820-3437 4v on PPBUS_G3H

[jfursdon]

So this board turned up with dead short on PPBUS_G3H turned out to be C7431 PPBUS_G3H then gave a flickering voltage from 8.5v to 4.0v but now only puts out 4v
changed U7100 have checked pins 27 and 28 get 20,2 ohms is the next move an SMC change?? or am i missing something

 

[JohnB8812]

Do NOT touch the SMC this is likely not the issue. What is the resistance between pins 17/18 of the ISL?

 

[2informaticos]

Remove F7140 and check voltage at L7130.

 

[jfursdon]

i got 2.2 ohms on this, is this another sensing circuit?

 

[jfursdon]

just removed f7140 and get 8.56v on L7130

 

[JohnB8812]

Please don't use comments we can't see them right away. If you get 8.56 at L7130, then you either have a blown fuse or a short on pin 2. What is your resistance to ground on pin 2 of F7140?

 

[2informaticos]

If you get correct voltage at L7130 removing F7140, then your problem is on the system side of PPBUS_G3H rail.
Post resistance to ground on that side.

Check for short on any big coil (secondary supplies),
Will be better to post diode mode to ground on all, as in PPBUS_G3H too.
Switch to ohm mode only for less than 010 value.

 

[jfursdon]

firstly thank you both so very much for your help!

so on pin 2 of F7140 i get a diode reading of 0.14 V.



So i (with your help)have found the fault!!!!!!


when testing for a short i soldered my positive end to the fuse f7140 as this was fairly large. i must have affected the fuse because i just tested it and it gives me a reading of 3 ohms soldered in a plain piece of wire and get 8,56v and fan spin!!!! just off to run up and down the stairs now!!

but question what reading in diode mode/resistance makes you start to wonder about a short circuit?

thanks again
john fursdon

 

[2informaticos]

Was not really short, as you got 4V with F7140 in circuit.
But voltage in from of F7140 goes to correct level when fuse removed.
That denotes an issue after the fuse, on the system side (pin 2/F7140).