Is the PSU (AT, Delta Electronics, 145W) faulty or underpowered?

Do you have another PSU to test with?

Hello Andrea, and welcome 😀

There is a number of things that can be wrong with your system. Can you give some information?
Has it worked (POSTed) before?
Did it get any upgrades? Obviously, the RAM has been upgraded, is the CPU the original one that came with the system? (a Pentium MMX will work on a non-MMX compatible board, but not for long)

Check if the jumper settings are correct (unknown motherboard, so hopefully it has some jumper tables on the silk screen).
Check that the bus speed is set to 33Mhz, the multiplier to 5x, the cpu type to P55C (split voltage) and the core voltage to 2.9V.
Check for physical damage to the board (cracks, burns, leakage, scratches, etc).
Clear the CMOS (usually a jumper near the CMOS battery).

Open up you PSU and check for bulged or leaking capacitors (don't use it if the capacitors are in bad shape). 145W should be just fine, PentiumMMX systems do not draw that much power.

Try disconnecting everything from the PSU and the mainboard. Then connect only the ATX power to the motherboard, the PC speaker and the CPU + heatsink (no fan). So no RAM, no drives, no add-on cards, etc. Turn it on by shorting the power switch pins in the mainboard with a screwdriver.

If these components are OK, then (with most motherboards) you should hear a beep code from the PC speaker, indicating no RAM. If this is not the case, try adding a single stick of memory and check if you get a beep code for no videocard.
Still no beep? Check if the PC speaker is functional. Then start by swapping out components (I would start with the PSU, then the CPU).

Also, could you upload a photo of the board? This could be very helpful in troubleshooting.

sounds like power supply, could also be bad caps or both.

Id check the system board for bad caps that look bulging or leaking. If I don't see any that doesn't mean they are not bad, however then Id try testing it with another power supply. Start from there.

also check the ram and the video card are seated properly. A lot of time its something this simple.

Hello guys and thank you for your replies.
I remember doing such a kind of troubleshooting many years ago (10-15) on a PIII machine, in that case the culprit was a faulty GPU...

Unfortunately I don't have any other spare PSU available (at least AT) I'll take a look online.

This machine is new to me, I've got it from eBay for a bunch of euros, no upgrades, just gave it a deep clean (I assume nobody did it before).
It came with WinMe installed ad it was able to POST and boot the OS. After that I decided to erase the HDD to prepare it for a fresh Win98 install.

Later on (now I'm away from home) I'll post few screenshots from Everest (it came preinstalled on the previous owner's Windows installation) just to give you more info about specs.
However I believe CPU and MoBo are stock, this machine used to show the Acer logo while powering on, there is an "Intel Inside - Pentium MMX" sticker on the case which matches the installed CPU (well, it might have been customised but I don't believe so, AFAIK it used to be an office PC).

Here are the basic specs:

"AcerPower"
- Intel Pentium MMX processor
- ALi Aladdin M1543 chipset
- 2x 64MB Samsung RAM sticks
- 1x NEC 3,5 FDD
- 1x Samsung CD-ROM optical drive
- 1x Quantum Fireball CR 8GB IDE drive
- Intel PRO/100 LAN adapter PCI (now removed)
- ESS AudioDrive 1868 soundcard ISA (now removed)
- Delta Electronics 145W AT PSU

The graphic adapter is integrated on the motherboard, ATI Rage II+ PCI according to WinMe's device manager.
Only a PS/2 keyboard and a ProView 17" CRT monitor are connected.

I tried to do the following:

- Remove everything but the RAM; dead, no video signal, only CPU and PSU fans spinning, no boot

- Remove everything, including RAM and disconnect CPU fan; dead, no video signal, no beeps or boot, only PSU fan spinning

- Disconnect and reconnect the PSU to the motherboard; same

- Remove CMOS battery; same

- Remove CMOS battery then reinstall a single RAM stick; system POSTed, video signal ok, RAM test (not MemTest+, just the BIOS built-in RAM test) and lack of FDD/keyboard error

- Insert the other RAM stick, reconnect CPU fan, optical drive and HDD; system POSTed, video signal ok, CD-ROM alive and HDD spinning ("insert system disk" error as the hard disk is wiped)

- Switch off, disconnect power cord, wait few minutes then turn it on again; again no life, no video signal, no HDD spinning and so on

- Then switch off, remove CMOS battery again, disconnect and reconnect IDE drives; now it seems to be working "fine"

Of course this behaviour is not normal, however I've tried to reinstall Windows Me as a test and everything went fine, the PC was running smoothly and installation was successful... now it's switched off and I'll try to power it on again as soon as I'll be back home.
Never seen anything like that before, haha.

Unfortunately this BIOS is very "basic" not so many options to play with, no voltages/fans rpm page, no cpu/memory clock and frequencies page, just few pages for system info, basic settings and IDE drives setup, nothing else...

P.S. MoBo speaker is working correctly

I'm finally back home, so this is the motherboard:

https://ibb.co/vQZkVtY

Now it's clean and I see no bulged or leaking capacitors, only the CPU fan needs a replacement (noisy and worn bearing).

This is another view of the mobo:

https://ibb.co/wWW8KRP

Few specs from Everest Home:

https://ibb.co/Ms1Ys4R
https://ibb.co/5kKKvMZ
https://ibb.co/XZ0LpjC

However, when I came back home I tried again to power on the machine (it was "working" before shutdown) and nothing, no video signal, no hdd spinning, only fans were alive. So I decided to reseat every component (again) and now it seems to be working (again)...

Fun fact: if I keep the BIOS power management functions disabled and the power button on "ON" position, cutting down the power using the small switch located on the power multi-plug instead of using the PSU's power button (of course after receiving the "it is now safe to turn off the computer" message) and then powering it up again using the same small switch everything is fine and the machine starts up correctly... very strange, maybe something is wrong with the PSU's power button.
Anyway, I've ordered a replacement AT PSU, I no longer want to rely on this one, I hope this will solve the issue...

SteyrPuch wrote:

very strange, maybe something is wrong with the PSU's power button.
Anyway, I've ordered a replacement AT PSU, I nolonger want to rely on this one, I hope this will solve the issue...

I think a new PSU could help. If it really is the switch, keep in mind that AT PSU usually don't include a switch.
You could try some contact spray to clean your switch contacts (don't forget to rinse with alcohol afterwards, as contact spray is slightly corrosive)