820-01814 Won't power on, PP5V_G3S shorted

[barry101]

Re: 820-01814 Won't power on, PP5V_G3S shorted

L7600 pin 1 to PPBUS_LS_OTH5V 5 to 6K and rising

L7600 pin 1 to PP3V3_G3H-T 18K

Measurement taken at L7600 pin 1 pad since L7600 is lifted at that end.

Did the MOSFET readings show anything?

I have some 820-3332 donor boards that have a TPS51980 as opposed to the TPS51980A on this board. Ok to use that version of the part?

I'll try some Diode Mode comparisons to the 820-3332 donor board and see if that shows anything useful. Will get back later today.

[barry101]

Re: 820-01814 Won't power on, PP5V_G3S shorted

L7600 - 1 to PPBUS_HS_OTH5V 6K and rising
L7600 - 1 to PP3V3_G3H_T 18K

I have some 2012 820-3332 donor board with TPS51980 vs. the TPS51980A on this board. Is it ok to use that part?

Before proceeding to remove the TPS, I will try some diode mode comparisons to the 820-3332 board and see if that shows anything interesting. I'll report back later today.

By the way, did the MOSFET reading show anything of interest?

[barry101]

Re: 820-01814 Won't power on, PP5V_G3S shorted

Sorry about the duplicate message, didn't realize the thread went on a new page, thought the first of the two messages didn't go out.

[barry101]

Re: 820-01814 Won't power on, PP5V_G3S shorted

Performed diode mode comparison for the TPS51980A on this 820-01814 board vs TPS51980 on a good 820-3332 board.

5V side discrepancies:
Pins 2 and 8 read 0 volts instead of .41 and .40 respectively since PP5V_G3S is shorted.

Pin 7 read .31 instead of .80 but would have read differently if L7600-1 was not currently lifted due to the short.

3.3V side discrepancies:
Pin 17 and 18, P3V3G3H_CSN2 AND _CSP2 respectively, read .40 and .71 instead of .13 and .52 respectively. However, I am having trouble getting any kind of stable reading off L7660-1 and -2. This would be consistent with the theory that there is damage internal to U7650.

Pin 24 P3V3G3H_DRVH goes through R7685 and then to U7660-3 (gate) read .90 instead of .62.

All other pins were within .05V of good readings and are probably not an issue.

[barry101]

Re: 820-01814 Won't power on, PP5V_G3S shorted

Removed U7650 TPS51980A. PP5V_G3S still shorted to ground.

Tried injecting voltage again on 5V side without TPS chip. L7660 still got hot.

Lifted L7660 pin 2 in an attempt to stop any path through L7660 to see affect on 5V side. PP5V_G3S still shorted to ground.

Injected voltage again at L7600 and, I couldn't believe it, but L7660 still gets burning hot almost immediately. There is no voltage on L7660 pin1. Pin 2 is definitely not touching the pad. Checked with a voltmeter and an oscilloscope - nothing there, only one pin attached to board and the inductor is getting hot. Even looked under the inductor with the microscope and nothing there.

Checked schematic looking for any other place where the 5V and 3V3 could interact and nothing seemed to jump out.

Electrically, it makes no sense. I'll try again when fresh in the morning. I will completely remove L7660 and then see if anything else on board gets hot. Maybe the 3V3 filter caps on the other side of the board are getting hot and conducting heat through the board to L7660. There has to be a reason L7660 got hot even with only one pin attached.

[mon2]

Re: 820-01814 Won't power on, PP5V_G3S shorted

Post a pic of the area of the board with L7600.

Also a pic of the area of the board with L7660 that is getting hot with the injection.

At this time, do not see the link between the injection and the heating up of L7660. The parts on the opposite side of L7660 are caps used by that half of the removed TPS (reports?) regulator.

[dellxps15]

Re: 820-01814 Won't power on, PP5V_G3S shorted

2,2A * 0,6 volt is 1,38 watt....

lower the current (psu lab has green and red light, tri to stay on the green spot before light turn red and see what get hot.

if whole pcb is getting hot, it should drain more then 15-20 watt at least.

ps: at that 1,38watt , does psu light turn red ?

wet that area with isopropil alchool, probably a 0201 caps

edit: wet whole board ofcourse with ipa and pay attention to small evaporation. 0201 caps are harder to spot.

[barry101]

Re: 820-01814 Won't power on, PP5V_G3S shorted

Hi,

Attached is a photo showing the L7600 and L7660.

However, I found the problem! I removed L7660 completely and the empty pad was getting hot. Using some alcohol, found that C7605 was shorting PP5V_G3S to ground. Removed and replaced C7605, reinstalled original L7600, L7660 and U7650 and problem solved.

Since C7605 is right next to L7660, I suspect the heat felt in L7660 even when lifted was being thermally conducted from the neighboring C7605 pads. I'm sure I felt C7605 with my finger earlier and it wasn't hot and it didn't register on the infrared gauge at the time. It was really hot after removing L7660.

Mon2, I would like to express my appreciation for your help, guidance and patience and also offer my thanks to dellxps15 for those suggestions.

[mon2]

Re: 820-01814 Won't power on, PP5V_G3S shorted

Dreaded tantalums.

Well done!

[barry101]

Re: 820-01814 Won't power on, PP5V_G3S shorted

All of that from a connecting a thumb drive!

[dellxps15]

Re: 820-01814 Won't power on, PP5V_G3S shorted

congratz for the repair