Laptop AC adapter Dell PA-1900-28D dead

********************************************************
Laptop 90W AC adapter (slim type) (19.5V, 4.62A)

Brand: Dell
Model: PA-1900-28D
Family: PA-3E

Platform: Liteon PA1900-27D, Rev.: B
********************************************************
Hello!

I'm working on mentioned laptop power supply.

Symptom:
- Dead, no voltage on the output.

The primary based on switching controller chip LTA804N (datasheet not available), which is quite similar like chip TEA1751T which has also the same pinout (datasheet at the attachment). The theory of operation is described on page number 6.



Schematic is available on the forum: www.elvikom.pl/schematic-power-supply-dell-pa-1900-28d
I also found a complete description of TEA1751T based power supply (at the attachment). Schematic is on the page number 25, and it is quite similar like mine.

320V present on the main primary filter capacitor. Power switching transistors Q001 and Q050 checked desoldered from the circuit and they are ok. No short circuit on the secondary.

The voltages to ground on all pins of LTA804N chip:
1 -> The voltage swing between 15 and 23Vdc with a frequency of about 0.7Hz (measured with oscilloscope, oscilogram on the picture below)
2, 3, 4, 5 -> 0V
6 -> 15mV
7 -> 3.03V
8 -> 0V
9 -> 1.78V
10 -> 35mV
11, 12, 13, 14, 15 -> 0V
16 -> 317V

Oscilogram of voltage on pin 1 of LTA804N chip:


What I've done up to now:
- Primary controller chip LTA804N replaced with brand new one -> no changes

I simply can not find any bad component.
Please, help me with further diagnostics.

Replace C12, C13, C14 not check ... just replace.
D23A replace.
Check transformer is all ok ?

I recomend to replace all electrolitic capacitors with smallest values (1uF, 10uF, 22uF ...) .

B rgds
Mark

inventco napisał(a):Replace C12, C13, C14 not check ... just replace.
D23A replace.

All listed components replaced -> exactly the same issue (voltages on all pins of the chip unchanged)

inventco napisał(a):Check transformer is all ok ?

Both transformers (T1 and L2) checked with ohmmeter -> no interrupted windings

inventco napisał(a):I recomend to replace all electrolitic capacitors with smallest values (1uF, 10uF, 22uF ...) .

On the primary side are only two electrolytic capacitors (C3 and C13). C3 is mains 400V filter capacitor, which is more than likely ok, another C13 replaced with new one.

Thank you very much for reply!
Any more idea?

Hi again.
D23 ?
What is the signal on pin 13 TEA ?
What is signal on gate of main transistor ?

I there any signal on FB (feedback) coil ?

Maybe transistor is dead (not switching).

inventco napisał(a):D23 ?

D23A replaced with new one, like I mentioned in post #3.

What is the signal on pin 13 TEA ?

No signal.

What is signal on gate of main transistor ?

No signal as well.

I there any signal on FB (feedback) coil ?

There is also no signal.

inventco napisał(a):Maybe transistor is dead (not switching).

Transistor is more than likely ok, but it is no signal on the gate.

I measured the signal on pin 5 (LATCH) of the chip - when I plug the cable in the wall socket I get 3V pulse for 35ms, than it falls to 0V and stays there: