Power Mac G4 MDD "Mirror-Door" won't power up

[Toasty]

Re: Power Mac G4 MDD "Mirror-Door" won't power up

Welcome to the forums jex.

Best wishes on schematics. No one here has ever reported finding one. All are proprietary and schematics are not released. Remember, these units are considered "not serviceable" and schematics would be "in-house" only.

If you need help in diagnosis, others here and myself have these units and have repaired them. Indicate your problem(s) and we'll try to help.

Toast

[tj2]

Re: Power Mac G4 MDD "Mirror-Door" won't power up

Does anyone have a list of caps in this Samsung power supply? (capacitance, voltage, dimensions, and perhaps original brand/series, or replacement brand/series, or ESR and ripple)

I'm looking at repairing, and probably fully recapping, one of these but I am under some time pressure as we need to get it working by the time the semester starts. The Mac with this power supply hosts a data acquisition board for a laboratory experiment. I'm at least going to order a new fuse, a new primary cap -- the old one is a 390 uF / 420 V CapXon -- as well as new switchers, Q1 & Q2, but I'd like to order new caps for the rest of the PSU at the same time. Fortunately, my department will pay for the parts.

As I haven't had time to check it over thoroughly after pulling it out, I'm not sure exactly what's wrong. The only evidence is a blown fuse. I don't see any burned components or scorch marks on the board. I gather that besides checking the switchers, I should check the bridge rectifier, and perhaps the thermistors. I'm glad to have found on the forum the correct switchers without pulling the heat sink first.

[tj2]

Re: Power Mac G4 MDD "Mirror-Door" won't power up

To answer my own question, these Samsung PSUs contain the following capacitors. As the GL caps are not standard for that series, I estimated the ESR (@ 20 C & 100 kHz) and ripple from others of the same dimensions in the series.

1x CapXon HP 390 uF/420 V 35x42
5x CapXon GL 2200 uF/10 V 10x25 57 mΩ 1400 mA
2x CapXon GL 2200 uF/16 V 10x30 35 mΩ 1815 mA
1x CapXon GL 1000 uF/35 V 13x25 36 mΩ 1950 mA
1x CapXon KM 470 uF/35 V 10x20
1x CapXon KM 47 uF/25 V 5x11
1x CapXon GL 470 uF/16 V 8x12.5 92 mΩ 745 mA
2x CapXon KM 56 uF/35 V 6x11
1x CapXon KM 2.2 uF/50 V 5x11
2x CapXon KM 10 uF/50 V 5x11
2x CapXon KM 22 uF/50 V 5x11
2x CapXon KM 100 uF/16 V 6x11

I replaced the 390 uF / 420 V cap by a Nichicon GW 390 uF / 450 V. I replaced the CapXon GL 2200 uF/ 10 V by Panny FC and I replaced the CapXon GL 2200 uF/ 16 V by Samxon GD from the badcaps.net store. I replaced the other CapXon GL series by Nichicon HE. The CapXon KM appear to be general purpose caps, which I replaced by whatever I had on hand, which was a mix of Panny FC & FM and Nichicon HE.

The only problem I could find with the PSU was that the 390 uF CapXon HP had failed open and popped the fuse. Everything else seemed fine, but I did replace the switchers as I had originally found that one tested shorted so I ordered a pair of ST W20NM50. Later the switchers tested fine. The Mac now runs so the experiment now has a working data acquisition system.

[Toasty]

Re: Power Mac G4 MDD "Mirror-Door" won't power up

Good Job! Knew you could do it.

One note: SMQ is a less expensive ($4) than GW. GW has a -slightly- higher ripple rating (160mA), but anything over the HP's 1760mA is excellent.

"originally found that one tested shorted "
-and-
"Later the switchers tested fine"

MOSFET could have been latched in the "on" state.

Toast

[tj2]

Re: Power Mac G4 MDD "Mirror-Door" won't power up

Thanks, Toasty! For my efforts our lab tech has obtained three more Macs as backups. Two are MDD Macs with the Samsung PSU -- one working and the other with another failed open primary cap in the PSU. The other doesn't work but is a different model and has an AcBel PSU. I'm going to recap the two Samsung PSUs, but I think that I'll give a pass for now on the other Mac. The department is going to owe me for even more capacitors!

