IBM AT not powering up.

[Electricdreams]

I'm a little puzzled with this one.

I'm trying to downsize and when I fished this one out of storage at my mothers house (in the spare bedroom not the loft) I thought it would be handy as my main DOS computer in my lounge as it has a special video card which connect to a scart lead.

When I first powered it up, a short click can be heard and the the LED lights up then goes out very quickly. The fan doesn't rotate.
So I removed the PSU and tried powering up the PSU, the fan still doesn't run. I suspected the PSU. (even tried the good power line trick)

So I found a old 386 with a same AT PSU (with a big red switch) and installed that. Tried it again, nothing.. unplugged it from the motherboard and it runs!

Is it the wrong psu for the computer? I thought they were all the same or is the motherboard at fault? I'm puzzled as I thought the 286 motherboard is generally reliable..

Anyone had this trouble before..?

[paulsherwin]

I think the PSU is shot, probably because of bad caps if it worked before storage. The AT PSUs don't interact with the motherboard in any way and just supply volts on the rails when switched on. Possibly the PSU is shutting down when presented with any sort of load.

[Electricdreams]

But the new PSU runs fine with the 386?

[paulsherwin]

Are you saying that the suspect PSU runs the 386 correctly? I thought you were saying that it worked when disconnected but not when the P1 and P2 connectors were plugged in. If it does work in one computer and not another, then you must have a problem with the mobo or peripherals which is pulling down one of the voltage rails and tripping the protection circuitry. This is pretty hard to do - I once melted quite a bit of wiring insulation in an old Epson AT by shorting the 12V rail.

[Electricdreams]

Forgive me I may not have been explaining properly.

The PSU from the 386 works fine when it was in the 386. I checked it before I took it out for the IBM AT. But when plugged into the IBM AT it stopped working.

I hope it's not to do with the battery leaking. I might have to take the motherboard out and have a look on the underside.

[Electricdreams]

Right I have just been unplugging all the cards out and working backwards and it was the first card that caused the problem, the fixed disk and floppy card. I'm disappointed by this as I need it for the hard drive. Would it be the three little caps on the board be at fault?
I can see one electrolytic and two orange caps that I haven't seen before..

[Electricdreams]

the orange caps are barrel type with a cone on the positive end.

And would you believe it, the original PSU works now!

[Dave Moll]

Originally Posted by idlerbiker

the original PSU works now!

At least that shows that the fault in your disc controller card hasn't killed the PSU!

[Electricdreams]

I found the schematics online, checked the orange caps, they all are 22uf 16v which I have already, replaced them all three and it runs perfectly again.

Thanks Paul for the tip re: cards

[jay_oldstuff]

sounds like those three where tantalum capacitors, I tend to think of them as the modern equivalent to hunts moaldseals, i had one let go on an old 286 motherboard once and go whizzing past my ear.

Jay

[evingar]

Originally Posted by jay_oldstuff

i had one let go on an old 286 motherboard once and go whizzing past my ear.

Jay

Indeed, I had an incandescently hot one fly out of a PCB I was working on at work once, whizzed past my right cheek. It just missed the top of the head of one of my co workers and embedded its self in the workshop carpet tiles. Quite frankly, if it had hit anyone in the eye, they would have stood every chance of loosing their sight. I treat tants vary warily now. I guess age has one advantage though, at least now I invariably wear glasses for such work, so I'm afforded a bit more protection.