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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 5th Nov 2021, 10:00 pm   #1
Joe_Lorenz
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Default Apple Power Mac G4 Samsung Power Supply Issue

Hello,
Dear Mods, please move or delete if that computer can't be seen as "vintage". I do as there seems to be no more support from the manufacturer.

One of my sons put the hefty thing on my bench. I protested "no valves in there, I cannot do anything about it!" but I will have to: That special psu PSCF401601B (C) made by Samsung for that Power Mac "Mirror Door" series seems to develop the bad habit of destroying itself one day or another, and most but all companies offering "repaired" or "refurbished with guarantee" units seem to have gone out of business.
Firstly, the 8 amp 5x20 glass tube fuse is blown. Next it has got an NTC (SCK-048) that should limit the in-rush current. Well, just some grime and two blackend wires left. Must have made a little "bang!" Following the paths I come to the primary bridge rectifier GBU8J, well 600V 8 Amp or so I think. This one is dead short.
What now: Should I replace these identified as defective components and give it a try? Or does someone have experience fixing these "machines out of hell" and can give some advice what to check else before firing it up again?
Ah, yes, one more fact I have noticed: None of the electrolytics shows signs of stress or blown caps or so.
Thanks for any advice!

Regards, Joe
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Old 5th Nov 2021, 10:25 pm   #2
ColinB
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Default Re: Apple Power Mac G4 Samsung Power Supply Issue

I'd be checking carefully for any low resistance type faults after the bridge, check the main smoothing cap, the switching devices and controller chip as a starting point, and put right the faulty parts you've already identified.

On the plus side, anything that has caused that much devastation due to its failure should be fairly obvious and hopefully easy to identify.

Good luck!
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Old 6th Nov 2021, 10:51 am   #3
JonSnell
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Default Re: Apple Power Mac G4 Samsung Power Supply Issue

Rule of thumb for all Samsung products.
Replace the electrolytics and blown components.
Always use 105°C capacitors, 125°C when available as the power supply runs hot.

It is likely that a main smoothing and feedback path capacitor have gone leaky/dried up and may have taken the driver and output transistor with it along with the surge protector.
Apple are not good at supporting older equipment., they just want to sell new.

Does this ring any bells?

Problem: the main cap failed open and ultimately shorted out one of the MOSFETS and then fried a thermistor managing surge coming in on startup.


Here is a list of capacitors that are suspects;
1x 390 uF/420 V
5x 2200 uF/10 V
2x 2200 uF/16 V
1x 1000 uF/35 V
1x 470 uF/35 V
1x 47 uF/25 V
1x 470 uF/16 V
2x 56 uF/35 V
1x 2.2 uF/50 V
2x 10 uF/50 V
2x 22 uF/50 V
2x 100 uF/16 V

The last one I repaired was over 10 years ago and is still working, everyday.
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Old 6th Nov 2021, 10:44 pm   #4
Joe_Lorenz
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Default Re: Apple Power Mac G4 Samsung Power Supply Issue

Hello gents,
thanks for the advice! In the meanwhile I think I will have to replace all those electrolytics to get it going for longer than a week. Yes, there is at last one MOSFET with a solid short and I wish to cure the root of the evil instead of symptoms only.
Regards, Joe
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Old 5th Dec 2021, 3:46 am   #5
Mitch-W
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Default Re: Apple Power Mac G4 Samsung Power Supply Issue

I can confirm that when I restored a G4 MDD PSU 4 years ago I found few capacitors with higher than normal ESR reading. Since it was on the bench, I decided to replace all the tiny electrolytic caps. I was surprised that a lot of them had ESR twice or thrice the value that is still acceptable. I left the larger ones as they read fine.
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Old 9th Dec 2021, 10:45 pm   #6
Slothie
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Default Re: Apple Power Mac G4 Samsung Power Supply Issue

Although you have identified the bridge rectifier as gone its probably worth testing all other diodes and power transistors on the mains side for shorts before applying power again, sometimes the semiconductors can fail causing a cascade effect with all devices in its path, some of which might still look OK. Some people recommend putting a high wattage incandescent bulb in series with the mains input if that can be done safely the first time, as a sort of "soft start" in case of any other shorts.
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Old 9th Dec 2021, 11:14 pm   #7
Joe_Lorenz
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Default Re: Apple Power Mac G4 Samsung Power Supply Issue

Hello gents,

you are so right! I have been able to sort quite a handful of suspicious or guilty components right now, starting with that 390µ/420V electrolytic, leading to shorted MOSFETS. Quite astonishing that I have a 60 years old radio, still in daily use with all the old tatty capacitors and that "almost new" computer psu is in such miserable condition!

Digikey will supply spare parts, that's the positive point here! I hope I will be able to post a success note some day.

Regards, Joe
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