Vizio D55-f2 backlight/power supply 20v/24v rail output high 26v

[Nonya333]

no backlight on set.

the 20v rail fuse on the mainboard was blown. i replaced and backlight and tv worked for about a day before flickering and out again

input on mainboard for led should be 20v to fuse but is reading 26v at input and <3v output.

power supply output should be 20v/24v from connector and output is reading >25.6-26v

any suggestions?

i also tested backlights with tester and they were all same steady voltage and on

output switch transisters seem to be reading fine in circuit.

will check capacitors esr next

[Nonya333]

Re: Vizio D55-f2 backlight/power supply 20v/24v rail output high 26v

is it normal for there to be aboit 26V on the 20v/24v power line and if not what would cause it to be so high?

[dokma]

Re: Vizio D55-f2 backlight/power supply 20v/24v rail output high 26v

You can check my thread where I've managed to fix 19V spikes on a 5.3V rail. My issue was a faulty switching voltage regulator on the power supply. Perhaps your issue is something similar.

https://www.badcaps.net/forum/showthread.php?p=991809

[Nonya333]

Re: Vizio D55-f2 backlight/power supply 20v/24v rail output high 26v

i will check it out. thank you for reply.

[Nonya333]

Re: Vizio D55-f2 backlight/power supply 20v/24v rail output high 26v

on the ps board on output there is a power mosfet to the 20/24v power rail/connector to main.

coming in from transformer there are 3 double schotky rectifiers.

there is a heat sink with bridge rectifier and what i am guessing is voltage regulator since i cannot see untill i desolder heat sink.

was your voltage regulator on hot side?

[Nonya333]

Re: Vizio D55-f2 backlight/power supply 20v/24v rail output high 26v

there are 2 outputs to the led lights from the mainboard.

the one with less pins still shows half as positive and half as negative but the output is reversed

positive is showing up on negative and negative on positive

i poked around everywhere i am thinking it is the ic chip that controls mosfets

any help would be greatly appreciated

thanks

[R_J]

Re: Vizio D55-f2 backlight/power supply 20v/24v rail output high 26v

How about posting some high quality, in focus pictures so we know what we are dealing with? It seems the led's are driven from the main board so I doubt the power supply is at fault, more likely you have an open led in the panel. If the power supply provides 24v and it reads 26v it is likely due to the fact the backlights are not working. If they were working the higher current draw would lower the voltage to 24v

[Nonya333]

Re: Vizio D55-f2 backlight/power supply 20v/24v rail output high 26v

please excuse the hackjob of the 15ohm resistor and wire for a fuse i could not find another 5R1 resistor.

when i plug in the connector cn6101 upside down with positive on neg side etc there is backlight and display is working however half does look brighter.

cn6101 is where polarity is reversed

[Nonya333]

Re: Vizio D55-f2 backlight/power supply 20v/24v rail output high 26v

i also removed mosfet pq4804 because i am getting inconsistent reading than other mosfets and pq4807 reading slightliy lower resistance as well

[Nonya333]

Re: Vizio D55-f2 backlight/power supply 20v/24v rail output high 26v

i tested leds with tester to led directly without pcbs and on cn6102 the voltage was higher but each line from connector to strip produced steady voltage on all strips

i did not remove panel to access strips because i am not that good at removing and reassembling lcd without damage

[budm]

Re: Vizio D55-f2 backlight/power supply 20v/24v rail output high 26v

https://www.badcaps.net/forum/attach...9&d=1601919019
Is that Brown Green Silver Gold resistor you installed? That is 0.15 Ohms.
That is current sensing resistor connected to the Source pin of the MOSFET's PQ4501, 4502 so you cannot just put in any value resistor it's place.
Was the 5R1 resistor bad? What are the color band of the resistor?

Where is the wire for the fuse?

[Nonya333]

Re: Vizio D55-f2 backlight/power supply 20v/24v rail output high 26v

top circled smd is the resitor i replaced. there is another 5r1 smd slightly lower.

the circle on right is where i put a jumper wire for the smd fuse. difficult to see it is a tiny piece of wire

[budm]

Re: Vizio D55-f2 backlight/power supply 20v/24v rail output high 26v

top circled smd is the resitor i replaced. there is another 5r1 smd slightly lower.

the circle on right is where i put a jumper wire for the smd fuse. difficult to see it is a tiny piece of wire

The 5R1 resistor close by is part of the RC snubber network for the Diode PD45??.

I cannot see how you can have a SMD resistor and SMD fuse on just two solder pads, you are not making any senses to me.

[Nonya333]

Re: Vizio D55-f2 backlight/power supply 20v/24v rail output high 26v

there is no smd fuse i just soldered a jumper wire

[Nonya333]

Re: Vizio D55-f2 backlight/power supply 20v/24v rail output high 26v

and replaced a 5r1 that was bad with the bigger 5ohm smd

[Nonya333]

Re: Vizio D55-f2 backlight/power supply 20v/24v rail output high 26v

what could cause the polarity to be reversed at cn6101?

[Nonya333]

Re: Vizio D55-f2 backlight/power supply 20v/24v rail output high 26v

bad pu4501? i cannot find data sheet for this ic

[budm]

Re: Vizio D55-f2 backlight/power supply 20v/24v rail output high 26v

1) OK, i see the Fuse F4501 now.

2) When did the fuse blow?

3) Was the 5R1 resistor blown open circuit?

4) Need to see the pictures of the whole backside the TV showing how all the boards and the backlight LED's are connected together.

[Nonya333]

Re: Vizio D55-f2 backlight/power supply 20v/24v rail output high 26v

the fuse was blown prior to me touching anything. it was a white smd with the letter "Y". i also could not find info on that.

i found a bigger package smd fuse rated at 2a and replaced that. the set worked for approximately 5-10 hours and then fuse blew again

the 5r1 resistor was showing up as a capacitor on my cheap esr meter and showing higher resistance i believe. i still have it i will have to double check

[nomoresonys]

Re: Vizio D55-f2 backlight/power supply 20v/24v rail output high 26v

You have to use the same type fuse that came out, it matters if it's a fast blow or a slow blow also.