Missing Diode on Mac mini 2,1 (mid-2007)

[superdupernerd]

Hi all!

I knocked a diode off my logic board while trying to remove the CPU heatsink clips. Oops. Now I have a couple of questions I need to figure out to get things working again. Refer to attached picture.

1) I found the missing diode's location, but I'm not sure about the correct orientation. I noticed an open bracket-like symbol around the dislocated diode (in red outline) and its intact twin (in green outline). I'm assuming this is a hint at which way to orient the diode. Would this be a correct assumption?

2) Also, now the diode is missing, so can anyone tell me what spec I should be looking for to replace it? I noticed it says S1, but there are a million different diodes matching that designation. Information I have on the diode's twin: 0.796 v drop in one direction and OL in the other direction. Also, this is on the side of the motherboard with very little clearance, so it looks like a surface mounted piece a couple millimetres in height, in case that helps in advising on the correct specs or part number for a replacement.

The motherboard is part 820-1900-A by Apple. I tried looking for a schematic online, but no such luck. I'm hoping the missing diode's label, D7924 (sorry it isn't very clear in the photo) can provide helpful hints in restoring functionality.

Thanks!

 

[velocityg4]

I found another high res photo of your motherboard model. D7924 appears to be in the same orientation as you have pictured.

[Sprzedam]Ukad NVIDIA G86-631-A2 i płyta APPLE 820-1900-A

Sprzedam układ NVIDIA G86-631-A2 oraz płytę główną APPLE 820-1900-A. Części uszkodzone, idealne do naprawy. Wysyłka gratis! Cena 200 zł.

www.elektroda.pl

 

[Needleroozer]

Hi all!

I knocked a diode off my logic board while trying to remove the CPU heatsink clips. Oops. Now I have a couple of questions I need to figure out to get things working again. Refer to attached picture.

1) I found the missing diode's location, but I'm not sure about the correct orientation. I noticed an open bracket-like symbol around the dislocated diode (in red outline) and its intact twin (in green outline). I'm assuming this is a hint at which way to orient the diode. Would this be a correct assumption?

2) Also, now the diode is missing, so can anyone tell me what spec I should be looking for to replace it? I noticed it says S1, but there are a million different diodes matching that designation. Information I have on the diode's twin: 0.796 v drop in one direction and OL in the other direction. Also, this is on the side of the motherboard with very little clearance, so it looks like a surface mounted piece a couple millimetres in height, in case that helps in advising on the correct specs or part number for a replacement.

The motherboard is part 820-1900-A by Apple. I tried looking for a schematic online, but no such luck. I'm hoping the missing diode's label, D7924 (sorry it isn't very clear in the photo) can provide helpful hints in restoring functionality.

Thanks!

I looked up the IC the diode connects to - it’s an ISL6227CAZ switching controller.

ISL6227CAZ-T Renesas / Intersil | Mouser

ISL6227CAZ-T Renesas / Intersil Switching Controllers VER OF ISL6227CA-T datasheet, inventory, & pricing.

www.mouser.com

From the datasheet’s example circuits, it looks like the cathode on the diode connects to the BOOT1/BOOT2 pins (6 and 23 on your IC).
Their suggested diode is a BAT54W Schottky rated up to about 30V reverse bias and 200mA continuous (600mA burst).

https://www.diodes.com/assets/Datasheets/ds30065.pdf

If you can find a similar Schottky in the same package as your diodes, you should be good to go. Or if you are feeling ambitious, you can get something like that SOT-323 package and put it on diagonal...

EDIT - Your 0.8V measurement for Vf isn’t really consistent with a Schottky’s low Vf (typically 0.2V or so)... In this application, though, it looks like these are just protection diodes and so the specifics are going to be less important.

 

[superdupernerd]

Thanks to both of your for the super helpful guidance! And Needleroozer, you were right about the voltage drop. It's actually around .285 in one direction and O.L in the other.

I was using my multimeter wrong when I measured before. I'm definitely in that "a little bit of knowledge is a dangerous thing" stage of electronics repair. Lol. I had the dial set to diode, but the sub-setting selection was on auto. On my multimeter, the diode switch position is also shared by an ohms symbol as well as an audio symbol. So who knows what auto mode was picking out of the three options on that dial position. Oops!

Hope I didn't damage the good diode! Time will tell. I'll buy 2 diodes just in case.

 

[Needleroozer]

Thanks to both of your for the super helpful guidance! And Needleroozer, you were right about the voltage drop. It's actually around .285 in one direction and O.L in the other.

I was using my multimeter wrong when I measured before. I'm definitely in that "a little bit of knowledge is a dangerous thing" stage of electronics repair. Lol. I had the dial set to diode, but the sub-setting selection was on auto. On my multimeter, the diode switch position is also shared by an ohms symbol as well as an audio symbol. So who knows what auto mode was picking out of the three options on that dial position. Oops!

Hope I didn't damage the good diode! Time will tell. I'll buy 2 diodes just in case.

I’m almost certain your other diode is fine; the meter shouldn’t have hurt it (but it’s always good to have spare parts). My guess is that the third option on that meter mode is a continuity tester: touch the probes together, and the meter should beep. This means that it has detected very low resistance (typically less than 200 Ohms) between the probes.
This mode is very helpful for tracing out connections and testing connectivity.

