Problems with IBM 5150 Power

Chuck Guzis cclist at
Tue Jul 28 13:47:53 CDT 2009

On 28 Jul 2009 at 14:12, Joe Giliberti wrote:

> It is very likely not a PSU fault. I wasn't sure on this, but could
> the PSU behave differently with and without load??

The 5150 PSU will not operate without a load.  At least a diskette 
drive must be attached.  Unlike other later power supplies, the 16-
64K 5150 uses an AC fan, so it will keep running even though the 
supply has shut down, so don't use the fan sound as an indicator of 

If the PSU has measurable output (within tolerance of +5, +12, -5 and 
-12) on all outputs with just a diskette drive load, connect the PSU 
to the motherboard.

What wasn't clear from your narrative was the "Nothing at the ISA 
slot" statement.   Does this mean that you measured correct voltages 
at the motherboard connector (with the motherboard attached) but not 
at the ISA slot?  If so, there's something really strange going on; 
most likely a broken PCB trace or cold solder joint.

On the other hand if the output goes away when you attach the PSU to 
the motherboard, you'll want to start checking for shorts.  Quite 
often, this is a motherboard capacitor gone bad.  Your ohmmeter can 
indicate which supply line is at fault.


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