820 3115 board not switching on shorts found at U6990

[leonattpcs]

Computer just stopped switching on.
Just beeping around the board for now and checking voltages:
Found short at u6990 c6999 c6994 c6996 - replaced them all, short gone, but still not switching on, no green light no red light
Looking at u7000, 16.5v is going to pin 2, 16.1v is getting to pin 26 from from q7080, less than 1v is getting to pin 1 from q7085 so am I replacing this and checking again? I only ask as the schematic does not mention voltages coming from q7080 or q7085 only going into them.
Thanks

I am also thinking isl 6259 needs replacing, but I want to know bout q7085 first. Checking pins 27 - 28 of u7000 and pin 19-20 all is well.
Thanks

 

[jason_microsoldering]

PP3V42 g3h present?

 

[leonattpcs]

First mention of it I see on the schematic is pin 5 at U6901 and it is not there. Checking it at pin 2 of u7000 and it is not there either. The place I get anything near that is 3.71v at U6990 pin 2 and pin 8. For what it is worth, when power is connected and I am testing for voltages, there is a ticking/spark noise coming from the board maybe an inch below F4260.

 

[leonattpcs]

After more looking:
Q7080 pin 4 gives out 16.8v and the same voltage gets to pin 26 on U7000
Q7085 pin 4 gives out 6.8v gets to pin 1 on U7000 with the voltage at 0v - 0.4v
Looking at the details on the schematic for q7080 and q7085 I am thinking Q7080 is good and Q7085 is bad. I am looking on scrap board to replace Q7085 and fit a new U7000 and see what happens.

 

[jason_microsoldering]

Stop doing anything until you fix pp3v42 first. What is the voltage drop to ground on pin 2 of L6995? What is the voltage there?

 

[leonattpcs]

That is about 0.95v

 

[jason_microsoldering]

really... so, which of the two questions i asked are you answering?

 

[leonattpcs]

I am answering what is the voltage there. Each side of the country l measures 0.95v.

 

[dukefawks]

Resistance to GND on L6995?

 

[leonattpcs]

Red on ground, then black on each side of the coil gives a number of about 180

 

[2informaticos]

A normal value, in diode mode, is around .300 in that sense; much more inverting probes (black to ground).
Post voltages at U6990, 3, 4, 7, 8, 10.
Check for corroded traces, resolder U6990 pins.

 

[leonattpcs]

Black to ground raises the number to nearly 190, but is little up and down.
U6990 pins:
3 - 15.1v
4 - 16.6
7 - 16.6
8 - 3.75
10 - 0v
Reflowed the pins
3 - 15.1v
4 - 16.6
7 - 16.6
8 - 3.75
10 - 0v
No corroded traces

 

[dukefawks]

190 is in diode mode? I need resistance in Ohm.

 

[leonattpcs]

Black on ground red on pin 1 - about 105 Ohms
Black on ground red on pin 2 - about 101 Ohms

 

[2informaticos]

That's not good at all; you must get more koms.
Even on diode mode can't be less than 290 (red on ground).
Reflow SMC and chech again; possible crap under it.

 

[leonattpcs]

Ive been thinking smc as well. Will reflow and check again. Computer isn't liquid damaged, but wadda you do.

 

[dukefawks]

100Ohm sucks as it is too low, but not low enough to cause anything to heat up. Did you check the infamous cap under the foam next to the BIL connector? You can inject 3.3V on the bus and see if your light turns green.

 

[leonattpcs]

I haven't heard of that no. Where is that on the board?
Thanks

 

[dukefawks]

Like I said, under the foam pad next to BIL connector. There is only one cap there that is on 3V42 rail. Maybe better to first inject external 3V3 on the rail and see if green light comes.

 

[leonattpcs]

I have found the foam by J6955, I will look for the cap and voltage.