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IBM 5150 Throws Breaker When Switched On

J

JonPointer

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Aug 10, 2018
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Fort Collins, CO
I have an IBM 5150 that my dad passed on to me. It worked for several years, but now, when it is switched on, it throws the breaker for whichever room it is plugged into. I'm a mechanical engineer with some electrical experience - soldering, using a meter, wiring, etc. - could anyone recommend how I should proceed in troubleshooting what has gone wrong, or a useful resource?
Thank you!
 
The obvious next step is to disconnect the drives and motherboard power leads from the Power Supply. Then turn on the power supply and see if the breaker is thrown. If yes, then PS is shorted. If no, then you might have a short elsewhere. But from the description, my initial reaction is power supply.
 
Yes, it will almost certainly be the power supply.

There are some 'X' capacitors on the mains side of the power supply acting as noise suppressors. Almost certainly this has gone faulty.

Have a look for anything burnt or charred first. Use your eyes with a bright light and a magnifying glass.

I assume you have an electronic breaker arrangement (ELCB or RCD) in your house that is tripping?

Dave
 
That's going to be something really weird. Isn't the 5150's power supply internally fused on the Line side? Your short is going to be after the main switch but before that fuse, or that would of already popped.
 
daver2 said:
I assume you have an electronic breaker arrangement (ELCB or RCD) in your house that is tripping?
Click to expand...

They're more common in North American installations than they used to be, but by and large, they tend to take the form of a specially-designed receptacle placed in kitchen areas, garages, lavatories, etc. and not throughout the house or in the main distribution panel, so the protector would trip locally, but not the breaker in the distribution panel.
220px-Residential_GFCI_receptacle.jpg


Now, if you look at the fairly-standard 120V-only North American 5150 PSU circuit diagram here, you'll see that the power switch follows the power line input filter network. So a short on the AC input board of capacitors C1 or C2 will have precisely the effect described. For testing, both can be removed--they serve as EMI suppression and are not a critical part of the power supply.
 
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Thank you everyone. It was actually plugged into a standard outlet. When turned on, the unit is tripping the breaker for the room at the house breaker box outside.
@Chuck(G) Thank you for the circuit diagram. I see the AC input board (picture below) and I see C1 and C2. Are you saying that I can remove these two capacitors from the AC input board and it is OK to then turn it on? Then if it works, the problem was in those capacitors and I replace them? Thank you so much for your help!
AC-Board.JPG
 
JonPointer said:
Thank you everyone. It was actually plugged into a standard outlet. When turned on, the unit is tripping the breaker for the room at the house breaker box outside.
Click to expand...
Presumably, by your "When turned on", you are referring to the red power switch on the side of the PSU. If so, referring to the diagram at [here], the fault must be following the red power switch, i.e. excluding C1 and/or C2 as the problem cause.

Because the PSU's F1 fuse is not blowing, I guess that your circuit breakers are more sensitive than a slo-blo 2 amp fuse .

Did you do what was suggested in post #2, which includes disconnecting any monitor plugged into the PSU's rear panel ?
 
A simple test is to unplug the AC board from the power supply main board. If the breaker still blows, you have narrowed the problem considerably.
 
Yes, anything plugged into J3 (the monitor) is before the internal 2A fuse, so this would be a candidate.

I would be tempted to remove fuse F1 (2A) and see what happens then. However, if you have a multimeter, I would be inclined to be taking resistance measurements between:

Live and Neutral.
Live and Earth.
Neutral and Earth.

With F1 removed, all three of the readings should be higher than 100 kOhms.

Follow the wiring in the schematic referenced in post #9 - or look at the full thing at http://minuszerodegrees.net/5150/psu/5150_psu_from_sams_computerfacts.pdf (remembering that your particular PSU may not be the same as the reference).

Dave
 
For clarity - when the red switch was turned on, the 15A breaker for the room, out at the breaker box, was tripping. I relocated to a protected outlet in the bathroom, so it would trip instead of out at the house panel.
Now for the good, but confusing news. I had the power supply out and open and blew out dust and some little bits I heard floating around. Took the power supply by itself, plugged it in, turned on the red switch, and the fan started and the outlet didn't trip. Next I put the power supply back in the unit and connected all of the connectors. Then turned it on and, again, all worked fine, no trip. Next I hooked up the monitor, tried again, and no trip. Finally, I attached the keyboard and put in the startup disk - and it all started up and worked fine.
So, sorry for all of this, but it appears to be working now. Not sure what happened. I guess I will make sure that everything is cleaned out, no bits floating around, and put it all back together.
Any other ideas? Thank you all again!
 
Ah, the old "metal mouse" :)...

I do this quite a lot with amplifiers when they don't work... Turn them off; blow all the dust, debris and dead spiders out of them; spray a bit of contact cleaner on everything and put them back together again - they are usually good for another five years then!

Dave
 
I have heard that electrolytics that haven't been used in a long time can still work, but need to be reformed. There is a process to do it not to accidentally damage things, but in your case, you might have just gotten lucky. People warn not to just plug in and flip switches on something not used for a very long time. Possibly was only the primary side of the PSU? and your breakers saved it.
 
It's interesting in that power supply. C1 and C2 appear to be the considerably more reliable metal film or polyester type.
 
NeXT said:
It's interesting in that power supply. C1 and C2 appear to be the considerably more reliable metal film or polyester type.
Click to expand...

Yes, those are not the standard Rifa "go boom" caps. Those caps should be rated X1/X2 safety "across the line caps", though I've rarely seen them in that package style you have. I know the markings on the caps are facing down (or rubbed off) but I wonder if those are indeed safety caps - as in, since they are connected "across the line" to both sides of the AC mains (even when the switch is "off") they are supposed to fail in an open state and never fail shorted. Would just be an interesting thing to check...
 
In yesteryear times, it was the Y-rated caps that went between the line and chassis ground and X-rated ones that went across the line, after the fuse. Y fails open, X fails short (or supposed to). The Y-rated cap was usually a disc ceramic and the X-rated one, a "foil in oil" one. You can substitute a Y-rated safety cap for an X-rated one, but not the reverse.
 
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