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More than likelly the Switch Mode Power Supply board is not powering up. Often caused by bad or leaky electrolytic capacitors.
Nowadays, the switchmode power supply module converts the incoming AC power to the lower DC voltages required by the other circuit boards.
It uses lots of Electrolytic Capacitors to clean up noise in the DC power. But one Electro Cap is used to store a charge for turning on the switch mode power supply. Being Electrolytic they are prone to leaking & bulging so they don't do their job. They won't hold a charge. By pulling the plug and restarting, you have found a way to overcome the problem. But only temporarily. When the Cap goes fully bad, you won't be able to start the TV any more. Suggest you order a switch mode power module for it or take it to a tech who will replace the module. It's usually in a metal shielded small box full of ventilation holes.
I have repaired many switch mode PS but most tech's these days don't bother. They just get a replacement board
If you have the technical ability you could find the faulty part, unsolder it and replace with a new one for $1.50
If you take the back off you might see a couple of electrolytic caps that might be bulged on the power supply board. The power supply board is getting weak, and if you replace the electrolytic caps that will fix the problem. You can also replace the power supply, and you can check at www.partstore.com for a power supply.
this could be caused by vented capacitors in power supply..........look for domed tops on the electrolytics(small can shaped devices)on power supply board. These are bad and need replacing. 1000uf 10v. Replace with high-temp(105 degrees C) electrolytic cap. I use 25v or greater. Good success........meder'sTV
Everyone has problems with SMPSs; mine started in 1971 when IBM was first using them; they were literally blowing up and throwing molten copper several feet.
Anyhow, if you have the benefit of a capacitance meter with DF function, check the main filter caps all of the smaller caps.
The quality and durability of electrolytics is bad and getting worse and once one of these fails anywhere in a feedback path, the supply will not start.
HF is hell on electrolytics.
The (ferrite) transformer in a switcher measures very low on the Ohms scale; depending on power handling and service frequency, primary <<100 Ohms, secondary from 0.? - 10 Ohms between taps, because they operate at higher frequencies.
The 50 volts from the diode bridge is way too low; you probably have failed 100-200 uFd. electrolytics right close to them although this is fairly rare since they are only working with 120 Hz.
From a 120 VAC line, you should have ~ 160-170 DC volts at no load if the caps are still working as they should and you have no notable resistance between the AC and the diodes.
Well-designed switchers are wonderfully efficient and very reliable if the maker didn't cheat on component quality.
I've repaired literally thousands as a sideline in a now retired business and after studying them and adding a ceramic cap.or two and better quality electrolytics, they will run for decades.
At one time, I had standard repair prices on ~ 70 different models.
My guarantee was 'lifetime' and that was limited only because I retired.
Your shop has probably checked the remote and the mating receiver in the set so I would bet on (or against) the power supply.
They need to check all electrolytic caps ranging from about 1uF to 100uF in the main power supply circuit with a meter that can measure both value and DF (losses) and preferably at 1kHz minimum.
Those that are found failed (shrunken value & high losses) they should be replaced with low ESR types rated for higher frequencies.
Additionally, they should bypass any caps so found with ceramic capacitors of no less than 0.5 uF.
These caps will prevent the destruction of the replaced electrolytic caps again.
If the shop does not own such an instrument, find another shop.
If the set runs at all, your flyback and output transistor are good. If the set shuts down after a few seconds and the sides are wavy, it usually means that the main B+ in your power supply is too high and not regulating properly. I do not know what kind of power supply that model uses, but if has an old-style STR regulator (like the STR30130) then it is probably shorted, and if the set uses a switching power supply then look for an open or leaky electrolytic cap on the primary side. If the power supply is working correctly and the set shuts down, check for an open safety cap (the large 1.6 - 2Kv cap connected to the collector of the horizontal output transistor).