ViewSonic VA926, 2 seconds to permanent off

[kloons]

Good day!

Todays patient:
ViewSonic VA926 - Model Number VS11886
PWB - Digital Decade BLM1700P60720 - Model PI-LT03

Symptoms:
After plug-in 220V, lights up with clear picture from PC.
After ~1-2 seconds, back-light and picture shuts off completely (no image in background).
Power led turns orange(standby) or off.
Pressing power button does nothing at all (led stays orange or off, depending on state it was).
Time to time after ~5-10min monitor tries to power on, but again for 2 seconds and off.

Caps (originally Lelon brand):
2x 16v 1000mF , swapped to another ones - no change (C816 and C829)
1x 16v 680mF , swapped to 16v 1000mF (don`t have spare 680) - no change (C817)
1x 50v 470mF , visually ok (have no 50v caps to fit in) (C802)
1x 50v 220mF , visually ok (have no 50v caps to fit in) (C901)
Big One, i believe 450VA (it`s glued to board with captions), visually ok (C804)

Diodes seems to work the way they must.

Did not have no luck to find schematics in internet for the board.
Searched here for pwb or similar monitors - no luck.

No signs of overheat.

I`m open to suggestions, where to start, what to measure?

Thank You!

[budm]

Re: ViewSonic VA926, 2 seconds to permanent off

C802, C901 are the filter caps for the backlights inverter circuit, you can use 35V rating caps for testing.
You can lift one leg of the Diode D801 off the board to see if the monitor will stay on without it going into standby, you just will not have backlights, this is to verify if the problem is in the inverter section or not.

[kloons]

Re: ViewSonic VA926, 2 seconds to permanent off

Hey - Thank You for quick response!

Replaced C802 (35V) and C901 (35V) - still same symptoms, no changes.
Lifted D801 leg - no backlight, but the same symptoms.

The moment its turning off, there is short "cpk" like spark sound, but i cannot determine from where - don`t like the idea putting my face near faulty board when starting up

[budm]

Re: ViewSonic VA926, 2 seconds to permanent off

OK, you need to check the voltages on that white connector of the power supply board goes to the logic board next.

[kloons]

Re: ViewSonic VA926, 2 seconds to permanent off

2x +5V ok
BL-ADJ - 0.03V
On/Off - Zero
Two grounds

[budm]

Re: ViewSonic VA926, 2 seconds to permanent off

ON/OFF (inverter on/off) should >2.5VDC when power switch is activated, <1V when off or in standby mode.
BL-ADJ (BACKLIGHT DIMMING) I expect to be >2V for mid level, 5V = max lamp brightness.

[kloons]

Re: ViewSonic VA926, 2 seconds to permanent off

You suggest faulty control board?

[budm]

Re: ViewSonic VA926, 2 seconds to permanent off

Yes. you should at least verify that the ON/OFF pin does go to >2.5VDC when it works.

[budm]

Re: ViewSonic VA926, 2 seconds to permanent off

BTW, You can force on the ON/OFF pin using 500~1K resistor between the ON/OFF pin and the 5V pin BUT YOU MUST disconnect that wire going back to the logic board first, you may need to have another resistor for the BL_ADJUST for the backlights to light up (wire must also be disconnected).

[kloons]

Re: ViewSonic VA926, 2 seconds to permanent off

Ok, will try to check this out today.
Thank You for detailed answer!