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Old 10-14-2019, 12:43 PM   #1
John McGivern
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Default ACER 5742 Dead M/Board

Hi, i am trying to repair this motherboard.
Ny daughter spilled red wine on it.
Adaper 19v
First mosfet PQ51 drain side 19v, nothing on gate or source.

Questions, 1. Do I have to install RAM for testing?
2. Current sense resistor missing R020 -2512-1%.
3. How difficult is it to solder this component?
4. Can I test without this or is value critical?

I have a 30v 1amp variable power supply for testing.

Many thanks in advance.
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Old 10-14-2019, 12:53 PM   #2
techtiger
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Default Re: ACER 5742 Dead M/Board

Most probably your 1st Fet is gone since it’s P channel it should cross 19V but less than Drain
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Old 10-14-2019, 03:15 PM   #3
piernov
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Default Re: ACER 5742 Dead M/Board

Why is the current sense resistor missing?
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Old 10-14-2019, 04:41 PM   #4
John McGivern
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Default Re: ACER 5742 Dead M/Board

Must have come off when disassembling but never noticed it anywhere.
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Old 10-14-2019, 05:09 PM   #5
piernov
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Default Re: ACER 5742 Dead M/Board

It looks like it has been removed so wanted confirm. It's required anyway.
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Old 10-14-2019, 10:31 PM   #6
John McGivern
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Default Re: ACER 5742 Dead M/Board

What is best way to solder this component very tight tolerance 0.5 ml trace gap? Bit vary of shorting traces.
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Old 10-14-2019, 11:41 PM   #7
mcplslg123
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Default Re: ACER 5742 Dead M/Board

Either with a finetipped solder iron or hot air gun.
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Old 10-15-2019, 01:44 AM   #8
John McGivern
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Default Re: ACER 5742 Dead M/Board

Thank you mcpisig123.
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Old 10-15-2019, 03:08 AM   #9
piernov
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Default Re: ACER 5742 Dead M/Board

The current sense resistor is a quite large component. A decent soldering iron and some flux will have no trouble soldering it.
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Old 10-15-2019, 03:26 AM   #10
John McGivern
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Default Re: ACER 5742 Dead M/Board

Thanks piernov.
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Old 10-15-2019, 03:51 AM   #11
John McGivern
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Default Re: ACER 5742 Dead M/Board

Should I leave the memory out while testing?
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Old 10-15-2019, 04:01 AM   #12
John McGivern
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Default Re: ACER 5742 Dead M/Board

Pj15/16/18 all shorted to gnd.
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Old 10-15-2019, 08:19 AM   #13
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Default Re: ACER 5742 Dead M/Board

Quote:
Originally Posted by John McGivern View Post
Pj15/16/18 all shorted to gnd.
PJ15/16 are supposed to show very low resistance to gnd.(measure it in resistance mode and not diode mode). thats not a worry for now. PJ18 is your B+ line(19V)-it is shorted. Open the jumper and apply low voltage like 1-2V on the shorted side if you have a bench power supply.
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Old 10-16-2019, 07:45 AM   #14
John McGivern
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Default Re: ACER 5742 Dead M/Board

Hi, I only have 1 amp will this be enough?
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Old 10-16-2019, 12:56 PM   #15
KvnTM
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Default Re: ACER 5742 Dead M/Board

Quote:
Originally Posted by John McGivern View Post
Hi, I only have 1 amp will this be enough?
Should be enough to heat up the shorted component. Pour some alcohol on the board and look where it evaporates quickly.
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Old 10-16-2019, 02:10 PM   #16
John McGivern
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Default Re: ACER 5742 Dead M/Board

KvnTN, thanks for replying, just a novice. I applied 1.8 volts to pj18 side 1 this was on pq62 drain only. R020 is missing so wil have to wait till it arrives to test. Many thanks.
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Old 10-17-2019, 03:58 PM   #17
John McGivern
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Default Re: ACER 5742 Dead M/Board

Tested all coils diode mode least measurement was 0.024. Hopefully R0.02 will arrive tomorrow. I have first and second mosfets to fit at the same time. Will it be best to remove Ram while testing? Thanks again.

Removed solder from pj18. Pq62 reads1.2v other side of pj18 shorted.

Last edited by John McGivern; 10-17-2019 at 04:01 PM..
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Old 10-18-2019, 10:04 AM   #18
John McGivern
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Default Re: ACER 5742 Dead M/Board

Hi Guys, soldered Pr352 this connects to Pj18 which is open. This is side 2 which is shorted to ground. I injected 1volt at side 1 of Pj18 this was on drain of Pq62 nothing on gate/source.
Injected 19v to Pq51 this was on drain/gate/source and also on Pq52 source but nothing on gate/drain. Cheers.
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Old 10-18-2019, 01:19 PM   #19
techtiger
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Default Re: ACER 5742 Dead M/Board

your 1st FET is not switching you need PACIN GATE PASS to pin 4 to allow same voltage to cross over there is a body diode in side forward bias PQ51 whatever the voltage on DRAIN it will pass voltage to SOURCE but less than the DRAIN PU18A is the starting point of PACIN unless you don’t have the PASS it will not work

Last edited by techtiger; 10-18-2019 at 01:20 PM..
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Old 10-18-2019, 01:53 PM   #20
John McGivern
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Default Re: ACER 5742 Dead M/Board

Thanks techtiger will change PQ51 tomorrow.
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