Insignia NS-LCD19-09 (PS OK, Inverter problem?)

[valvashon]

For some reason I'm just not going to let this 19" TV defeat me. I guess because I have a schematic to work from I feel like I'm oh so close.

History- while adding a turn-on circuit I let a small screw roll under the power board. Worked through all of that circuitry, probably replacing a MOSFET and PWM controller that were not bad but I was following the service manual. Finally figured out that the power supply is in working order when the inverter section (for the backlights) is disconnected. When the +12 supply to the inverter is disconnected the +12 and +5 will come up properly. The measured resistance value across the +12 and ground of the inverter circuit is approximately 5 ohms which can't be correct.

See attached, I traced out the circuit and the only logical place for the resistance to drop to that level is across one or both of the inverter MOSFET's, Q805 and Q806 if one or both are shorted internally. C811 and C812 are both good and have been removed. With these out the resistance is still about 5 ohms. +12 is highlighted in yellow and ground is in red. As you can see the only place they could meet up with such low resistance is Q805 and Q806, unless C801 or C802 and IC801 somehow became shorted.

Thoughts on replacing the MOSFET's? Anything else I could test to confirm that they are bad? If only one is bad and I pulled it out of the circuit should the +12 rail come back up and light up the other MOSFET, thereby lighting up half of the screen?

Val

[mon2]

To test a mosfet, remove all power. Wait a while to allow for the capacitors to discharge. Meter in resistance mode.

check the resistance across each mosfet:

source & drain
source & gate
gate & drain

checking for a low resistance which will mean the mosfet is defective. Post each measurement.

[valvashon]

To test a mosfet, remove all power. Wait a while to allow for the capacitors to discharge. Meter in resistance mode.

check the resistance across each mosfet:

source & drain
source & gate
gate & drain

checking for a low resistance which will mean the mosfet is defective. Post each measurement.

Mosfet #	1	2	3	4
Source & drain	4.4 ohm	0 ohm	4.4 ohm	.3 ohm
Source & gate	222 ohm	104 ohm	7.0 m ohm	5.2 k ohm
Gate & drain	227 ohm	104 ohm	5.4 m ohm	5.2 k ohm

These are some all over the place resistance readings. The short and almost dead shorts tell me I broasted some FET's during the "incident". The schematic and specs call for the FET's to be "AM4502C-T1-PF" with 11 amps maximum N-channel continuous drain current but the actual ones on the board are FDS8958A FET's with 7 amps maximum continuous drain current.

I think I found the AM450 type on aliexpress so I'll be ordering some of those.

Val

[valvashon]

Replaced the shorted MOSFET's with eBay specials from across the ocean. Marked FDS8958A, they could of course be counterfeit but as you can see, the TV now works. Would love to now thank everybody for their help but of course the saga continues. The set will turn on and come up with a picture, but then turn off after 5-10 seconds. Subsequent turn on's will produce a flash of a screen, then turn off again. It takes 4-5 tries to actually get it to turn on and stay on. When it does it will stay on all day. This was before I put my auto turn-on circuit into the equation. Now, instead of having to turn it on manually it will auto restart but exhibit the same behavior- on-off-on-off, etc., various lengths of on time before it turns back off again. Once the TV is up and running you can turn it off and on manually and it works as it should.

I do get a series of red blinks when it powers up (this was the case before, too) and now get red blinks as it shuts off which may be normal but I don't remember this. Am I looking at something weak in the power supply (like a cap) or knockoff MOSFET's that can't handle the job of finishing the power-up? Tomorrow morning I will pull the wires from one of the backlights and see if that affects the startup.

I could always try to obtain the specified MOSFET's with their higher current handling capabilites.

Any ideas welcome. I can make a video of what it's doing if it will help.

[nomoresonys]

Might just be the problem, you don't want to use ones with a lower current rating.

[valvashon]

Might just be the problem, you don't want to use ones with a lower current rating.

The FET's are marked the same as the ones that came out of there although of course they could be counterfeit. Digikey shows this part number as a "last time buy" starting in 6/2024 so fakes could already be popping up. Schematic shows the higher current ones but the lower current ones were on the board before I started shorting things out.

The set would power up and stay powered up (the first time on startup) when I had one or the other backlight connectors pulled. Dim screen during startup but then it went black. Could still hear audio; I assume that the screen going back with one backlight out is normal.

Monday morning I will warm up the power supply with a heat gun to see if that allows it to start. I still think it might be a power supply issue. The set restarted 20-30 times this morning, gradually staying on longer but never actually staying on. That's making me think it's a PS warm up issue.

[diif]

If the set is giving a picture that suggests the MOSFETs are fine.
If heating the boaerd has an effect then I suspect it's the caps, but i don't think it will and it's tired CCFLs

[nomoresonys]

Be careful not to overheat with a heatgun, I just use a hairdryer it's plenty hot enough to find a marginal/faulty cap.