If another PSU just works fine, its probably some PSU safety-self-shutdown.
Maybe there is only some minor stuff like dead capacitors and the like.
Can you please post photos?

And, if you are accustomed with working with line voltage electronics, and won't electrocute yourself while doing that, taking some meaningful pictures from the PSU inside could be very helpful to find out what is wrong.

Thanks for the reply.
I dont know how comfortable i am with working on PSUs, but I guess I can inspect it for bad caps without dying.

Here are some pics:

The PSU apparently got some special screw you screws I cant open.

I started pulling cards, and I found the culprit. The PC starts without the HD/Floppy controller card.

What could be wrong with it?

Here is a pic:

Still not sure whether it is the PSU which possibly can no longer deliver its full rated output due to aged capacitors or an actual defect in the card.

To establish whether it is the card you should try to take off other loads and try with the card installed.
For example, the stickers on the PSU indicate that only 2.8amps should be drawn by the HDD. However there were 5 1/4 FH drives that were rated at 4+ amps at 12V.

The first thing I did was pulling the molex conectors from the diskette and hard drives. The cards where still in. This is the only card that changed anything.

Then just replace the card with another MFM controller.
Take care it is a WD one, so you retain your data. Does not need to be fullsize.

Edit:
Also check that the cables are put correctly onto the drive.
Good MFM cables had a little coding thing so you could not plug them on the drive incorrectly.
But either that coding strip got lost (it was often just a piece of plastic sheed put into the plug, which fell out more or less easily), or there was never one. In that case it was sometimes possible to plug in a way that shorted out, due to large tolerances on the PCB fingers.

Yeah Im afraid I have to do that. I have found a similar one on ebay. But It would be really nice if I could fix this one somehow. My card have some extra cords on it for some reason compared to the one on ebay.

Edit:
Yeah I have checked with the card plugged in the ISA without MFM cables connected. Same happens.

Edit2:
Same also happens in different ISA slots.

I have a 286 XT and also had problems getting the PSU open. It uses screws with hollow hexagonal heads with a dimple that prevents insertion of normal keys. Fortunately I had a tool capable of opening them, but in the process (it was cheap soft metal junk) it wore out and became useless. What is the name of this screw and what professional tool do I need to open it?

SVD wrote:

My card have some extra cords on it for some reason compared to the one on ebay.

Which ones? I only see the FD/HD and LED connectors.
Maybe post the image of the card you saw in the bay so we can make sure it is compatible.

Errius wrote:

What is the name of this screw and what professional tool do I need to open it?

A few years ago I bought a cheap set of "tamper proof bits" (ebay search words) and they included hex and torx bits with that hole in the middle.
As I just throw away the sh!tty "tamper proof" screws and use normal ones instead, that cheapo set still serves fine even though the bits are of quite soft metal (compared to quality ones).

SVD wrote:

I started pulling cards, and I found the culprit. The PC starts without the HD/Floppy controller card. […]
Show full quote

I started pulling cards, and I found the culprit. The PC starts without the HD/Floppy controller card.

What could be wrong with it?

Here is a pic:

55536483_367573390514022_6404556211324190720_n.jpg

Are those axial tantalum caps I see ? I have heard tantalum caps have a tendency to short when they go bad .

I had the same problem with a new old stock Everex EGA card. It had a shorted tantalum (silently failed... it didn't blow or smoke) which caused my 5150 to have a pretty similar problem. It wouldn't turn on. After replacing that cap (I just used whatever standard electrolytic I had laying around that was a similar value) the card has worked fine for three years now.

If i were you I'd check for continuity across the leads of each tantalum cap. If any are shorted, replace them.

retardware wrote:

SVD wrote:

My card have some extra cords on it for some reason compared to the one on ebay.

Which ones? I only see the FD/HD and LED connectors.
Maybe post the image of the card you saw in the bay so we can make sure it is compatible.

The yellow lines near the big chips. Link https://www.ebay.com/itm/Vintage-IBM-MFM-Fixe … vIAAOSwowdckW9w

@darry @Ozzuneoj
Ahh thanks, i measured 1,1 Ohm on the big yellow cap C46 right in the middle of the 8bit part of the ISA slot. Is that normal?

EDIT
0 Ohm seems to be 1.0 ohm on my measurement thing, so I guess the cap is 0,1 ohm. Thats a short I would think.

SVD wrote:

The yellow lines near the big chips. Link https://www.ebay.com/itm/Vintage-IBM-MFM-Fixe … vIAAOSwowdckW9w […]
Show full quote

retardware wrote:

SVD wrote:

My card have some extra cords on it for some reason compared to the one on ebay.

Which ones? I only see the FD/HD and LED connectors.
Maybe post the image of the card you saw in the bay so we can make sure it is compatible.

The yellow lines near the big chips. Link https://www.ebay.com/itm/Vintage-IBM-MFM-Fixe … vIAAOSwowdckW9w

@darry @Ozzuneoj
Ahh thanks, i measured 1,1 Ohm on the big yellow cap C46 right in the middle of the 8bit part of the ISA slot. Is that normal?

EDIT
0 Ohm seems to be 1.0 ohm on my measurement thing, so I guess the cap is 0,1 ohm. Thats a short I would think.

I mainly just look for shorts (0 ohms; or a beep on my continuity\diode tester). Beyond that, I can't say for sure. I've just been learning this stuff by experience... I'm definitely not an engineer. 🤣

I see it doesn't really have that many tantalum caps. I think the bullet shaped cylindrical ones like C45 and C46 are tantalums, but the picture isn't quite clear enough to tell what type the others are. I don't think the small-ish yellow ones that are sort of irregularly shaped are tantalums. I think those are called "resin dipped ceramic", which should be nothing to worry about. I'd check any other bullet shaped caps. I don't think any caps should should ever be shorted, so it should be easy to tell if one is causing this.

SVD wrote:

The yellow lines near the big chips.

Factory patches, it is a bit older version than that in the bay. The latter one already has them incorporated in the PCB.

SVD wrote:

Ahh thanks, i measured 1,1 Ohm on the big yellow cap C46 right in the middle of the 8bit part of the ISA slot. Is that normal?

Solder off one leg of the cap and measure the caps' resistance again.
The small resistance of 0..1ohm often is just the oxide layer of meter tips and solder surface.
If the cap measures that way with one leg off (to exclude other shorts causes), then it is indeed the culprit.

Hi again, just to update. I have finally come around and changed the cap. Now its working! So thanks for all your help bringing this beast alive 😀

The new cap is the second to the left of the battery wires. Also I have installed a BOCARAM/AT card with 1 MB, so now I have over twice the RAM I will ever need 😁

Good to hear! Might have to look at my EGA card now and see, if the caps are okay 😀

Oh do I want a 5154...

Yeah its always the caps 😀

The 5154 got really brilliant colours ^^ They seem to be a bit expensive and hard to get tho, none on ebay at the moment. I got it with this AT computer and a Cherry G81 keyboard for 200€, someone was getting it for 150 but i overbid them, still cheap I think. I thought it was a 5153 until i went to pic it up. 😁

The contrast knob does not seem to work tho. But its at an okay level luckily.