820-2936 PP3V42 being pulled down.

[bridges_4u]

Hi there,

I got an 820-2936-B motherboard which was originally dead when came in. So replaced U7000, U6900 & Q7035. Now i have got 12.16v on PPBUS_G3H and 3.425 volts on pin 1 of L6695, but just 3.415V on pin 2 which is a PP3V42 line. I am certain that SMC is not being turned on as per the current PPBUS_G3H voltage.

The issue i am trying to resolve is the voltage difference on pin2 of L6995. Any ideas as to what could be pulling it down???

[mon2]

Re: 820-2936 PP3V42 being pulled down.

Remove all power. Meter in resistance mode.

Measure the resistance to ground on pin # 2 of L6995.

The voltage drop is not much and may be ok as-is.

What is the resistance ?

[bridges_4u]

Re: 820-2936 PP3V42 being pulled down.

The resistance to ground on pin 2 of L6995 is 0.562k ohms. On pin 1 i have 3.443 volts and 3.415 v on pin 2. It is a big difference.

[mon2]

Re: 820-2936 PP3V42 being pulled down.

pin 2 of L6995 is 0.562k ohms

So ~560 ohms to ground. That is low.

The rail travels all over the logic board. Start with reviewing the caps on this rail.

Flux and remove C6999. Then measure the resistance to ground again on pin #2 of L6995.

[bridges_4u]

Re: 820-2936 PP3V42 being pulled down.

I have removed C6999 and C6994 and checked the resistance again. It is still showing the same i.e, 0.560k ohms. I was getting 3.425V a few days ago until i changed Q7030 which was short. Once it was replaced the PPBUS_G3H came up to 12.16V, but the PP3V42 is being pulled down...! Moreover, I do not have the green light on the magsafe, which is already apparent due to the low voltage..!

Should i try and inject voltage? I am very curious as we have the SMC on that line. I just want to screw it up.

[mon2]

Re: 820-2936 PP3V42 being pulled down.

No. Do you feel comfortable to remove the part you have replaced already? Lots of flux and remove it.

Then measure the same resistance again to see if it changes. Do not power up without the part.

[bridges_4u]

Re: 820-2936 PP3V42 being pulled down.

I am absolutely comfortable doing that. I dont think there was a bridge formed while soldering in Q7030 because there would not be 12 volts if it was shorted. Strange...?

[mon2]

Re: 820-2936 PP3V42 being pulled down.

The royal pain is that someone is guilty of pulling down this line but who? So just suspecting this reworked area.

The magsafe adapter is an original? Able to support this model? (power rating)

Contacts on the magsafe pogo pins and mating side are clean? We always use IPA to clean the contacts as they do tarnish over time.

[bridges_4u]

Re: 820-2936 PP3V42 being pulled down.

Hi mon2,

The adapter is indeed original. Just resoldered the FET once again. But no change. I always care and clean any circuits that i work with IPA.

[mon2]

Re: 820-2936 PP3V42 being pulled down.

What is the voltage to ground of SMC_BC_ACOK ?

[bridges_4u]

Re: 820-2936 PP3V42 being pulled down.

Hi mon2,

Some progress has been made. I removed u7000 and discovered a bridge cause by the solder. I redid the soldering job on u7000 again. This removed the fluctuations at pin 2 of L6995. But the voltage has now dropped at L6995. If i measure at both ends of L6995 I get 3.325 V instead of 3.425V. SMC_BC_ACOK is 3.182V.

[mon2]

Re: 820-2936 PP3V42 being pulled down.

The 3.425V value is required to power the parasitic one-wire magsafe communication device inside the magsafe power adapter.

This is the voltage of SMC_BC_ACOK which is used to power the security device and also to light up the magsafe head to be green or orange. It is a low value for your case. Needs to be higher.

Remove all power, measure the resistance to ground of PP3V42_G3H.

What is the measurement?

Attach the power adapter.

Also measure the voltage to ground of pin # 3 @ D6990. What is the reading?

[bridges_4u]

Re: 820-2936 PP3V42 being pulled down.

Pin #3 @ D6995 measures 16.8 volts. Resistance of PP3v42 to gnd is 0.570k ohms. Something interesting happens as well. If i remove F7040 i get 3.421 volts on pin 2 of L6995. When F7040 is inserted back, the PPBUS comes back to 12.16 volts but PP3v42 @ pin 2 of L6995 is back to square one.

[mon2]

Re: 820-2936 PP3V42 being pulled down.

Good that you tested this permutation. Suspecting that D6990 is leaky.

Remove all power. Meter in diode mode.

Red meter probe on pin #1. Black meter probe on pin #3. Meter should conduct. Reading?
Reverse the probes and the meter should block. Reading ?

Repeat for pin # 2 and #3 combination. With red lead on ANODE (back of arrowhead); meter should conduct but block other way around.

Confirm it.

If available, replace this part and test again.

[reformatt]

Re: 820-2936 PP3V42 being pulled down.

PP3V42_G3H is rated at 300ma max, so should be able to drive a load as low as 12 ohms. Not sure if that is your issue.

The abscence of one wire comms to the magsafe and the low PPBUS_G3H voltage would lead me to be looking at the SMC to see if it is running etc.

[bridges_4u]

Re: 820-2936 PP3V42 being pulled down.

Hi mon2,

The pin 1 & 2 of D6990 is giving readings in diode mode even when the probes are interchanged with GND. Bizzare.!!! I might have to remove D6990 and test it. Will try that. Any other pointers???


@reformatt. Hi there. It certainly is the absence of the one-wire circuit and the low PPBUS that is causing the issue. We might be poking a finger at the SMC once the investigation is done. But there is not short on those lines and I cant pump voltage in as SMC is on that line which would not take more than 300mA. Bugging me....!

[mon2]

Re: 820-2936 PP3V42 being pulled down.

If available, replace this dual common cathode diode.

For testing, meter in diode mode. No power.

Red meter lead on the anode (back of arrow head); black lead to the common cathode (the regulator Vin).

There should be a voltage reading as the diode conducts in this forward direction.

Reverse the leads and the diode should not conduct.

Repeat for the next leg. Post each measurement.

[bridges_4u]

Re: 820-2936 PP3V42 being pulled down.

Hi mon2,

I noticed something interesting. R7005 has gone to a high value from 20 ohms to 293 ohms. Do you thing this is connected to our problem.I shall post the readings of D6990 shortly.

[mon2]

Re: 820-2936 PP3V42 being pulled down.

Yes, this value should match the original. Please replace it.

[bridges_4u]

Re: 820-2936 PP3V42 being pulled down.

Hi mon2,

I have ordered the diode and resistor and will update in the next 3 - 4 days once I install it.