LA-E921P no power

[ictc]

Diode replaced from a doner board, voltage now around 2044 but some times rised up till 20.7 than stabilized around 20.6. resoldered the 10ohm resistor in place and measured VCC... same identical situation as before... Could it be that the IC is struggling to manage the output of the second MOSFET, perhaps due to a varying demand for current from Vcc? It might be that, for some reason, it shuts down and then immediately attempts to restart, only to encounter the same conditions forcing it to vary its current draw again. This could create a sort of loop, which would then manifest as fluctuating current consumption and cause the measured Vcc voltage to vary continuously?

[mon2]

With no power to the board, meter in resistance mode, what is the resistance to ground of pin # 28 = Vcc?

We are checking if perhaps the 1uf @ PC314 is shorted. That is, once yo mate the dual diode & PR312 (10 ohm) resistor, the voltage should still remain stable. This is based on the assumption that there are no shorts on PU301 and that PU301 is not defective. Regardless of the issues with other parts, the VCC leg of this circuit should be stable.

Can you also test PQ302 (if not done already)?

With meter in resistance mode, test:

source / drain
source / gate
gate / drain

pins. Each should be hundreds of k ohms or higher if the (battery) mosfet is ok.

[ictc]

with PR312 in place, negative probe on ground positive on vcc ..6.85Mohm, reversing probes 0.5Mohm. Without PR312 negative probe on ground 8.3Mohm, reverse 1.94Mohm
PC314 not shorted
PQ302 seems to be ok no low resistance in any probing combination.
No low impedance on any of the pin of PU301

[mon2]

Each mosfet resistance test should be hundreds of k ohms or higher - is this correct?

[ictc]

I removed PQ304 and measured VCC again... Perfectly stable! Could it be something related to the PQ304 load side? I wondered. The MOSFET has also been replaced with no luck. Without it, the IC seems to function properly, providing the correct gate voltage and showing a perfectly stable VCC.

Next, I tried removing both current sense resistors (PR335 and PR304), and, as expected, I got 19V_P2.

What's still driving me nuts is the 4.27V reading I get on the +19VB side...

[ictc]

still measuring Mohm value on +19VB... Apparently no direct short on this rail but someting get weird when powerd up. Can we exclude problems on PU301 side? Injecting voltage on this rail without a short would it make any sense?

[mon2]

If the resistance is not low then voltage injection will not help. Voltage injection is helpful to smoke out shorted parts.

[ictc]

If the resistance is not low then voltage injection will not help. Voltage injection is helpful to smoke out shorted parts.

Yes, I agree. However, how can I determine what is happening at +19VB that prevents the charging circuitry, which seems to be functioning correctly, from delivering voltage to the rest of the board? If there is no short circuit, what should I check next? The only assumption I can make at this point is that the short might not be on the main rail. However, I haven't been able to identify any shorts on any of the coils.

[ictc]

To further narrow down the issue, I selectively removed the jumpers connecting the main power rail to various sections of the circuit in order to identify the faulty section responsible for the malfunction, which causes the +19Vb power rail to shut down due to the primary power circuit entering protection mode. This method allowed me to pinpoint the jumper labeled PJ403, associated with the 3V3 generation circuit connected to the PU401.

However, no short circuit was detected at the output of the 3V3 generation circuit. This suggests that the issue might extend to other components or sections sharing the same power rail, making it more challenging to pinpoint the exact cause. How would you proceed from here?

[mcplslg123]

Measure the resistance to GND on +3VLP aka LDO?

[ictc]

the resistance value on +3VLP is 32Kohms measured on PC411 with red probe on ground, viceversa I don't get a fixed value.
Considering that PJ403 powers only the circuit of PU401, I decided to reconnect that jumper and open PJ401, which is instead related to +3VALWP. However, at that point, I once again lose the +19VB.