75 inch roku tv Standby light flashing 18 times no power up. Bought repair kit from shop jimmy. Replaced 5 different boards. Same problem. Any ideas?

You have bad LED backlights

This explains the problem.

https://www.fixya.com/support/r29790007-repairing_own_television

..

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just cause your power supply is tested good doesnt make it good it could be a shorted panel or more likely the leds behind it unplug everything except the remote eye mainboard and buttons from the power supply and see if the light stays on if it does then i would look into the led strips also check the flash's i dont remember if samsung has a set amount of flashes = a code

Picture tube is out or fuse inside tv is blown..

Did you try repairing the PS with a repair kit first then replace both the boards after that?

Did you get a new T-con board in the kit? Have you talked to Shopjimmy about your problem? They may replace the boards under warranty.

These blinks [flashes] are fault indication (Error Codes). From the number of blinks occur at one time switch ON, you can get an idea about the faulty circuit section inside it. This will make repair easy. These are called blinking codes. Make sure about the exact number of blinking of the standby/power LED, by switching On & OFF the set, for at least two times. Count it carefully. If you wish to get some details, check the site linked here. It has details about blinking codes, and fault locating procedure, to many popular brand TVs and other devices.
http://electronicshelponline.blogspot.com/
http://electro-medical.blogspot.com/
Blinking codes details of many popular brand TVs are given. Surf the site with patience.
If blinks continue without any pause, the main board is faulty.
Counting of number of blinks to Philips TVs is entirely different to that of other brand TVs. It is not a direct count procedure. The site have details about how to count the number of LED blinks [Fault Codes] to Philips brand TVs are given; along with details of the faulty circuit location.
Search the site by type in the "brand name" to your device, in the "Search box at the top right of the display window of the Homepage" to get gathered related posts.

You might have a bad B board. What's worse is that the power switch is connected to the Q board which works in conjunction with the B board. Replace B board, You can send your B board for repair to a company called modus link pts. They are the largest part supplier of after market parts and repair.

This is what I found
You may want to have a Tv repair shop look at this .

Standby light flashes 10X

Symptoms / Customer Description
Standby light flashes 10X
[Primary Symptom Code: 2111 ]

Solution

G1 and G2 boards are ok.

Found Q box was causing loss of DTL MODULE 5v.
Replaced Q box to resolve.

Part #
A1058689A