iMac 20" PSU dead, OK to use 6.3v caps instead of 10v?

[berniedd]

I recently acquired a 2 GHz 20" dead iMac. I opened up the PSU (4PC46) and saw a bulging C40 (2200 uf, 10v). It is a Ltec branded cap, fed from T2 thru a big diode D20. From there, it feeds a small ferrite choke. The main leads to the psu connector harness are far away (at least 2 inches) from this circuit. I have on hand now are low ESR Samxon 2200 uf but 6.3 v caps. Do you think I could use these instead of 10 volters?

I have recapped many psus and motherboards in the past, all Windows desktop machines. I've used 6.3 volt caps in place of 10 volt units without issue, as the circuits involved here were 3.3v and 5v output circuits. But this the first time I've tackled Apple and I simply do not know what voltages are to be expected around the PSU. By the way, there are other, normal looking Ltec caps 10v, 16v 25v. I plan to replace all caps bigger than 1000 uf capacity (using identical voltage replacements) to be safe.

Also I noticed Q1 on the foil side of the board (T1 is directly underneath it on the other side of the board) to have a 1 mm diameter rough dot at the center, making the markings unreadable. None of the other similar transistors Q2, Q3 Q4 etc near it are marred like Q1. I suspect it's burned out. What type is it? I hope I can easily find a replacement transistor.

[Toasty]

Re: iMac 20" PSU dead, OK to use 6.3v caps instead of 10v?

You can, but I wouldn't.

That's the 5vsb filter cap and is right off the standby transformer. This PSU varies from all the others in that there is no pi filter on the 5vsb here. Hence, no second cap to further smooth the output. I recommend staying with the spec'd cap voltage or if anything, stepping up to a 16v cap.

Replace -all- caps except the 2 mains. This is a PFC supply.

Toast

[berniedd]

Re: iMac 20" PSU dead, OK to use 6.3v caps instead of 10v?

Thanks. Any comments on Q1? 2N2222? Can you recommend a beefier replacement? Fortunately M2 seems ok.

I read in your post here:

https://www.badcaps.net/forum/showth...highlight=imac

to especially replace 4 caps. I'm having difficulty identifying these on the board itself. C1 (or is it C11), C37, C69. If C1 is really C11, only 3 caps then. Correct?

Viewing the whole thing, I count 14 caps total (excluding the 2 mains) that I would have to replace.

[Toasty]

Re: iMac 20" PSU dead, OK to use 6.3v caps instead of 10v?

No - 4 caps. Between the 2 mains caps and the optos. One (C69) is likely buried in the grey "muck" between the 2 mains caps.

15 is the correct count.

Please confirm the values of C1, 11, 37, 69 for me.
Do a "cap map" before you start pulling them, being sure to note polarity.

Toast

[berniedd]

Re: iMac 20" PSU dead, OK to use 6.3v caps instead of 10v?

Thanks, Toasty. It's getting late, I'll catch up on this tomorrow or next day. All this is a lot of work, and I really want to get the iMac working soon for my daughter. I know someone selling his PSU for a 17" iMac. Both are 180 watts. Except for the location of the AC in and Kensington lock, they are very similar. I think I may be able to shoehorn his in my iMac chassis. Will the PSU for the 17" work in my 20" ?

[berniedd]

Re: iMac 20" PSU dead, OK to use 6.3v caps instead of 10v?

OK, I took a look before wrapping today up.

C1 = 22 uf 50v
C11= 2.2 uf 50v
C37= 100 uf 50v
C69= 47 uf 25v

[Toasty]

Re: iMac 20" PSU dead, OK to use 6.3v caps instead of 10v?

Thanks! Matches what I have. I thought I had slipped a decimal point in the C1 / C11 values, but I had it correct.

For bench testing, the 2 are interchangeable. For "back in the case" usage, they are not. The power cord connection is in a different position because the chassis layout is different.

Toast

[berniedd]

Re: iMac 20" PSU dead, OK to use 6.3v caps instead of 10v?

