Sony KD-49XF9005 boot loop: power board tested good, main board replaced

At a glance

• Device: Sony Bravia KD-49XF9005, 49 inch 4K TV
• Symptom: Sony logo, then a black screen and a reboot loop. Later completely dead with no standby light.
• What we tested: the power board, on our £40 board assessment. No fault found.
• Cause: the main board, Sony part number 1-983-249-31 (BM2A)
• Fix: a replacement main board, fitted by the owner. The TV is working again.
• Service: UK-wide by post, with tracked return

The Sony Bravia KD-49XF9005 is a 49 inch 4K LED TV. This one had a fault that got worse over time. At first it would show the Sony logo and then go to a black screen. Later it would sometimes play the start-up sound with the screen still black, then reboot and go round again. A factory reset made no difference. Eventually it went completely dead, with no standby light at all, but after being left for a few days it would come back and start the same loop.

The owner had already done some useful groundwork. They had taken the back off, found the main board's part number (1-983-249-31), and shone a torch at the screen during the black-screen phase to look for a faint picture. There wasn't one, which suggests the problem was more than just the backlight.

Two boards, one symptom

A boot loop like this has two likely suspects, and the photo above shows both: the power board on the left and the main board on the right.

• The power board. When its capacitors weaken, the supply can sag as the TV starts up. The TV gets as far as the logo or the start-up sound, drops out and reboots. Weak capacitors also fit a TV that goes dead and then recovers after a rest.
• The main board. It runs the TV's software. If it can't load that software reliably, the result looks much the same: logo, black screen, reboot.

The owner sent photos of both boards. Nothing on the main board looked burnt or blown, and none of the power board's capacitors were bulging, leaking or scorched. But capacitors can dry out without any outward sign, so photos alone could not clear the power board.

The owner's photo of the power board's large smoothing capacitors. They look healthy, but a capacitor can look fine and still have lost much of its capacity, which is why the board needed testing.

Test the cheaper board first

The power board is the more common fault and the cheaper fix, and it can be posted on its own. So rather than guess, we suggested our £40 board assessment on the power board only, and the owner posted just that board.

On the bench it was inspected on both sides, and the capacitors, diodes and resistors were tested, including those in the standby circuit. There were no shorts and no blown fuses. Powered up, every output came up and held at the correct level: the 3.3V standby supply, the 12V rails and the LED backlight supply.

The result: no fault found. The power board was healthy.

Why "no fault found" was the useful answer

A healthy power board points firmly at the main board. On boards like this, a boot loop usually comes down to the memory chip that holds the TV's software, or the main processor. Both are fine-pitch chips soldered underneath their bodies, and replacing them is not an economic repair. A replacement main board is.

So instead of a repair bill for the wrong board, the owner got a clear answer and a route to a fix. We found UK listings for a replacement board with the matching part number, and suggested confirming the connectors and software version with the seller before buying.

The main board's own label: BM2A, part number 1-983-249-31. Matching this number is how you find the right replacement.

The £40 covered the testing and tracked return postage for the power board, with nothing further to pay.

The result

The owner bought the replacement main board we had pointed them to, fitted it themselves, and let us know the TV is up and running again.

Replacing the power board first would have been the obvious guess. It would have cost more and fixed nothing. One test on the bench settled which board was at fault before any more money was spent.

Questions about this fault

Why does my Sony TV show the logo, go black and restart? Usually one of two boards. Either the power board's supply sags as the TV starts up, or the main board can't load its software. Testing the power board on the bench tells you which, before you spend money on parts.

Can a Sony TV main board be repaired? Often, if the fault is a standard component. If it's the memory chip or the main processor, a replacement board is the realistic fix, and we'll tell you that straight away.

How much does it cost to find out? Our board assessment is £40, including tracked return postage, and it's credited against any repair. Power board repairs are £49 flat.

Is your TV stuck on the logo, rebooting, or dead with no standby light? See our TV and monitor board repair service. Power board repairs are £49 flat, and when it isn't clear which board is at fault, the £40 assessment finds out first.