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Repairing a power board for a Vizio P75-C1

Tha fios agaibh

Aug 11, 2014
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2,241
It's possible to follow the traces to see how it's connected and surmise It's value by the components It's connected to. More than likely a component such as that nearby transistor on the heat sink shorted causing the failures.
Sometimes you can get an unknown value from a component by looking for other components of the same type and connection.
Is this cap connected similarly to transistor of the same type?
Screenshot_20211216-053621_Chrome~2.jpg
 

73's de Edd

Aug 21, 2015
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Sir Brogan . . . . .

Whut you talkin' 'bout Willis ?

How about re examining that board . . .with brain fully in gear and . . .re evaluating . . .

HERE is what my learned eyes are seeing . . . .
FOR SURE . . . . 2 RED DOTTED defective components, and possibly, the RED DOTTED POWER FET K11A650 also,

This board is constituting 1 of 5 power sections on it, that, in all . . .develop upward of 200 Potts of wower to operate its behemoth 75 incher of a screen..

YOUR ONE DISTINCTIVELY BAD POWER SECTION. . . .

Initially in looking at the far top left corner, I am seeing a 450 VDC E-cap rating, therefore I'm expecting a 340'ish VDC supply being in use.
Looking at our sections PALE BLUE markup of its power transformer, with its primary and a HEAVY current at 12 or 10 VDC secondary.
I see power coming in at either RED X markup . . . .If the top C9103 is 400VDCish rated, I am expecting power from it and passing down thru J931jumper wire and into the transformer primary and the other transformer primary connection, being the RED arrow path and up to the Drain of Q9101.
At no extra charge . . . . some voltage peak and spike protection was factored in via the BLUE disc ceramic cap and two diodes and two resistors in their cluster, combinationally serving as voltage snubbers.
The Q9101 conducts in accordance to its Pulse Width Modulated drive being received at its Gate . The complete power loop is then fully completed to ground via the R9101 source resistor.
KNOW WHUTS . . . .YOU AIN"T GOTS NO SOURCE REE-SISTOR NO MO' !
You say . . .
The resistor looks like it's red, yellow, grey, gold, green.
Which makes it a 24.8 ohm, 0.5%.

Au contraire . . . . .

Its Red- Yellow- Silver-Gold-Gold and looking more like a 2<---> 1 Watt unit and it has a value of decimal 24 ohms @ a 5% tolerance.
Check out Mousers or Digi Keys product PDF sheets for physical size to exactly zero in on wattage ratings.

Now the chicken and egg conundrum . . . . . did the C910(8) (?) ceramic cap break down and start arcing across its its end discs, with those extra random arcs / surges, thereby upsetting the previously metered duty cycle . . . . .in stretching it out . . . . at the Power FET and it then shorts from its S-D and blows open the R9101 Source resistor.
I might think so, as I believe that if you will carefully pull C910(8) (?) ceramic cap and measure its resistance, I DON'T expect its dielectric and blue conformal coating surrounds carbonization, possibly even being able to pull down to the low resistance value, in order to be able to pass adequate current / power to blow open R9101 .
Now move on over to Q9101 and see if a Drain to Source shorting is being present .
For a like relative comparison, move directly over to the left to another power supplys Q9801 and ohm out its D-S leads.

What is the now carbonized and un viewable C910(8) (?)'s value ?
Well stay over there and check out the VIOLET and RED flagged disc ceramic HV caps and see if the one VISIBLE VIOLET C9123 on our supply is not being the same spec as the VIOLETS on the left supply. You can now place C9123 back into a proper vertical alignment.
( Ten Hut ! . . .DRESS RIGHT DRESS ! )
NOW ! . . . .the left supply has Q9801 and the same snubber capacitor being across its SOURCE and DRAIN. That will be what you will need to replace your defunct unit with . . . . . .with my educated guess of it being an ~ .001-005 @ 1 KV.
BUT I would suggest that replacements, be of a step up to 3KV rating , and also being used at all other 4? FET units D to S, if this same 1KV voltage rating value cap are being used there .
Question . . . . .did this failure knock out the MAIN ceramic power supply fuse . . . that's F9901 . . . at extreme top left corner ?

To err is human . . . . . . . . and by looking at one lead of that original R9101, you're DEFINITELY human !
Too early of a lead pull or not a hot enough reheated solder joint . . . . as you totally pulled out a foil rivet with one lead of R9101.
Not to worry . . . though . . . as this is not being a double foil sided board with it / rivet also serving as a via. . . . .from foil to foil.
BUT, instead the rivets surround gives a bit more surfactal solder bonding to a lead of a hot running part.
You can compensate by leaving a high solder build at the foils side, around the resistors lead.

IN THE FUTURE . . . . . . . place boards working side . . . . .TOP UP.
If a lead is bent over or skewed, initially heat it up and get lead(s) vertical.
Work with a soldering iron tip so well tinned, that it looks like it is chrome plated.
ADD new solder to the applicable old joints until being in such excess, that they form round solder balls.
Grip the component side with gloved thumb-fingertips, or using a thermally isolating silicone pad, or a mini binder clip
upload_2021-12-16_6-29-10.png
or a scribe / Phillips mini screwdriver, placed under the wire loop and then levering it up from the PCB, one lead at a time.

Then a well tinned soldering tip speeding from joint to joint until both liquefy, should have the part almost dropping on down and out or with a slight coaxing from the above handling aids.

NOW . . . . . .what say ye ?



And to thank . . . . . six muntz agoe . . . . . i dun cutent evin spels .


SELECTIVE BOARD MARK-UP . . . . .

VIZIO-P75-C1-Power-Supply.png


73's de Edd . . . . .


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.Someone told me I was gullible … and I believed them.



.
 
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73's de Edd

Aug 21, 2015
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Sir Brogan . . . . .

That was for the pic that I supplied . . . . . .now, for YOUR board . . . .
I will just read YOUR boards resistor of photo #2
First . . . . but it being of no consequence . . . . is the body color of a pale pink? / salmon ? . . .wouldn't you say ?
Then, the first band is being a definite RED and then a YELLOW band and then a SILVER band and then a GOLD band . . .and then a final (inconsequential) GREEN band = 20 tco that, in our situation / application, equates with pubic hair on a gnat.

End result . . . . .the replacement resistor needed is 0.24 ohms at a 5% tolerance.
And if you need to know, you get below that pink conformal coating and take razor blade scrapings of the actual resistance element and you will find CARBON dust from a spiral of a deposited or fired carbon resistance element.
Or METAL shavings from a metal films element.
Or a visible, 3 dimensionable, spiral wire from a wire wound element .
! BELIEVE ME ! *
*( Can I have about a 1,000 backer uppers LIKES / comments now ?)

How are the units other situations now . . . . .?
.
 
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Tha fios agaibh

Aug 11, 2014
2,241
Joined
Aug 11, 2014
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2,241
Sir 73's de Edd,
Can you explain this Green band 20tco?

There's 5 bands, why not use the 5 band resistor code?
My eyes often mistake grey and silver let alone hair on a gnat.
 
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