Very dirty PC. Stripped it down and cleaned dust out of everything both with brush and shot of canned air. Reinstalled MB, memory, front panel switch plug. video card and front usb connection. Green power light on MB is on. Tested PSU with tester and no -5 V. Press power switch and nothing happens. No cpu fan starting, no diagnostic lights at back of computer (A,B,C,D lights). Tried replacing with standard ATX Power Supply (not Dell) and still nothing. Before disassembly computer would start and then freeze probably from all the dirt. Any help would be appreciated.
It could be a grounding problem. Improper grounding would cause the start and then freeze. Maybe a missing standoff, a loose screw or a loose wire. Try taking the motherboard out, place it on a thick magazine or cardboard, hook up the power supply, the and the video card and see if it will power up. You might have to use a screw driver to toggle/short the power switch to get it to come on. Grab a large metal structure to remove any static charge you may have before touching components.
The dirt shouldn't be a big issue, other than fouling the moving parts, like the fans.
The power switch on the case maybe stuck.
Otherwise, the motherboard could be fried. With no beep codes or error messages, it is almost certainly a bad board, CPU, or PSU. With no information, the only way to tell for sure is to replace one at a time.
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