 36.. turn on and measure the V on them at MB side

Yes, thats a good test. If you have a sharp probe tip + steady hands, can try the exposed contacts on the LCD cable connector. Another place might be those 2 board through holes coming off the pins. Will need to remove tiny bit of insulation to expose the copper trace. I have very sharp (like sewing needle) probe tip that can be pushed and turned a little to remove the green insulation. Anyway, can try sewing needle. Don't press too hard since they are fragile small copper traces. I guess careful scrap from Xacto knife tip will do too.

[Nicktronic]

Re: Samsung QN75Q60TAF Boot Loop fixed but no picture

@howardc64, the 12 ohms was measured on the main board, i believe you mentioned R89 earlier being an inductor. It was all caps around that.

@diah, mask on, 35/36 at MB side is 3.272VDC on both pins to gnd

[Diah]

Re: Samsung QN75Q60TAF Boot Loop fixed but no picture

@diah, mask on, 35/36 at MB side is 3.272VDC on both pins to gnd

yours Photo

with out masking.... you test to turn on after
A- remove SPI IC 601

[Nicktronic]

Re: Samsung QN75Q60TAF Boot Loop fixed but no picture

Sorry I don't know if i understand..... So, take the masking off, turn on tv. Measure 36 and 35 on buffer board as marked in picture?

After, remove SPI IC601?....Test again?

[Diah]

Re: Samsung QN75Q60TAF Boot Loop fixed but no picture

Sorry I don't know if i understand..... So, take the masking off, turn on tv. Measure 36 and 35 on buffer board as marked in picture?

After, remove SPI IC601?....Test again?

no need to measure at all. just remove the spi and the mask then turn on

[Nicktronic]

Re: Samsung QN75Q60TAF Boot Loop fixed but no picture

ok