820-00875-A and PP3V3_S5 line

[jasko_jacker]

Hi
I need help on an Apple 820-00875-A board, it doesn't turn on and I noticed that the voltage fluctuates from 0V to 3.3V on the PP3V3_S5 line. If I inject 3.3v (with the board powering and the coil isolated from the generator) the consumption is 8 mA.

[mon2]

Carefully review the voltage to ground of P3V3S5_EN @ pin 21 on U7650. What is the measurement?

With the low current on the downstream / consumer side of the power rail, there is no short on that section.

Remove all power. Meter in resistance mode. What is the resistance to ground @ L7690 (best to check each of the PCB pads for this component)?

Next, meter in resistance scale. With no power to the board, measure the resistance between pins 17 & 18 on U7650. You are checking the resistance of the current loop path (CSP2 & CSN2). Post each measurement.

[jasko_jacker]

Hi
the resistance detected by both sides of the L7690 is (with the tester in manual mode) 1.5 M, not stable (it tends to drop); while in auto mode it is not detectable. The resistance between pins 17 and 18 of u7650 is 1.5712 Kohm in auto mode
Thanks

[mon2]

The high resistance to ground on L7690 pads confirms there is no short on these rails.

The current sense loop resistance looks to be ok since R7693 and R7692 are in parallel. Both being 3k16 => should be 1580 ohms between pins 17 & 18. As a double check, confirm the resistance between pin 18 on the IC and the PCB pad # 2 of L7690. This should be ~3k16 ohms. No power during these resistance checks.

Interested in a few other resistance checks for this regulator.

No power -> meter in resistance mode.

1) One meter probe on L7690, pin #1 pcb pad (PP3V3_S5). Other meter probe @ pin 16 of U7650 (VFB2). What is the resistance ?
2) One meter probe on L7690, pin #1 pcb pad (PP3V3_S5). Other meter probe @ pin 3 of U7650 (RF). What is the resistance ?

Next, confirm the voltage to ground of the enable pin of this regulator @ pin # 21 (P3V3S5_EN).

It is possible that U7650 (TPS51980A) is defective. Louis mentions this fault often in his YT videos.

[jasko_jacker]

Hi,
Thanks for your help, from what I understand u7650 goes on off, the only stable voltage is pin 21 P3V3S5_EN while all the other signals fluctuate. PP5V_S4 is not present. With pad 1 of L7690 disconnected the P3V3S5_EN voltage is present so there is probably a feedback signal which fluctuates and has repercussions on the chip itself since if the chip were to go into protection it would become hot,, I don't know how correct an idea is. I see a pgood signal but it refers to PP5V_S4
R7693 -> 3.156K
R7692 -> 3.147K

U7650 PIN 18
L7690 PAD 02 PAD1 DISCONNECTED
RES 3.156K

U7650 PIN 18
L7690 PAD 02 PAD1 CONNECTED
RES 1.571K

U7650 PIN 17
U7650 PIN 18
RES 1.571K

L7690 PAD 01
U7650 PIN 16
RES 13K

L7690 PAD 01
U7650 PIN 03
RES 180K

L7690 PAD 01
GND
RES 1.6M

[mon2]

Since the enable pin is stable, suggest to replace this TPS51980A @ U7650. Have some here but to get to you would have to fly in, have some good pizza and fly back. Perhaps you have a local source for these or a donor board?

[jasko_jacker]

Yes, I could try to replace the chip but since it is an old laptop I don't think the shop will accept the cost of repair. However I found this post which might be useful. https://boards.rossmanngroup.com/thr...-problem.3252/

[mon2]

What is the voltage to ground on ppbus_g3h ?

is it stable?

[jasko_jacker]

Yes, misured on F8400 13V stable PPBUS_G3H

[jasko_jacker]

Solved, but I still don't know why. I checked all the recommended voltages including SMC_PM_G2_EN or PM_SLP_S4_L and they were all correct. In this case I would normally have thought there was a problem involving one of the two voltages that the U7650 creates, such as a damaged component that uses them.
However, I desoldered the U4000 memory because I wanted to use it in another board and I noticed that now the P3V3S5_VSW and P5VS4_VSW voltages were stable, I re-soldered the U4000 and the board started working again. probably (since nothing repairs itself) if it was a software problem something was reset, if it was a hardware problem (a BGA chip that had some balls with problems) the repair will certainly take a short time. I'm not very expert in Apple systems and I'm now starting to dedicate myself to repairing this brand