IBM PS/1 model 2155 refuses to start

[ECarnby]

I've got an old computer that suddenly won't start. It's an IBM PS/1 model 2155. I got it new in 1993 and used it until 1996, when it was replaced and went into storage. I dug it out (literally -- it was buried in a decade's worth of forgotten crap) a few days ago. It started fine, loaded DOS, loaded Win3.1, it ran the games I wanted to play, and it shutdown without issue. Later that day, it did all that again. The next day, I pressed the power button and nothing happened.

No POST, neither on-screen nor beeps.

The hard drive doesn't spin. The floppy drive is never accessed, even if you take the hard drive out. The CD-ROM drive does nothing, but it will open and close when you press the eject button. The keyboard lights up for an instant, then goes dead. Unplugging the keyboard, mouse, and gamepad changes nothing. The only things running are the power supply fan and the on light.

I've made sure everything is well seated. Both sticks of RAM are firmly in place. The sound card, modem, and scanner card are fully inserted into the riser and the riser into the motherboard. None of the cable connections are loose.

The old CMOS battery was slightly corroded, but changing it didn't help.

I don't have any means of actually connecting the hard drive to a different computer, but I have tried just attaching it to a different power supply and it spins up and sounds normal.

Taking out either stick of RAM doesn't change anything, neither does taking out both of them (still no beeps). Removing all the drives doesn't change anything. Removing the riser and all three cards on it doesn't do anything.

I checked the power supply with a multimeter and it's working fine.

I admit that I'm not a hardware guy and have already exhausted everything I know to try. Does anything leap out to those of you more well-versed in the subject?

 

[barythrin]

You might check the motherboard for any capacitors that have a bulge or hole in the top. Have you reseated the processor? Does the keyboard do anything if you press caps lock/num lock (do the lights turn back on or are they not responding)? Usually the capacitors can dry out if they don't get electricity for a while but it also sounds like possible a bad processor. How handy are you with a multimeter?

 

[ECarnby]

The capacitors look okay. I don't really know what I'm looking for, but they all look the same, so I assume they're okay. There's certainly nothing obviously wrong.

The CPU is soldered on. There is a socket on the motherboard for a second CPU that will override the built-in one, but there isn't one installed.

Apart from the brief flash when you first turn the system on, the keyboard is totally dead. Caps lock, num lock, and scroll lock do nothing.

I would describe my knowledge of multimeters, and electronics in general, as fairly rudimentary.

 

[modem7]

The hard drive doesn't spin

I don't have any means of actually connecting the hard drive to a different computer, but I have tried just attaching it to a different power supply and it spins up and sounds normal.

The kind of hard drive fitted to a PS/1 would normally spin whilst it has +12 volts supplied to it. It sounds like your power supply, for whatever reason, may be supplying +5 volts, but not +12 volts. That situation would result in the symptoms you describe:
1. Motherboard not starting because of lack of POWER GOOD signal from power supply.
2. Keyboard momentarily flashing its lights (part of keyboard's self test) on receipt of +5 volts.
3. Hard drive not spinning up because of lack of +12 volts for spindle circuitry.

I checked the power supply with a multimeter and it's working fine.

What exactly did you check/measure?

 

[ECarnby]

I unplugged the molex connector from the CD-ROM drive (since it was the easiest to access) and stuck the black probe of the multimeter into one of the middle sockets and then touched the red probe to either side socket. They read 5 and 12 volts. I don't know how many amps its putting out -- I know how to measure that with a multimeter, but I couldn't figure out a way to actually do it.

 

[modem7]

So +12V on at least the CDROM drive power connector.

The hard drive behaviour is interesting. You know from your experiment that the hard drive spun up when it was connected to a different power supply. It would be good to check the +5V and +12V on the hard drive power connector in the PS/1 (with the drive connected). If both present, then there must be something coming in on the hard drive control/data cable that stops the drive from spinning up, and you'd be able to prove that by disconnecting the cable.

If there is both +5V and +12V on the hard drive power connector, I think the 'POWER GOOD' signal from the power supply is the next thing to investigate. I'm unfamiliar with the PS/1 series, and as such, I don't know whether or not their power supplies generate a POWER GOOD signal. Most (not all) other IBM personal computers do, and it's that signal in effect, that starts the motherboard (releases it from a reset state).

 

[ECarnby]

I've discovered that if I unplug the power cables from the motherboard but leave the drives connected, both the floppy drive and hard drive start spinning when switched on.

Checking the voltages on the motherboard cables is difficult as the sockets are very small, but power good shows around 6 volts.

 

[ECarnby]

Evidently a problem with the motherboard. I replaced it with another out of a different PS/1 2155 and now it's working fine again.