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#51 2014-08-07 02:04:13

techknight
Member
Registered: 2014-05-22
Posts: 449

Re: Mac Portable motherboard repairs

Next century... And send me a PM ;-)

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#52 2014-08-17 00:48:07

techknight
Member
Registered: 2014-05-22
Posts: 449

Re: Mac Portable motherboard repairs

More tips for non-backlit M5120 motherboard issues:

I figured something else out. The non-backlit models that are hit or miss on being able to power on, and the charging indicator is erratic or stuck in an on, or off state is the LTC1040CN is bad.

That chip is a window comparator, it has 2 op-amp comparators internal to the chip. it has 3 outputs, But the trick is, the outputs are latching. Meaning, they dont reflect the input states until the chip is strobed. since strobe is tied to ground on the LTC1040 in the Hybrid, That means the internal oscillator of the chip has to strobe at the frequency at which the oscillator is set. So if the chip doesnt oscillate, it will latch to whatever the states are on power-up and thats it. So the thing will be stuck on charging, or not charging. Or be stuck off...

Comparator 1 is the undervoltage lockout. if the voltage is below a certain setpoint, the 1st comparator wont latch. This causes the 5V reg to be 0v. When the voltage comes up to acceptable levels, the next time the chip cycles, it latches in a good state. Bringing power to the unit.

Comparator 2 is the battery charge indicator. This goes high when the ac adapter voltage is higher than the battery voltage. When this goes high, it turns on power to one of the op-amps which controls the charge current/flow and monitors battery condition. Also changes the ADC Scale in the PMU, as well is tell the PMU that its in a charging state. Once the charger is removed, or the battery is equal to AC Adapter, the chip will latch back into an off state.

Thats how it works. Problem is, This oscillator circuit composes of a 100nF SMD cap, and either a 1 meg or 10 meg resistor which is underneath the IC. This type of RC combintation sets a very slow oscillation time, almost once per second. Downside is it makes it real sensitive to internal leakage. Somehow, the chip starts to become leaky internally and its oscillator stops. Once this stops, that means the machine can no longer latch in charge states, or sometimes wont latch in undervoltage lockout so it will appear "dead".

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