1996 Power Macintosh with a dead PSU?

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1996 Power Macintosh with a dead PSU?

I have a 1996 Power Macintosh that does nothing when I flip it on.  I pulled the PSU board and it's dead silent.  No sound, and a trickle of 2 volts on one 5 volt rail.

I pulled the filter caps and they tested out fine.  I see 115 volts AC on one lead for each cap, but nothing on the otehr side.

I can keep pulling and testing caps, but I suspect it might be something else.  Any tips / refernences for these boards?

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maybe

If the design is anything like a Power Computing power supply I had, there is a large and hot resistor used to limit fan current (and thus noise). It was quite blackened. But the cause of no output is more likely to be something else.

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Dead PSU from a 1996 Power Macintosh

Right.  At the moment, my logic tells me the issue is on the hot side of the board.  Since this board is the typical CRT / logic PSU combo (and since neither side is putting out anything), I'm suspecting the issue comes before the transformers.

Note:  The fuse wasn't blown, and from what I understand, a blown fuse is often the result of a failed switching transistor on the hot side.  

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5400?

Oh, this comes from an all-in-one style like the PM5400? That could be a clue.

On a switching PSU without PFC, the line in goes thru an EMI filter (looks like you checked that), then gets rectified, the DC is smoothed by large (and very dangerous!) caps, then goes to the switching transistor(s) and the pri side of the transformer. I would always try to run a PSU on a safety isolation transformer if doing power-on measurements because the possibility of shock is real (those pri side caps also hold dangerous charge after being powered off if bleeder components are inadequate).

If you have access to the pins of the main MOSFET, you can test them with a DMM's diode mode. Most common FET failure is a gate to source short, which will read 0V on a diode test. Please be careful of the pri side capacitors (discharge them with a screwdriver etc)

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Dead PSU from a 1996 Power Macintosh

I'll look in to it.  Wouldn't a FET short blow the fuse, though?I realize I should have posted this at the start, but here's a pic of the PSU board:

 

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Tanzania ??

Does it have a Tanzania motherboard?  Other problems related to the motherboard battery can mimic power supply problems on those? I am more familar with those types of problems in PM4400 and the Motorola Starmax series.  I think a lot of Mac clones of that era use the Tanzania motherboard. 

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Dead PSU from a 1996 Power Macintosh

I've no idea about the Tanzania thing.  I can pull the mobo and look up Apple's part number...?

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The only Apple-branded Mac

The only Apple-branded Mac with a Tanzania motherboard was the Power Mac 4400.

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Dead PSU from a 1996 Power Macintosh

Mine is a 4620, I believe.

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That's not a model.

That's not a model.

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Dead PSU from a 1996 Power Macintosh

My mistake.  It's a 5260.  Here's the service manual:

https://www.ordinosaures-parc.org/images/PDF/apple/Mac/powermac.perf_5260.5280.pdf

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