SIGLENT Power Supply Repair SPD3303X-E: 0-30v Powers itself off at random. 

Hi There, Well if it doesnt light up at all what i would do is start at basics being, to varify if its just a backlight usse check to see if the output still works & the buttons light up when pressed, if they do it looks likely its a LCD backlight ussue so read on, check all the fuses are ok. From there connect an ohm meter across the mains plug Live & Nutural pins (with the unit unplugged of course) press in the power button & should read from open circuit to a lowish hundreds of ohms resistance with the power button pressed in, all this non invasive test does is to confirm the fuses, cord, switch, & primary coil of transformer are in circuit. Obviouly under this test if you can only read open curcuit then there is a break somewhere. Secondly if all that works out you can measure across the rectifier diodes / smooting capacitors on the main PCB towards the rear with the heatsiks on it. This may be difficult due to the cramp space, but the object here is to confirm if there is DC voltage on the smoothing caps. Thirdly the little bridge rectifier as in the video that powers up the front board this supplies one of the voltage regulators which you can measure, using the heatsink TAB as the ground reference for your meter, also the regulator IC runs fairly hot if its getting & giving out a supply, also there is a single diode near the this on the top edge of the PCB which supplies a yellow / orange coloured thermal fuse next to it, this also supplies many circuits so check. You will be able to see the small connector suplying the board with a connector near the bridge rectifier this supplies about 2x 6v AC supplies to this board to supply the little bridge & the single diode. Note the -ve (0v) line reference for measurments here is isolated from the (ov) on the main board / high current supply outout as there are different secondary coils on the transformer. The little bridge rec as per the video supplied other voltage regulators on the PCB near the processor chip, as identified in the video, also there is a Buck circuit / charge pump located near the processor & display connector which supples high voltage to the LCD back light. Check components round there. the surface mount inductors should give a dirty ringing sound when the Buck is working it sounds like low volume white noise.

Well TBH i do not know if Siglent did change the design, i would not worry about it as it may take many years for this issue to show, i don't think its a common problem because there are not any other videos on youtube about it so it may be an issue to only a small number of supplies & could be down to a bad batch of bridge rectifiers that affected only a small number of supplies that may have been returned under warranty. At least you know how to fix should it happen.

I have two SPD3303C and both have the small rectifier but one has a small heat sinks glued to it. This version with the heat sinks has experienced the problem