I'm going to have to replace Q1, what transistor should I use? I's marked W1P, and in smaller characters, "54" at right angles to the W1P mark.

[Toasty]

Re: iMac 20" PSU dead, OK to use 6.3v caps instead of 10v?

Please update your profile with your location. Makes it easier to answer as some areas of the world can't get these numbers easily...

MMBT2222A

[Toasty]

Re: iMac 20" PSU dead, OK to use 6.3v caps instead of 10v?

A little help for those lost in the SMD marking psychosis....

http://www.s-manuals.com/smd/

[berniedd]

Re: iMac 20" PSU dead, OK to use 6.3v caps instead of 10v?

MMBT2222A it is. Thanks a bunch.

[berniedd]

Re: iMac 20" PSU dead, OK to use 6.3v caps instead of 10v?

I can't find any surface mount MMBT2222A locally. It seems I can shoehorn a regular 2N2222A transistor as a replacement, as there seems to be enough space between the PCB and the metal cage. Comments?

[Toasty]

Re: iMac 20" PSU dead, OK to use 6.3v caps instead of 10v?

Any port in a storm....

Just make sure which "flavor" you use. They vary in collector current (Ic) and collector-emitter voltage (Vceo). Stick as close to the parameters in the attached sheet as you can - 1A, 40v.

What suppliers do you have there? How many do you need? These are very cheap and a 1st Class letter to you is only around $2.50 from here.

Toast

[Toasty]

Re: iMac 20" PSU dead, OK to use 6.3v caps instead of 10v?

Any Luck?

[berniedd]

Re: iMac 20" PSU dead, OK to use 6.3v caps instead of 10v?

No luck on finding an MMBT2222A yet from the local stores I've gone to.

[berniedd]

Re: iMac 20" PSU dead, OK to use 6.3v caps instead of 10v?

OK, I finally rounded up the parts to repair this API4PC46-290 thing. Got a replacement caps kit that featured all Japanese caps. Also I now have MMBT2222A surface mount transistor on hand. I went to replace the MMBT first, but it was difficult to remove. Not only are the parts very small, the proximity of the other parts to that transistor wasn't helping. Even after repeated attempts with my 40 watt fine-tipped soldering iron, I couldn't it get off the board. I crushed that transistor with my pliers, but still there were some remnants of the transistor body that wouldn't let go. I finally got it off by using a fine flat jeweller's screwdriver as a chisel against a corner of the remains, and what do you know, the transistor body was glued to the board. I know, 'coz when it finally let go it took with it the green paint on the board under it and left an exposed copper tracing. My "chisel" didn't move over that area.

Now I'm in a bind. My eyes aren't very good, and even with a magnifying glass I have difficulty verifying if there is an actual copper foil bridge that connects R4 (marked 512 on its body) and the collector of Q1, the MMBT2222A. It's the only leg on that side of the transistor. Solder at the end of the copper foil at the end of this resistor has settled into a circular shape that just touches the foil where the transistor leg should be soldered to the board. I took a look here before I started desoldering the MMBT and I thought there was a gap between the 2 foil patterns. Now I'm not so sure this is so. Toasty, could you look at your units and verify for me? Much obliged.

[Toasty]

Re: iMac 20" PSU dead, OK to use 6.3v caps instead of 10v?

R4 [512] bridges Base to Collector on Q1.

The one to watch is R3 [432] that it does NOT bridge to the Q1 Emitter leg.

T

[berniedd]

Re: iMac 20" PSU dead, OK to use 6.3v caps instead of 10v?

Thanks, Toasty. For those who may be interested, I did a little research also and came up with this link to the schematic on that part of the PCB, thanks to Santaken:

https://www.badcaps.net/forum/attach...8&d=1306410193

[Toasty]

Re: iMac 20" PSU dead, OK to use 6.3v caps instead of 10v?

Cap values are wrong on that schematic:

C1 - 22@50v
C11 - (missing from schem) 2.2@50
C37 - 100@50v

[berniedd]

Re: iMac 20" PSU dead, OK to use 6.3v caps instead of 10v?

Good catch, Toasty!