I'm afraid a lot of the time, the flow charts are not worth the paper they are written on, they were designed by people who "imagined certain fault conditions" and they don't take into account many other variations that occur in reality, though they were primarily made to try to assist people with a limited understanding of circuit theory. But a lot of the time, because of the limitations of them, a technician will fall off the bottom of the flow chart.
The main thing repairing an SMPS, is to understand how the circuit works, and set up a test system where safety is the number 1 priority, because you often need to test things on the line side of the supply, in many cases where the circuit is powered. The over 300V DC on the main storage capacitor is potentially lethal too, so the utmost care is required.
There are some un-powered parts tests you can do though such as on diodes & resistors and in circuit capacitor ESR testing. And check the continuity of the inductor L601 (depending on the design, sometimes these small inductors can go O/C)
Don't go willy-nilly replacing parts on hunches they might be defective, only replace a part if there is reasonable evidence on testing that the data supports the part is defective. Or you will create a real mess and probably introduce more problems and fail to repair it. You will see this sort of thing happen a lot with blind parts replacements.
One good bit of news, this VDU has an excellent manual with good schematics and waveforms too.
The first thing before powering it for tests, is to power it via an isolating transformer. The actual common voltage reference for everything on the primary side is the negative terminal of the output of the bridge rectifier and main filter capacitor, so if you want to scope anything like the drive to the output mosfet (gate voltage) or its drain voltage driving the transformer primary, that is where the scope's earth clip would connect to the negative terminal of the bridge. And unless you have a scope with completely isolated inputs (like the TEK 222ps) then you have to power the board via an isolating transformer, you cannot otherwise place the scope's earth clip there. Also, if you look at the drain voltage on the mosfet it is 580v pp, so you cannot use a x10 scope probe, there are only good to around 400v, You need to buy a x100 probe, rated at 1500 to 2000v , these are cheap now on the bay.
There are a number of possibilities why the supply could be shut down. And the IC's internal oscillator not running.
If you look at the start up circuit, the IC is initially powered via R603 and R630, probably to the tune of around 17V that you measured. Then (if the IC is driving the output mosfet and all else was normal) the 18V supply derived from the transformer and Q601 takes over. That is obviously not happening. This implies that the mosfet has no gate drive, or is open circuit, or the current sensing resistor in its source connection R610 is open circuit (check that on the meter, unpowered test). Also the + supply to the transformer primary comes via R602, R624, D605 and R827, if any of these were open that would also result in loss of energization of the transformer and have the equivalent effect of an open mosfet or open R610 or no gate drive for that matter.
(To test the mosfet itself, ideally that is removed and set up in a simple test circuit with a load, a simple low wattage, 5 or 10W auto lamp works and a 12V supply, the source connected to negative, the lamp in the drain to supply positive and place a 1meg resistor in series with its gate and check when the other resistor terminal connects to +12v the lamp turns on and when connected to negative it goes off)
If all those parts are ok, then to confirm that the IC is in shut down mode more likely (or possibly failed), requires a powered test and the scope/isolation transformer to look at the drive waveform at the gate of the mosfet or on pin 2 of the IC.
If the drive waveform is absent, this does not mean that the IC is definitely defective. Apart from receiving the current sense signal developed across R610, it also can be shut down via feedback from two optocouplers monitoring two of the output rails.
So, check the basics first, on passive unpowered tests. In circuit test on resistors for example, the value should never read significantly higher than the marked value, though it may often read lower due to other pathways in the circuit. If those tests reveal nothing definite, then the powered tests with the isolation transformer and scope with the x100 probe are indicated.
