Most likely bad capacitors in the power supply. Do you have another to test it with?

retropol wrote:

until I get finally beep and display, computer is starting, I hear fan, hdd starting operations or something, but no display and no beep, so need to switch off and try again...

Sounds like a bad video card, actually. Or maybe the monitor is the culprit and not the computer.

RTFM of your motherboard to find out what the beep(s) means. Furthermore check the clock battery, it might be almost empty.

@SpectriaForce:

Read his post again. There are no beeps when the issue occurs.

Cold boot problems are usually capacitor related, check the capacitors around the VRM. Checking inside the PSU may also be worth a shot.

derSammler wrote:

@SpectriaForce:

Read his post again. There are no beeps when the issue occurs.

TS is not very clear about the issue. He has written about a beep at the POST if the pc does power on. Now that is usually a good sign if it is a short beep. The issue makes me wonder whether he gets any (other) errors on the screen. In any case reading the manual of your motherboard, checking the battery, swapping the graphics card and memory and checking the PSU would be wise suggestions.

retropol wrote:

i need 2-3-4 times on/off my 486 to get finally beep and display, what may be the cause ? […]
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i need 2-3-4 times on/off my 486 to get finally beep and display,
what may be the cause ?

- performance of power supply?

- capacitors?

until I get finally beep and display, computer is starting, I hear fan, hdd starting operations or something, but no display and no beep, so need to switch off and try again...

I had an issue similar to this one before. Which fan are you referring to that won't power on? The power supply fan, or heatsink? I once had an issue where the heatsink for one of my computers wouldn't power on, and so I fixed it by plugging the wire for the heatsink into it's proper pins, and the heatsink ran its fan again. But it looks like you were referring to the PSU fan.

hi, thanks for comments...

i was talking about both fans, psu and cpu... just everything starts and there are no POST beep error codes...

computer just do not start video display and there is no this expected short beep, so i need on./off/on/off/on/off/ on and finally i hear beep and display shows up...

looks like capacitors... as you suggests... will try another psu