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IBM 5155 Portable PC

latvija13

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Jun 8, 2012
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Hello all,

My brothers were cleaning out my parents garage the other day and ran across an IBM 5155 Portable PC. It appears to be in decent condition; however, when I try to turn it on, the floppy drives flash once and then nothing happens. I took off the cover and looked around a little bit inside. I was surprised to see how good of a condition the inside was considering the PC has been sitting in a garage for 15+ years. I'm wondering where to start with troubleshooting turning it on? Obviously power is getting to the computer since it at least flashes once. I was wondering if the fuse on the power supply could be bad?

Any thoughts would be great!

Thanks
 

Ole Juul

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What did you get on the screen? Does it have a HDD? Do you have a boot disk?

If you are booting without a disk and there is no HDD then when it checks the drive (light flashes) then it won't be able to proceed without an OS.

This is a great machine. I've got one, and it is nice to have an XT without taking up a lot of space.
 

latvija13

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What did you get on the screen? Does it have a HDD? Do you have a boot disk?

I'm getting nothing on the screen, and I don't believe that it has a HDD. From looking at the Repair Information PDF that I downloaded for the 5155 it looks almost exactly like the Interior View except four of the expansion slots are occupied instead of just two.

I do not have a boot disk so I'll have to see if I can it at my parents house among their junk. I wasn't sure if the screen should turn on at all if there was no book disk/HDD. I was expecting to find some corrosion or something to that nature inside the box but from what I can tell, it looks fine. Would the computer try to turn on and then just turn off without even attempting the POST with no boot disk or is that the single flash the I'm getting from the floppy drives? Is it simply looking for a book disk and since it's not finding anything, not powering on at all?
 

ibmapc

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Assuming you have the original motherboard in there, you should see the memory start to count up. Then the floppy drives should make some noise as they are tested. then it should beep once. Then drive a: should look for a boot disk. With no boot disk or hard drive, it should go into rom basic. So you've definately got at least one thing keeping it from coming on.
I would start by removing all of the expansion cards but leaving the speaker plugged in. Then power it up. The power supply fan should start up and after about a minute, you should get some beeps from the speaker (error codes telling you that there is no display adapter or floppy adapter). If the fan doesn't start, then you have a problem with either the power supply or the mother board. If the fan starts, but no beeps then the problem is probably with the motherboard, but still could be the psu. If you get beeps, then turn it off and plug in just the video card, and try it again. Keep adding cards (with power off) one at a time 'till it fails to start. The last board added is your problem. Note, don't turn on the psu without the motherboard attached. It won't hurt the psu, but it'll make you think that the psu is bad since the fan will not come on without a sufficient load attached to it. (Been there, done that)
 

pearce_jj

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Use a multimeter to check the PSU outputs for example on a floppy disk power connector - should be 12V between yellow and black, and 5V between red and black. If nothing remove all four expansion cards then re-test, and work back from there.

Just a word of caution - be VERY careful if you remove the shroud over the CRT (screen) itself, as with any CRT there are voltages in there which will certainly kill you if direct contact is made.

HTH!
 

cchaven

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If I remember the interior of my 5155, there isn't room anywhere inside for a hard disk unless you remove one of the floppy drives. So it sounds like, among other things, that it isn't finding any boot disk at all. As has already been mentioned, if you have the brightness and contrast turned down too much on the amber monitor you won't see anything either.

They are nice machines...I got rid of a dual floppy version years ago but still have one with a hard disk installed. I've run all kinds of OS's on it..MS-DOS/PC-DOS, CP/M-86, Concurrent CP/M-86, and the UCSD Pascal P-system.

Jeff
 

MikeS

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Just a word of caution - be VERY careful if you remove the shroud over the CRT (screen) itself, as with any CRT there are voltages in there which will certainly kill you if direct contact is made.

HTH!
Well, caution is always good, but "will certainly kill you" is a bit extreme; the only harm is usually from whatever you hit when you reflexively jerk your hand away.

The voltages (currents, actually) in the power supply are far more likely to hurt or (if you're extremely unlucky) kill you.
 

latvija13

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I would start by removing all of the expansion cards but leaving the speaker plugged in. Then power it up. The power supply fan should start up and after about a minute, you should get some beeps from the speaker (error codes telling you that there is no display adapter or floppy adapter). If the fan doesn't start, then you have a problem with either the power supply or the mother board. If the fan starts, but no beeps then the problem is probably with the motherboard, but still could be the psu. If you get beeps, then turn it off and plug in just the video card, and try it again. Keep adding cards (with power off) one at a time 'till it fails to start. The last board added is your problem.

I tried removing all of the expansion cards and then turning the power on after removing each card. I still got the same result, the 2 floppy drives flash once and then nothing. I'm guessing that the problem is with the PSU because the fan doesn't turn on either.

Use a multimeter to check the PSU outputs for example on a floppy disk power connector - should be 12V between yellow and black, and 5V between red and black. If nothing remove all four expansion cards then re-test, and work back from there.

Testing the PSU output on a floppy connector, the multimeter jumps to 12V when I flip the power switch, but then it drops down to 0 in less than a second. I'm wondering if that would confirm that the PSU is bad? I did notice that the PSU makes some sort of sound, when I turn the power on, almost like a tube TV turning off. I think I'll go ahead and poke around the PSU next.
 

mikey99

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Its possible you could have a shorted tantalum capacitor on the motherboard.
Try disconnecting the power supply connectors from the motherboard, and see
if the power supply voltage will stay up, check both +5 and +12 lines.

