Why BP ATX does not start?

Question

Do it yourself

There is a non-working 400 watt power supply. Initially, I replaced all swollen capacitors, including a large 200V470. The duty room is present - 5V. On PSON 3.8 in. When green and black are closed, it does not start.

Answer 1

Oh, there are many possible reasons.
1. Before starting, it is extremely useful to make sure that the power transistors and diodes are in good condition. Instead of a fuse, it is useful to turn on a 100-watt incandescent lamp - you can understand a lot by its glow (or non-glow), and even if there is really a nuisance, then you won’t get enough without such a fuse lamp.
2. The duty room provides not only 5, but also 12 volts to power the PSU control circuits - check. Sometimes these 12 are actually increased to 15 ... 20 - do not be scared, this is the norm, only 5 volts stabilizes in the duty channel.
3. Check the health of the PWM controller chip. There is such a nuance: if this controller is notTL494 (or its exact analogue 7500), then it is better not to repair such a PSU, but to use it for spare parts, since the rest of the controllers (from my own experience) are too capricious, burn out easily and are not worth the effort. If it is TL494, you can go further. The most superficial check 494: on leg 13 there should be a stable 5.0 volts, on leg 5 - a saw with a frequency of several tens of kilohertz, on 4 and 16 - zero volts, on 2 - 2.5 volts.
4. If this is all normal, but there is no start, try changing the TL494.
5. There is a short-term start, then after 1 ... 3 seconds a shutdown - then look for a short circuit in the output circuits.
This is a very short enumeration, in fact, large volumes and even more "hunting stories" have been written (and posted on the Web) on the repair of the ATX power supply unit.