820-3209 keeps burning current sensing resistors

Narek

Member
Hi,

stubborn board again. So I got it with not turning on problem. No liquid damage, but had replaced backlight IC before, the rest looks good. There is green and orange light on MagSafe, but fan doesn’t spin, just try to start and stops (Quarter fan spin). PPBUS_G3H ~4v, not shorted. I‘ve checked the area of U7000 (ISL6259) and adapter current sensing resistors R7021/22 were blown (resistance in kohm, the battery side was measuring 2,2ohm). I’ve replaced them along with ISL, resistance between R7021/22 got 20ohm. After connecting adapter had no fan spin though PPBUS raised to 8,56v. It turned out, that adapter resistors blew again, so I’ve replaced ISL and resistors again. This time board started, so I decided to put it in topcase and test it. Another adapter connection blew the resistors again, PPBUS dropped to ~4v... I’ve checked all components on page 53 of schematic... R7005 measures 20ohm, R7020 and R7050 are ok, C7020 is ok... I’ve replaced all set again and tried with battery only - no reaction. I’ve tried with different MagSafe boards and I/O cables - it keeps burning these resistors and ISL... I’ve also checked voltage rails for shorts and haven’t found any... just because board started after one of ISL/resistors replacement I believe power rails are not the actual problem here. Do you have any suggestions?
 
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2informaticos

Administrator
Staff member
Post diode mode to ground at pins 17/18 and 27/28 of U7000, as L7030 too.
Be sure no short between R7020 and R7050.
 

Narek

Member
Pin 17 - 0,136v
Pin 18 - 0,137v
Pin 27 - 0,619v
Pin 28 - 0,579v

L7030 - 0,136v

I guess problem is here?


R7020 and R7050 not shorted.

EDIT: I've removed F7040 and applied 8,56v on pin 2, board is powering on, fan is spinning, CPU gets warm.
 
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2informaticos

Administrator
Staff member
Do NOT do that again, as no reason.

"Pin 27 - 0,619v
Pin 28 - 0,579v"
You still have a problem with R7051/52, or traces.
 

Narek

Member
Not yet, after I replaced ISL and resistors with new ones I had to leave my office. I’ll power it up on Monday and report. Something tells me, that they will blow again... Thank you for your support, I really appreciate it. I wish you good weekend, will get back to this on Monday 😉
 

Narek

Member
I've plugged charger in and resistors blew again.. Resistance between pins 27/28 was 1,6kohm, PPBUS ~4,5v. Replaced all set again, first I got 8,3v on PPBUS and board was cycling (one of the coils clicked every half a second, couldn't recognise which coil exactly.), after repluging charger PPBUS went down to 4,5v again but R7051/52 are OK (20,5ohm between pins). There should be short somewhere, but interesting thing is, that when I have applied 8,56v it started normally.. I will try to reflow SMC, but what else can it be? mosfets?
 
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2informaticos

Administrator
Staff member
Post basic voltages for U7000; pins 1, 2, 3, 6, 9, 12, 13, 14, 15, 17, 19, 20, 27.
Be sure to have 20 ohms between pins 27/28; remove C7021/22, just in case.
 

Narek

Member
After reflowing SMC and replacing ISL + resistos there was no change, board wasnt turning on, just clicking. PPBUS ~8,4v. It turned out, that L3895 is clicking, voltage on it was jumping between 0 and 2v. While I was measuring the board it suddenly started... I replugged the charger and again is started. I have connected display, SSD and it boots normally, loads OS. PPBUS without battery is 8,37v, with battery connected 8,56v. Was turning on both on battery and charger. After few minutes it turned off, and started clicking again... PPBUS 8,36v, current sensing is fine, 20ohm betwee 27/28. Here are readings you requested:

Pin 1: 0,003v (burned R7086, after replacing it burns again, 0,464v in diode mode on pin 2, OL on pin 1)
Pin 2: 16,15v
Pin 3: 4,35v
Pin 6: jumping 0,7v - 1,1v
Pin 9: 0,037v
Pin 12: 3,42v
Pin 13: 3,39v
Pin 14: 3,39v
Pin 15: 0,059v
Pin 17: 8,38v
Pin 19: 5,06v
Pin 20: 5,03v
Pin 27:16,10v
Pin 28: 16,10v
 
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2informaticos

Administrator
Staff member
"I've removed F7040 and applied 8,56v on pin 2, board is powering on, fan is spinning, CPU gets warm"
I was thinking that works on battery alone...

Post PPBUS_G3H voltage on battery alone and bat+charger.
 

Narek

Member
I was thinking that works on battery alone...
I've applied direct voltage with power supply then



With battery only connected PPBUS is 7,98v, battery + charger jumps 7,5v-8,59v.

I've replaced R7086 and it blew up again (Q7080, Q7085 replaced too)
 
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2informaticos

Administrator
Staff member
"I've replaced R7086 and it blew up again"
Impossible, if you soldered correct 330K value.

Be sure to have correct resistors on board.
 

Narek

Member
I'm more than sure that it has correct value, I'm replacing it already third time and checking before and after soldering... I have just put new one there, it measured 330K. I plugged charger and board doesn't react, just clicking. I measure R7086 - blown, shows OL...
 

2informaticos

Administrator
Staff member
Considering the worst scenario, applying 20V on 330K resistor, means 0.06mA (60 microampers).
Do you really thing that resistor can get blown from such reason (1.2mW)???
 

Narek

Member
Yes, you are completely correct, I don't argue with that. I just post here what is happening on this board. I put perfectly measuring 330K resistor, triple check it before plugging charger in. After I plug it in nothing happens. I disconnect charger, check that resistor and it's OL.... I've done it 2 more times now. every time putting 330k resistor..
 

Narek

Member
Everything done and checked triple before connecting charger. Board has green/orange light, clicking every half second with quarter fan spin. With charger only PPBUS is 8,37v, with battery 7,99v, with both is jumping 8,17v-8,56v. Current sense is OK, U7000 pins 27/28 have 20,5ohm, 17/18 have 2,5 ohm.

R7086 was perfect 330K, after disconnecting charger it shows OL or 80K, after some time it's 330K again. This board is trolling me now...
 
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