Just to close the loop on this, the used power supply I bought on ebay works fine and the TV is as good as new. Only $20 and one less TV in the land fill. Thanks for all your help!

Thanks for the link. Since we can't be sure if that's the right part for U9101, and also since there are other parts that need replacing, I decided to go with a used PS board from Ebay for $21. The kit with 3 other boards could have been a good buy, but with shipping and tax, the total would have been over $60....

Looks like I will buying the repair kit instead of indivicual components because I can't find U9101 (SSC3S121A) from the usual suspects (Mouser, Digikey). Will post back here when new boards are installed.

Thanks for all the help, you guys! I will order the parts tomorrow and if I get it working, I will donate it since Best Buy is replacing the old one under warranty. I also hate to see things go to the landfill.

BTW, I can't believe they sell all 3 boards for only $33. It may be a wash buying the boards, vs. the parts I need....

Thanks and yes, I will replace the mosfet with the exact part if I can get it to show signs of life. You were right about an open resistor. The .3 ohm resistor was open. But after replacing that, still no joy. I had to get out the high powered microscope the read the number on U9101. It is SSC3S121A, if anyone else needs to know. Anyway, it is supposed to have 15V on Vcc, at least during startup and I measure about 1.5V. It is not heating up, according to my thermal camera. I will order a replacement U9101, but are there any other parts I should order? I will of course get the correct...

Thanks for the new info. I had the exact issue with one of my HP monitors. I had to remove the pcb containing several pushbuttons that controlled the menus. It wasn't a problem since I never have to change the settings. But based on the behavior of the air fryer, I'm not sure that was the issue. Too bad I scrapped it, or I would have done more testing. I would have just removed all the pushbuttons and used a jumper wire to test. Oh well. The other weird thing is that two previous units that were replaced under warranty had the exact same problem, but I don't see any others posting about...

Thanks for the suggestions. The power supply was good. The push buttons are discrete surface mount and all tested good. After letting it sit on the bench for a few days, the behavior changed to where only the rotary encoder would power it on (but not in the way it is supposed to). Anyway, this convinced me that it is unrepairable so I stripped it for usable parts and trashed the rest.

Thanks to all for your help!...

I didn't find any control boards on Ali, but saved a search on eBay, something may come up. Thanks so much for sharing your experience with STM32's; that saved me a bunch of time trying to learn how to program or even access it! Worst case is that I will strip it for parts....

I'm having a random issue with this otherwise nice double oven/air fryer. It sometimes does not respond to the controls. Sometimes only the upper control buttons work, sometimes they all work and sometimes none of them work. The power supply is good as the control board has a steady 5V and 3.3V. The pushbutton switches are all good as tested with multimeter. When the buttons are working, there is about 2.5V to the high side of the pushbutton. I'm assuming pressing a button grounds the input pin to the microprocessor. When the buttons are not working, there is about 0.3 volts on the high side....

Just to close out this thread, I removed the remnants of the old capacitor and the traces on the ground side were gone. So I mounted the replacement cap directly on top of the other one (PC4427) and it is working fine. Thanks for all the help! Will be donating to this awesome site.

Thanks, will do. I'll remove the mobo to make sure no damage on the other side and hopefully get a good connection point on the top side....

Thanks, useful information!...

Re: Emerson LC391EM3 - Constant High Pitch Sound

Thanks for the info and explanations! I found a 6.7uF SMD cap that is a little bigger physically, but was able to solder it in. All back together and ready for duty. Hopefully the difference in capacitance isn't going to be a problem...if so, should I just remove it and go without?

Ya'll are great!

Re: Emerson LC391EM3 - Constant High Pitch Sound

I fixed it! I found the service manual/schematic in another post for a different model, but same boards. Without that, it would have ended up in the trash. The problem was on the main board, specifically C3103 (a tiny surface mount capacitor near IC3008) was shorted. I found it by checking the resistances of the various power supplies (or rather their loads) and the P-ON-12V was 10 ohms. Pulled 2 other caps before finding the bad one.

I'm reposting the service manual in case anyone else is looking for it. ...

Re: Emerson LC391EM3 - Constant High Pitch Sound

Thanks neilc6. Unfortunately, my ps and main boards are different from those in that thread, so troubleshooting is tough, especially without a schematic. I did check resistance from the various power supplies to ground with the main board connected and disconnected. I did see low resistance (24 ohms) at the P-ON-12V when the main board was connected. Disconnected it was 200 ohms or higher, changing as the capacitor charged.

Re: Emerson LC391EM3 - Constant High Pitch Sound

I should mention that I pulled and checked all the parts that vinceroger69 mentioned and they all test fine.

Re: Emerson LC391EM3 - Constant High Pitch Sound

I have the same TV with the same whine when the main board is plugged in to the power supply. Did you ever find your problem? I see people on ebay that sell repair kits, one specifically for the whine and won't turn on. It includes what looks like 3 surface mount caps and is cheap enough to try, but I'm not even sure it is the main board or the power supply.