I thought about the SMQ, but for some reason the GW grabbed my attention.

I am careful when working with MOSFETs but I wonder how sensitive the switchers and the PFC MOSFETs are to accidental static damage.

[Toasty]

Re: Power Mac G4 MDD "Mirror-Door" won't power up

AcBels are fairly easy. A couple of small caps cause the start up failure. Usually most of the output caps are crap/blown. The primary's are fine (UCC's). Look for the threads in here. I did point out the failed start caps in there.

The PSU's interchange, so try the Samsung's in the AcBel unit.

MOSFET's can be very sensitive. The capacitance from your body can be enough to trigger them, although not necessarily 'in-circuit'. Not like the bipolar transistors of old, MOSFETs usually have to be driven 'off' (have their gate charge removed). One of the testing methods (in here too) uses your finger to reset the MOSFET.

Toast

[keithwbloom]

Re: Power Mac G4 MDD "Mirror-Door" won't power up

OK I am sufficiently inspired to try to tackle a fix on my Samsung 360W PS. I desoldered the main cap and it is testing open for continuity, but I only have a simple tester so I am not sure about other performance characteristics, and as a result, I was not sure if that was the end of it. So I then removed the copper winds on the fuse side of the board to find scorching between the (assumed) RF shield (yellow, plastic covered thingy) and a green thermistor (SCK 048). The thermistor is heavily scorched as well, and it appears there is actually a pinhole on the lower edge of it. Could this be the end of the trail? Can the main cap be the source of the scorching current through the thermistor? I would like to replace the main cap, but I am finding it hard to source a similar size and performance electrolytic cap, and I now realize I have to replace the SCK 048 thermistor as well. Any favorite online stores for this kind of small order?

Please forgive me for my "noobie-ness" — I last took electrical engineering in junior-high school industrial arts (shop) class 30 years ago. I am still handy with a tester and soldering iron for most home fix it things and would like to try to get this PS back online.

Best,

Keith

[keithwbloom]

Re: Power Mac G4 MDD "Mirror-Door" won't power up

Answered many of my initial questions when I found this thread on badcaps.net - https://www.badcaps.net/forum/showthread.php?t=8934

I think I will now work through the steps outlined for others before me and hope I have only a couple more components to replace. Help on source for main cap would still be very welcome.

Thanks,

Keith

[Toasty]

Re: Power Mac G4 MDD "Mirror-Door" won't power up

DigiKey has everything.

- UCC SMQ series cap, 390uF@450v - $5.25
- inrush current limiter, 4 ohm cold, current steady state 9.5A - $2.65
- ceramic fuse, 8A @ 250v, 5 x 20 - $0.36
- (2) MOSFETs, Q1 & Q2 - always replace as a pair - $5.25 ea.

Also, the caps that are hidden by the main heatsink as shown in:

https://www.badcaps.net/forum/showthread.php?t=10136

C135, 137 - 56uF@35v, 6.3x11 - HE, KY, FM (50v)
C141 - 47uF@25v, 5x11 - HE, FM


See also this thread: https://www.badcaps.net/forum/showthread.php?t=7165

Toast

[keithwbloom]

Re: Power Mac G4 MDD "Mirror-Door" won't power up

Everything on order. Thanks for the quick reply. Will post update when the parts are in!

[BP61001]

Re: Power Mac G4 MDD "Mirror-Door" won't power up

I have an AcBel power supply and it will start on the bench. All of the voltages look ok it would not startup my MDD. Would it be a good idea to change the caps and the switches? Or put it back into the computer and see if it works are not ?
Any help would be greatly appreciated.

[keithwbloom]

Re: Power Mac G4 MDD "Mirror-Door" won't power up

Success!

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. I followed the steps in the other forum post and presto!

Some pics and comments on the process:

Scorching evident on the yellow coated heat shield from the fried thermistor:


More significant scorching evidence once the heat shield and coils are removed:


Same area, different angle. Note the sub controller board in front of the main heat sink hiding the smaller capacitors ultimately replaced as a preventative measure:


Eeek!


