IBM 5155 wont turn on unless I detach P8 power cable

[Pekopome]

****Update! Found the solution here! http://minuszerodegrees.net/5150_516..._beeps_1_1.htm****

Hey guys. Haven’t turned my 5155 on for a couple of years. When I did it turned on for 4 seconds and went dead. The PSU fan does a half spin and then nothing. But when I unplug P8 power cable on the motherboard the PSU comes to life and turns on the (flickering screen) But of cause nothing happens beside that. Have tried to detach everything from the motherboard expect P8 and P9. But still get nothing when P8 is attached. When only P9 is attached and nothing other it gives me a lot of error beeps. Of cause suspecting a blown capacitor. But I cant see anything. When P8 is the problem does any of you have any advice of where to narrow down the fail? Can it still be the PSU that candy handle a type of voltage or?

Best / David

 

[Stone]

Most likely one of the tantalum caps on the ±12V rail is short. Minuszerodegrees has a good discussion w/diagrams to help with that cap identification process.

 

[SomeGuy]

It sounds exactly like a tantalum capacitor on the motherboard failing and shorting (but without going boom). Common mode of failure. Too often people blame the power supplies, but the power supply is just detecting a short and automatically shutting down instead of burning things.

Given that you already tested it powers up without P8 connected, it would be a tantalum capacitor along the -12v or +12v line. You might try checking continuity between the +/- 12v lines and ground to see which.

If it were me, I would start by clipping or unsoldering the tantalum capacitor right next to the power plugs on the motherboard on the -12v line, and if that does not do it, then the +12v one.

You can run the motherboard without the tantalum capacitors, although they should be replaced.

 

[Pekopome]

It’s alive! Haha brilliant. I just cut off c56 and now it spins up. Easiest fix ever. Why on earth did IBM install a capacitor it doesn’t need. Will it cause other problems down the line?

 

[retro-pc_user]

The site is giving me a 404 error.

 

[vwestlife]

Those capacitors filter ripple on the power rail, to improve stability and reduce radio interference (to help meet FCC regulations).

 

[modem7]

The site is giving me a 404 error.

A 'damaged' URL was posted.

The OP was following the procedure at [here], and in following the procedure, ended up at the page at [here].

 

[modem7]

It’s alive! Haha brilliant. I just cut off c56 and now it spins up. Easiest fix ever.

I have added the failure to the list at [here].

Why on earth did IBM install a capacitor it doesn’t need.

I presume that you wrote that, because on the web page that you refer to, is, "The two capacitors are not critical to the operation of this motherboard, and this motherboard can operate without them."

Not much different to the wing mirrors on my car. They are not critical to the operation of my car, and my car can operate without them.

But the wing mirrors are there for a reason.