 

[superdupernerd]

Makes sense. Hitting up the local nerd market tomorrow morning to get components for this as well as my PS3 phat capacitor replacement project. Very excited. Very ready to permanently ruin stuff. Now I'm fantasizing about buying broken practice boards. Lol. I'll keep everyone posted on my progress!

 

[superdupernerd]

My local nerd store doesn't sell surface mounted components. Eyeroll. All they had were these giant diodes.

But I found this. Will this work?

https://www.mouser.com/datasheet/2/427/bat54wsg-1767640.pdf

 

[Needleroozer]

My local nerd store doesn't sell surface mounted components. Eyeroll. All they had were these giant diodes.

But I found this. Will this work?

https://www.mouser.com/datasheet/2/427/bat54wsg-1767640.pdf

I think that should work, yes.
Electrically, it’s definitely fine, and comparing the package dimensions to those of the ISL chip makes it look like the SOD-323 package size is a match (the diode body is 2mm long, and the ISL’s SSOP is about 3.8mm on the short side).

 

[superdupernerd]

Ready for the next novice question? LOL. On comparing that area to the high res photo found by velocityg4, I noticed the outlined red items missing on my logic board. They are all right by where the diode was. Thing is, I found the diode, I didn't find anything else that got knocked from the board when my spudger slipped (suggesting they were never there).

I guess my question is, could it be possible that some variants of the logic board exclude those parts/don't need them for functioning? See attached photo. This was as close and high-ish resolution as I could get the photo with my camera. Sorry about the odd angle, but I wanted to capture the whole section.

For clarification, I am talking about C7931, C7920, R7950, R7951 and R7920. Maybe the 2007 didn't need those where the 2006 did? Shrug.

EDIT: Tried reading the chip schematics posted above by Needleroozer and arrived at the following. This is based on the assumption that the end of the chip with the dimple is the top per the schematic. From that, following the traces from pins 6 (Boot1) and 10 (vsen1) I concluded R7920 = Rbt1 0 ohms; C7931 = Cbt1 0.15uF; R7950 = Rfb11 17.8k; C7920 = Cfb1 0.01uF; and R7951 = Rfb12 10k. Is this read looking correct? Also, what about that huge gash in the blue stuff? Is that the end of this logicboards usable life?

 

[Needleroozer]

Ready for the next novice question? LOL. On comparing that area to the high res photo found by velocityg4, I noticed the outlined red items missing on my logic board. They are all right by where the diode was. Thing is, I found the diode, I didn't find anything else that got knocked from the board when my spudger slipped (suggesting they were never there).

I guess my question is, could it be possible that some variants of the logic board exclude those parts/don't need them for functioning? See attached photo. This was as close and high-ish resolution as I could get the photo with my camera. Sorry about the odd angle, but I wanted to capture the whole section.

For clarification, I am talking about C7931, C7920, R7950, R7951 and R7920. Maybe the 2007 didn't need those where the 2006 did? Shrug.

EDIT: Tried reading the chip schematics posted above by Needleroozer and arrived at the following. This is based on the assumption that the end of the chip with the dimple is the top per the schematic. From that, following the traces from pins 6 (Boot1) and 10 (vsen1) I concluded R7920 = Rbt1 0 ohms; C7931 = Cbt1 0.15uF; R7950 = Rfb11 17.8k; C7920 = Cfb1 0.01uF; and R7951 = Rfb12 10k. Is this read looking correct? Also, what about that huge gash in the blue stuff? Is that the end of this logicboards usable life?

1. Yes, it looks like those parts were previously populated, though it’s hard to tell from your latest picture. Looking at your first picture, I can see divots where the edge of the components used to be attached (e.g. C7031).
Apple does sometimes leave components unpopulated, though that doesn’t seem to be the case here - there are too many remaining support components here for this side of the regulator to be unused.

2. Yes, the dot in the end typically marks pin 1. This is also in the data sheet for the chip, under the packaging specifications for the SSOP.

3. Your mapping to the example circuit seems good, and I think that it is fine to use those values for everything but Rfb12.
Rfb12 is used in conjunction with Rfb11 to set the output voltage (page 15, EQ.3, “Output Voltage Program”).
Since Apple seems to use these regulators fairly generically (I found a few on a 2007 iMac logic board), they are probably not set to produce 2.5V like this example circuit is. You could measure Rfb22 and see if it is also 10K, but it’s not directly obvious what value you’d want here.
I would trace out Vout1 (pin 9) and see if you can find what it’s connected to and what voltage it expects. It might be a bit of a long shot since the connection goes under the IC and may pop up under a different IC...
This would be a good place to use your multimeter in continuity mode, maybe after soldering a wire to Vout1 so you don’t have to balance a probe on that pin.

4. The “blue stuff” is solder mask, a transparent film that is put down to keep solder from sticking anywhere that a component is not going to attach.
Looking at your first picture, it looks like just a surface scratch and the underlying copper should still be okay. If not, I only see one possibly affected trace, so it should be easy to run a wire to reconnect it.