Repair underway, three new caps in the positions under the main heat sink (underside):


Top view of area under main heat sink with new caps in place and sub-controller board not yet reinstalled:


Underside again, new MOSFET leads and main heat sink posts sticking through, before soldering:


Topside, viw of the new caps in place under the wing of the main heat sink, before reinstalling sub controller board:


Almost done. New main cap in place, transformer in left corner reinstalled:


The usual suspects (clockwise from top left): the main cap, the shorted MOSFET and it's mate, the four replaced smaller caps (I swapped out one of them on the sub controller board too), and the fried thermistor.


Whee! Been up and running for more than 24 hours, and I am very pleased with temperature and ambient noise too!


Now what to do with the thing... Anyone want a loaded G4 Dual 1.25GHz in excellent condition with a better than new power supply? ;-)

[jonathan7500]

Re: Power Mac G4 MDD "Mirror-Door" won't power up

Samsung's on no-start are a few small caps down behind the PWM board next to the center/larger heatsink with the primary switchers on it. The little board is mounted upright next to the heatsink and the caps to be replaced require you to remove the entire heatsink and components. MIND THE THERMISTOR!!! Desoldering station makes it a 5 minute job. Wick and iron, 10 minutes.

Again, check the primary cap as you have on the other G4. If it's CapXon HP, replace it. These can go "weak" and not blow the fuse or switchers.

Toast

Thank you, Toasty

I have a no-start issue on my MDD dual 1.42 and decided to give it a go to replace the capacitor behind the PWM board. Because I only have the 10uf 50V and 47uf 25V capacitor sitting around, I decided to just replace these capacitor behind the PWM board.

I have managed to replace one of the capacitor (47uf 25V)behind the PWM board and one 10uf 50V capacitor on the PWM board it self without de-solder the heatsink and PWM card. This is achieve by pulling the PWM board away from the heatsink to make an 8 mm gap. This allow me to change out the capacitor with a chain nose plier and soldering iron at the other end.

I then test the power supply and the computer start up normally when the "power on" button on the powermac was pressed. However, I don't know how long will this last as there are still a lot of Capxon capacitor in the power supply. I think I just enjoy it now until next round of "recapping".

[Toasty]

Re: Power Mac G4 MDD "Mirror-Door" won't power up

Glad you got it working. Did you replace the mains cap?

I've now gone to replacing all 5 of the small caps in that startup area.
It's just not worth the aggravation to do it twice.

3 on the main board:
C135, C137 = 56uF @ 35v
C141 = 47uF@25v

2 on the PFC/PWM controller board:
C129 = 22uF@50v
C130 = 10uF@50v

I do not recommend bending that board back to make room.
One false move and those SMD chips across the back detach easily.
In most cases you have to pull the heatsink -anyway- to replace the bad MOSFETs.

Toast

[Agent24]

Re: Power Mac G4 MDD "Mirror-Door" won't power up

The mains capacitor is a known issue on these isn't it?

[Toasty]

Re: Power Mac G4 MDD "Mirror-Door" won't power up

Yes. See posts 48-49 above and links within.

[Agent24]

Re: Power Mac G4 MDD "Mirror-Door" won't power up

It seems from those posts it only applies to the Samsung built model?
(Mine is the AcBel)

[Amuyamula]

Re: Power Mac G4 MDD "Mirror-Door" won't power up

Thanks guys! I got my 360W Samsung PSU up and running by following up on these posts! I am ever grateful!

[Mh4mac]

Re: Power Mac G4 MDD "Mirror-Door" won't power up

Hi, need a little help with the Samsung PSU in topic here. Does or can anyone tell me the transistor bands at location R170. It was damaged and is not readable for me. It is the one at the end of the heat sink that hides the caps in discussion.
Thanks a bunch.
Mike

[Toasty]

Re: Power Mac G4 MDD "Mirror-Door" won't power up

Orange-Black-Orange-Gold

30kΩ, 5%, 2 watt

Damaged? How?