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

[uffda_oi]

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

Toasty (and others)

Thanks for the help. I LIKE this forum!!

I've been hanging out here wondering if I could actually fix my power supplies. The helpful and well illustrated accounts of repairs got my courage up.

Yesterday I thought I had done in two Samsung PSUs. My wiseguy test lamp did a cute light routine (bright --> dim --> a few bright blinks --> off).

It did not light at all with the other Samsung I had worked on. So, given my winning streak, I switched out the big cap on my only working PSU (a third Samsung) and, lo, my wiseguy test lamp did the same cute routine. I figured I was cursed but put it back in the G4 1.25 Mhz MDD anyway. It fired right up. Success!

So I switched out the ailing AcBel PSU in my G4 1.25 Mhz dual MDD and the third Samsung also fired right up. This is the power supply with the broken x/y cap. I did a hack mend job on it yesterday but assumed that it had failed the light bulb test.

Now my G4 dual is very fast! The power light stays on and the computer doesn't have to be unplugged before each start. I still have plenty to do and feel very fortunate that there are experts here that take the time to help the less gifted.

I'll buy a few x/y caps in a larger future order. There is something about a $.50 order...

I LIKE this forum!!

[Toasty]

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

See:

http://search.digikey.com/us/en/prod...2414-ND/790170

DigiKey #: 445-2414-ND

[uffda_oi]

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

Thanks for the tip, Toasty. This hint should get me closer to lighting up my PSU (in a good sense, that is).

[Toasty]

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

It's an X or Y cap, depending on how it's wired.

Mains to ground - Y
Across mains - X

472M = 4700pF - 20% tolerance

250-400v as an X
400v as a Y

Toast

Furthermore:
Y1 is double insulated and insulation pulse tested to 8kV DC.
X1 is insulation pulse tested to 4kV DC.

Do some Googling. You could spend hours...
.

[uffda_oi]

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:

A belated "thank you" for this well-illustrated repair log.

I just finished "repairs" on two Samsung PSUs and broke the wires on the 4th component from the left in the first IMG (the blue disc in shrink-wrap).
I think, with the help of "wiki," it is a MOV (in LF2, next to and at 90º to TH2). The lettering says:

JEC
JD47 2M V1

250V 400V X1
400V Y1

The shrink-wrap in Keith's picture says 125º

I can't find a replacement for it at Digikey, et al, and would appreciate help with the ID of this component.

The PSU with the broken LF2 does not light the test lamp. The other PSU lights the test lamp in the following pattern:

Bright--> dim--> several bright blinks--> off

My test lamp was derived from Toasty's circuit. It uses a scrap of lamp cord. One line (red clip) goes through the bulb, the other (black clip) goes directly to the PSU.

It is my one success so far.

[Mh4mac]

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

I chipped it while removing the toms of white silicone the put all over the board. THANK YOU! This is the last Samsung I have and didn't have another to match with. Thanks a bunch Toasty ;-)

[Toasty]

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

Orange-Black-Orange-Gold

30kΩ, 5%, 2 watt

Damaged? How?

[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

[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!

[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)

[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

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

[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

[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".

[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? ;-)

[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

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

[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

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

[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