There could also be a short in the floppy drives, try disconnecting those also.
 

marcoguy

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Just a word of caution - be VERY careful if you remove the shroud over the CRT (screen) itself, as with any CRT there are voltages in there which will certainly kill you if direct contact is made.

HTH!

I refer you to this. I have actually been shocked really bad in the solar plexus by one of those. I was essentially giving a 21" CRT a bear hug to lift it up. It hurt like hell, but I'm still here typing away on this forum, aren't I?
 

modem7

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I tried removing all of the expansion cards and then turning the power on after removing each card. I still got the same result, the 2 floppy drives flash once and then nothing. I'm guessing that the problem is with the PSU because the fan doesn't turn on either.

Testing the PSU output on a floppy connector, the multimeter jumps to 12V when I flip the power switch, but then it drops down to 0 in less than a second. I'm wondering if that would confirm that the PSU is bad? I did notice that the PSU makes some sort of sound, when I turn the power on, almost like a tube TV turning off. I think I'll go ahead and poke around the PSU next.

Its possible you could have a shorted tantalum capacitor on the motherboard.
Try disconnecting the power supply connectors from the motherboard, and see
if the power supply voltage will stay up, check both +5 and +12 lines.
There could also be a short in the floppy drives, try disconnecting those also.

Note that the 5155 PSU will not operate unless it has a sufficient load.
Refer to http://www.minuszerodegrees.net/5155/psu/5155_psu.htm

Based on the info on that web page, see if the PSU will operate when the only thing connected to the power supply is the motherboard.
Of the two motherboard connectors, P8 and P9, try it first with only P9 connected.
 
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latvija13

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Note that the 5155 PSU will not operate unless it has a sufficient load.
Refer to http://www.minuszerodegrees.net/5155/psu/5155_psu.htm

Based on the info on that web page, see if the PSU will operate when the only thing connected to the power supply is the motherboard.
Of the two motherboard connectors, P8 and P9, try it first with only P9 connected.

Wow, that did work. With only P9 plugged in it does turn on. However, when I plug P8 in it goes back to the same problem. So based on that, what does it tell me? Is the PSU probably being overloaded?
 

modem7

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Wow, that did work. With only P9 plugged in it does turn on. However, when I plug P8 in it goes back to the same problem. So based on that, what does it tell me? Is the PSU probably being overloaded?
P8 supplies plus and minus 12 volts. As Mikey99 suggested, you probably have a shorted tantalum capacitor on your motherboard (overloading the PSU), and from your experiment, it will be on either the +12V line or the -12V line.

Refer to http://www.minuszerodegrees.net/failure.htm
The first step is to visually inspect the tantalum capacitors on your motherboard. If a tantalum has gone short, it may be showing visible indication of failure.
 

modem7

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The IBM 5155 was supplied with an early 5160 motherboard, the 64/256K one. The circuit diagram for that motherboard shows:

+12V line: Capacitors C56 (10 uF tantalum) and C55.
-12V line: Capacitors C58 (10 uF tantalum) and C57.

So if the cause is a shorted tantalum capacitor, it will in your case, be either C56 or C58.
Shorted tantalum capacitors are a very common problem on these motherboards.

Use of a multimeter on its resistance setting will reveal which line is short, and therefore point you to a particular tantalum capacitor, C56 or C58.

Alternatively, you could cut both C56 and C58 off the board, with the view to replacing them later. They help filter out noise on the +12V and -12V lines.
 

latvija13

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The IBM 5155 was supplied with an early 5160 motherboard, the 64/256K one. The circuit diagram for that motherboard shows:

+12V line: Capacitors C56 (10 uF tantalum) and C55.
-12V line: Capacitors C58 (10 uF tantalum) and C57.

So if the cause is a shorted tantalum capacitor, it will in your case, be either C56 or C58.
Shorted tantalum capacitors are a very common problem on these motherboards.

Use of a multimeter on its resistance setting will reveal which line is short, and therefore point you to a particular tantalum capacitor, C56 or C58.

Alternatively, you could cut both C56 and C58 off the board, with the view to replacing them later. They help filter out noise on the +12V and -12V lines.

Ok, I can see that C56 is the problem, C56 looks like it's burnt out while C58 appears fine visually. I'll see if I can find a replacement. Looks like I'll need to get a soldering iron :D
 

modem7

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Ok, I can see that C56 is the problem, C56 looks like it's burnt out while C58 appears fine visually. I'll see if I can find a replacement. Looks like I'll need to get a soldering iron :D
For now, you could simply cut C56 off the motherboard using a pair of side cutters.
 

latvija13

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For now, you could simply cut C56 off the motherboard using a pair of side cutters.

Should I be able to plug P8 after cutting off C56? The computer is booting up to "The IBM Personal Computer Basic Screen." Without any disks I doubt there's much more I can do for now. Thanks for the help everyone!
 

modem7

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Should I be able to plug P8 after cutting off C56? The computer is booting up to "The IBM Personal Computer Basic Screen." Without any disks I doubt there's much more I can do for now. Thanks for the help everyone!
Besides plus and minus 12V, P8 also supplies the POWER GOOD signal from the PSU. Without the POWER GOOD signal, the motherboard won't start. Obviously, because your 5155 is now booting to BASIC, you must have reconnected P8.
Yes, it it quite okay to have P8 plugged in without C56 being present. Ideally, you should fit a new C56 later.
 
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