Author Topic: Dead PSU from a 1996 Power Macintosh  (Read 628 times)

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Offline joelgraff

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Dead PSU from a 1996 Power Macintosh
« on: January 21, 2022, 01:58:36 pm »
Hello,

I've been trying to sort out a dead PSU on a 1996 Power Macintosh.  The board is typical for a Mac - there's a side for the logic board (with 5 / 12v power rails) and a side to power the CRT (left and right sides respectively in the marked up image).  The CRT is not powering up and I see maybe +2V on one +5v rail for the logic side.  I see line voltage to the line filter caps and nothing thereafter.

I pulled the line filter caps and tested them - they look perfect.  The fuse is not blown.  So right now, I'm thinking it's likely on the hot side of the PSU (since both the logic and CRT outputs are dead) and it's probably not a transistor because of the intact fuse.

Obviously, I'm not much of a power supply person, so I'm looking for tips or ideas on how to go about diagnosing this a little more efficiently than just pulling diodes and caps to test them...
 

Offline BrokenYugo

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Re: Dead PSU from a 1996 Power Macintosh
« Reply #1 on: January 23, 2022, 07:22:45 pm »
Mains filter caps are usually the last to die, if your problem is a cap it's probably one of the little ones on the hot side involved in some standby or bootstrap supply.
 

Offline Gyro

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Re: Dead PSU from a 1996 Power Macintosh
« Reply #2 on: January 23, 2022, 07:49:33 pm »
That black rectangular Sincera part is a thermistor (puck type held by spring contacts inside the enclosure). It's a possible candidate for going high resistance.


Edit: It might be related to the CRT degaussing coil, I can't tell from its position.
« Last Edit: January 23, 2022, 07:59:11 pm by Gyro »
Regards, Chris

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Offline Labrat101

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Re: Dead PSU from a 1996 Power Macintosh
« Reply #3 on: January 23, 2022, 07:58:25 pm »
Did you check the PFC coil . They fail or have a broken terminal .

"   All Started With A BIG Bang!! .  .   & Magic Smoke  ".
 

Offline Gyro

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Re: Dead PSU from a 1996 Power Macintosh
« Reply #4 on: January 23, 2022, 08:00:21 pm »
Does it have PFC?
Regards, Chris

"Victor Meldrew, the Crimson Avenger!"
 

Offline Labrat101

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Re: Dead PSU from a 1996 Power Macintosh
« Reply #5 on: January 23, 2022, 08:17:48 pm »
I think its the vertical board that sitting behind the large power caps .
I could be wrong . I don't have a schematic of that psu on hand .

EDIT .. Some serial No.,  Model numbers . type  . would help track down a schematic .
« Last Edit: January 23, 2022, 08:36:19 pm by Labrat101 »
"   All Started With A BIG Bang!! .  .   & Magic Smoke  ".
 

Offline joelgraff

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Re: Dead PSU from a 1996 Power Macintosh
« Reply #6 on: January 24, 2022, 10:10:32 pm »
It's a Power Macintosh Performa 5260.  The PSU board model is PWB-1801.

I did find a link to the service manual:
https://www.ordinosaures-parc.org/images/PDF/apple/Mac/powermac.perf_5260.5280.pdf
 


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