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

[budm]

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

You should check the IC that drives Q1 and Q2 since when MOSFETs shorted out they will take out the IC with it, it will damage the new MOSFETs if the Gate drive outputs of the IC stuck on.

[haroldblum]

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

Thanks for the info I will replace Q1 and Q2 also

By the way would you happen to know where I can find these parts?
ntc thermistor sck-048 4ohms 8amps
I will try looking up the transistors after I figure out how to get the nut and bolt off of them to de-solder
Also the fuse size was 240v 8amps can I use 120v 8amps

Can I use a 5ohm 8amp for the thermistor?

[lti]

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

It looks like the filter cap fails open on these. From page 2 in this thread:

IF the mains cap is a CapXon HP, it -MUST BE REPLACED-. There is a marked history of failures in this series of capacitor (even from a Ukraine electronics site via kc8adu) in which the cap either goes open or shorts then opens. It invariably causes the main switchers Q1 & Q2 to fail (STW20NM50FD) and the primary fuse (8A/250v, Slo-Blo, 5x20) to open (usually violently) as the blackened glass indicates. I recommend a replacement with a ceramic bodied fuse here (Schurter 0001.2513). Another failure can be the destruction of the inrush limiter TH2 (SCK-048).

Check everything else budm and Agent24 mentioned as well.

[Toasty]

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

Wow, this one's back from the dead.. lol

I probably have all the parts. I'll take a look and post back.

T

This is the Samsung unit, correct?

[Toasty]

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

Yes, have all the parts in stock.

See this post: https://www.badcaps.net/forum/showpo...8&postcount=49

I'm in NJ.

T

[Toasty]

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

You should check the IC that drives Q1 and Q2 since when MOSFETs shorted out they will take out the IC with it, it will damage the new MOSFETs if the Gate drive outputs of the IC stuck on.

A good precaution, but after several dozen repairs, I've never found this to be bad. ML4824IP1 - It's on the daughterboard next to the main heatsink.

https://www.fairchildsemi.com/produc.../ML4824-1.html

On the opposite side of that board is the quad comparator KA339.

https://www.fairchildsemi.com/produc...ors/KA339.html

T

[budm]

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

Just check the resistance between VCC pin 13 and pin 12, Pin 13 and pin 11 to see if it shows low resistance.
Then check GND pin 10 and 11, GND pin 10 and 12 to see if they are shorted to GND. Shorted to GND will not cause damages, just there will be no drives.

[haroldblum]

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

You listed ceramic fuse, when I opened the ps there was a 250v 8A Glass fuse

[Toasty]

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

I've seen them both ways. Ceramic is a better way to go.

As I said, I have all the parts.

T

[Agent24]

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

Ceramic ones don't explode and put shards of glass everywhere if they blow hard.

[Oily]

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

Toast

I've had this same issue with my Mac recently. Found C129 and C130 to be bad but changed all of the above along with the mains cap today and it's all working again now!

Thanks Toasty (and forum!) for the great info.

Dave

[Kansas Sparks]

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

Ahh, so there are newer posts in this matter instead of only the one ending in 2010. So, here's a double post (as I haven't read through this particular thread yet). But, I shall now read after posting this...

AND NOW… 7 years after the last post on this thread, I’m on here asking if there’s anything new in this process (as in: a *simple* PSU “remove old and replace with new” replacement for these old PSU’s), or am I destined to begin the recapping and test procedures noted herein? I’ve two of these (still), working G4 MDD’s that now… just after turning 14 years old, might indeed be waay past due for some “brain salad power-supply surgery”. Like to see ‘em both go another 14 years.

*as if “simple” is possible.

And maybe this is covered here, elsewhere (under Dual 1.25 and/or Dual 1.42), perhaps?

Okay, I’m very new here, start throwing things. Forgive me.

[Toasty]

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

Welcome to the forums!

As the old saying goes, "If it ain't broke..."

T

[luvtrumpsh8]

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

These are a fun fix. I haven't read this thread but used to work for a company doing these. The ACBEL was always caps, the 1000uf was the main prob not allowing it to start up, but good to do the others while you're in there, and the samsung ones were a little more difficult. it was always a diode or two and the mosfets. you have to remove the heatsinks.

[Kansas Sparks]

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

Thanks luvtrumpsh8, buz, & most definitely…Toasty!

Six months later, I'm back here (like a bad penny perhaps). Both my MDDs are still running strong, but realizing the near-absolutely inevitable… I've begun work on a failed AcBel 360W PS.

The “other” lengthy thread here, begining with a post from buz (April 2011), concernning his failed AcBel has proven absolutely invaluable in this, my initial attempt at this PSU resurrection. (Glad indeed that I drew an AcBel first, over the Samsung PS.)

There's a very good local source for the caps on the 12-item “shopping list” from the aforemetioned thread, but would greatly appreciate current comparison & cost info - using Digi-Key, if readily/easily available from anyone here, for the AcBel.

Already removed the C29 and C60 (visually swollen/puffed), TEAPO caps but figured that I might as well replace the entire list of 12 while I'm at it… and afterwards perform the blind, trial-by-fire test. If it doesn't “light-the-lights” THEN I'll begin other tests…

Again, hats off to Toasty… et al.

[Toasty]

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

It all comes down to which supplier has what caps in stock. The price per unit is typically very close if not exactly the same. The quantity discounts are where myself and others make our decisions as we buy in bulk. For onesie-twosies, there's no real difference.

Where the difference lies is in the shipping.

T

[Kansas Sparks]

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

Thanks Toasty.

Understood. Local source here has always been very inexpensive even for onesie-twosies. I'll shop using buz's last list, noting specs from Digi-Key and Mouser sources for the AcBel caps.

[Kansas Sparks]

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

Thanks again Toasty! Much higher resolution file is available if you'd like. Just email me. I'll most likely be in touch again, once I crack open a Samsung PSU for clarifications on a similar line-art project. Cheers!

[falcontech]

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

Hello to all of you guys, writing after a few years from the last post is a bit strange, however short, I have the same problem in practice too, when I press the power button on the g4 the light turns on for a moment but then vanishes and nothing happens, I removed the heatsink to check the hidden capacitors and I'm ok, no other capacitor appears swollen and therefore needs replacing, can anyone tell me which component I could replace, to fix the power supply? Thank you

[falcontech]

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

I also tried to change the big capacitor 390uf 450 volts but nothing