dell latitude 7490 la-f322p short on +rtc_cell

[kjchatman]

this board doesn't power up at all. there is a 1 ohm resistance to ground on +rtc_cell. 1v was sent through the rail and the sio (mec5105) got warm. The sio was iremoved and the short was gone. It was replace with a new sio and the short is back

[mon2]

Re: dell latitude 7490 la-f322p short on +rtc_cell

See attached.

Locate the PC3 cap that will be mated to the common cathode of the dual diode @ PD3.

Remove all power. Meter in resistance mode.

Red meter probe on the location in the attachment.

Black meter probe to ground.

What is the resistance to ground ? Is it 1 ohm ? If yes, remove capacitor PC3 off the logic board and measure again.

Next check the coincell +v location in the next attachment.

What is the resistance reading to ground for this junction ?

Is it 1 ohm ? If yes, remove the PC2 capacitor off the logic board and measure again.

[kjchatman]

Re: dell latitude 7490 la-f322p short on +rtc_cell

pc3/pd3 with the super iO removed reads open loop. I put the super IO back on and it reads 1 ohm. the resistance of the coincell +v is open loop regardless of the super IO being attached or not. why would a brand new sio cause the +rtc_cell to short

[mon2]

Re: dell latitude 7490 la-f322p short on +rtc_cell

How confident are you of the soldering of the new super I/o chip? Is it a bga package?

Consider to reflow it with lots of flux and heat to make it 'dance' with a gentle nudge of the tweezers at the edges. This should allow for the bga balls to properly align without adjoining shorts.

Is the replacement ic a known good part? Is it the same part? From a reliable vendor or donor board?

Check the nearby balls of this ic. Are there surrounding ground pins near the coincell pin under the super io chip? Solder shorts may be causing this fault.

[kjchatman]

Re: dell latitude 7490 la-f322p short on +rtc_cell

ordered it from a vendor in germany off of ebay an received it today. it's the same part as the original (mec5105). it was reflowed with genuine amtec flux and the Ic was perfectly centered within the squared outline

[mon2]

Re: dell latitude 7490 la-f322p short on +rtc_cell

Aligned correctly to pin #1 orientation?

[kjchatman]

Re: dell latitude 7490 la-f322p short on +rtc_cell

yes it is aligned correctly

[kjchatman]

Re: dell latitude 7490 la-f322p short on +rtc_cell

could it be the dual diode? the super IO seems to have low resistance to ground even on a working board

[mon2]

Re: dell latitude 7490 la-f322p short on +rtc_cell

Remove the dual diode and test it out of circuit.

In diode mode, should conduct in one direction and block in the other. Test each internal diode.