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Suspected 5150 power supply fault

j2theLj

j2theLj

Member
Joined
Jun 6, 2023
Messages
11
Location
Uppsala, Sweden
Hello, I'm new to this forum, and the reason is that I earlier this week came into possession of an IBM 5150 PC. It was sold to me untested as an XT and it wasn't until I got home and counted five card slots that I checked the label - indeed 5150. The machine has two floppy drives, but I thought maybe the hard drive of the XT was in a card slot.

Anyway, the computer doesn't power on. There is no sign of life from neither the 5151 monitor nor the disk drives. The PSU fan does spin however.

I did the minimum diagnostic configuration test from minuszerodegrees.net and there is no beep, nor does there seem to be any shorts on the voltage rails on the mother board. This leads the troubleshooting to the PSU which is a bit of a head ace since it won't power on unless there is a load.

My power supply is the one that mrmanse made schematics for.

I checked the fuse and it's fine. Without it the fan won't run either.

I tried to run the PSU with a load of four SCSI hard drives, but this did not make it kick to life (are four enough?). I measured the voltages under this 'load'. On the +5V it seems to start off at 3V and then within a split second go to 1.5V and then quickly go down to 157mV where it remains. The -5V is at about 3mV and 12V is at 70 mV.

I can hear a quick tick in at least one of the hard drives as the power is turned on, but that's all. Without the hard drives as load, the 5V on one of the molex connectors measures 5V for about a second before decending to 300mV and it keeps falling.

Does anyone have any pointers on how to go about PSU troubleshooting on the 5150, taken the need for a load to show if it actually works? Any particular known failure points?
 
Welcome to the forums. Possibly 4 HD's is too much load? I don't know what power supply you have or it's rating in watts. I still use the "car bulb" load test. Right now I need to check one of my PSU but don't have any of those bulbs. Just to see if it can start with a light load. Mine too dies if I connect "1" HD to it. This is not for a PC, but a Rainbow 100 (in my case). Good luck. Many here will be able to help you move forward on this.
 
On a 5150, the motherboard by itself should be enough load to start the PSU. In the minimum diagnostic configuration, that's all you have plus a speaker.

I'd go back and carefully follow the steps indicated by your symptoms: Your power supply fan runs but you did not hear the beep. Just put your cards and drives to the side for the present.

They may be defective, but you've got more basic problems to deal with first. Either your MB or your PSU is sick.
 
j2theLj said:
I did the minimum diagnostic configuration test from minuszerodegrees.net and there is no beep, nor does there seem to be any shorts on the voltage rails on the mother board. This leads the troubleshooting to the PSU which is a bit of a head ace since it won't power on unless there is a load.
Click to expand...
Presumably, the navigation that you followed was:
1. At [here], you would have clicked on {Click here if you did not hear the beeps}
2. At [here], you would have clicked on {Click here if the power supply's fan is turning (continuously)}
3. At [here], you would have clicked on {Click here if any of the 5 lines did not measure as expected.}
4. At [here], you would not have measured zero or a few ohms on any line.

DDS said:
On a 5150, the motherboard by itself should be enough load to start the PSU.
Click to expand...
"Should" being a key word. There has been at least one case where it wasn't enough of a load.

j2theLj said:
I tried to run the PSU with a load of four SCSI hard drives, but this did not make it kick to life (are four enough?). I measured the voltages under this 'load'. On the +5V it seems to start off at 3V and then within a split second go to 1.5V and then quickly go down to 157mV where it remains. The -5V is at about 3mV and 12V is at 70 mV.
Click to expand...
That really sounds like the PSU starting up and then quickly shutting down. Capacitance will mean that the voltage on the various output lines does not drop instantly to zero volts.
Possibilities:
* Faulty PSU.
* Good PSU being under-loaded.
* Good PSU being over-loaded.

Are four enough? A yes/no answer is not possible without knowing the start-up loading requirements of the make-model of power supply, and knowing the start-up loading presented by the SCSI hard drives.

Along with what tradde indicated, try one drive, then two, then three.

You have two floppy drives (presumably two full-height IBM ones). Try one of those, then two.
 
Thanks all! I'll be trying the PSU under different loads and then probably test components in the PSU with the help of the schematics if it still doesn't fire up. I'll be reporting back!
 
I managed to get hold of an AT PSU and the the 5150 started right up - I'm very pleased. This one too needed a load to get going. I tested the voltages before I hooked it up to the 5150 and there was nothing, but with three old hard drives connected it came to life. Shame that the AT PSU doesn't fit in the PC case... Now that I know that the fault definately is the original PSU I'll focus some efforts there when I get time.
 
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