820-00138 Customer used knock off charger

[tlough75]

So I have on the bench an 820-00138, and the customer stated their customer used a knock off charger. The charger melted and now machine is dead. I have it on the bench and it is drawing 20mA, PP3V42 is preset but PPBUS is 4.2V. I measured pins 27 & 28 on U7100 and have 30K ohms, with pin 28 being short to GND. Should I assume the ISL is dead and replace, or remove C7121 to see if short goes away? Options....

 

[tlough75]

So I replaced U7100 and it sounds like it blew again. The computer now turns on, but the resistance between 27 & 28 is .5M Ohms.

 

[2informaticos]

Did you forget to replace R7121???
Check R7122 yoo, but should be good.

 

[tlough75]

oops, yeah. R7121 is reading 625M and R7122 is 9.2K Will replace all three and report back. Weird thing happened too, when I turned it on something "popped" on the other side of the board.

 

[tlough75]

I took them off the board to confirm. R7121 is "Open" and R7122 is 9.2K, so I would say they are bad as well

 

[tlough75]

I forgot to conform they were ok after I changed the ISL. SO be 20.2 Ohms, Won't forget this time. I just hope nothing else is blown. I may use a donor ISL this time to test.

 

[2informaticos]

Your problem might be only bad C7121/22; don't change U7100 so fast...

 

[tlough75]

Got it, will change other components first

 

[tlough75]

Did you mean C7021 and C7022?

 

[tlough75]

Replaced R7021 and R7022 and resistance between pins 27 & 28 is 17.4K ohms

 

[tlough75]

I checked R7020 off board and is still 0.2Ohms

 

[tlough75]

never mind about caps, was looking at a different board view

 

[tlough75]

and I meant I checked R7120

 

[2informaticos]

Do you get correct resistance between pins 27/28 now (20 ohm aprox)?

 

[tlough75]

No, getting 17.7K

 

[2informaticos]

Please, this is classic and easy thing to solve.
What else than R7121/22 and their traces can fail there???

Check R7120 too, but shouldn't get burnt, maybe corroded.

 

[tlough75]

i did remove and check R7120, as mentioned above

 

[tlough75]

Don't give me answer, I will try to solve myself.

 

[2informaticos]

I also said, R7120 should be good.
Check the other things mentioned...

 

[tlough75]

Checked/replaced C7121/22, no change.