Syncmaster 2493 - LED not lit, monitor won't power up

[johnwartjr]

I am working on a Syncmaster 2493.

The screen is black. The power LED never illuminates.

I have tried another power cord.

It belongs to a friend of mine who said it made a loud quacking noise before it failed.

I have a DMM, an ESR meter, and good rework equipment. I tested all the caps on the power supply with the ESR meter and all are within 10% of their rated values.

Any suggestions where I might start?

Thanks!

[Pentium4]

Re: Syncmaster 2493 - LED not lit, monitor won't power up

Can you see a picture if you shine a flash light on the screen?

[selldoor]

Re: Syncmaster 2493 - LED not lit, monitor won't power up

Tested the fuses?

[johnwartjr]

Re: Syncmaster 2493 - LED not lit, monitor won't power up

Fuses test good.

I plugged it into a PC, and the PC detects the monitor, but the picture never comes up. With or without a flashlight shining on it. Turn the power switch to 'off' and it goes away on the PC, turn it on, it comes back on the PC, but the screen never illuminates and the power LED never lights.

The power LED does not light on the front of the monitor.

I attached some PCB photos.

[budm]

Re: Syncmaster 2493 - LED not lit, monitor won't power up

Check to see if you have 160vdc on that large filter cap (82uf/450v), and see if you have 5V at the SB PIN.
More pictures here:
http://s807.photobucket.com/user/bud...?sort=3&page=1

[johnwartjr]

Re: Syncmaster 2493 - LED not lit, monitor won't power up

5.25 at the SB pin, 176 VDC across the cap.

[budm]

Re: Syncmaster 2493 - LED not lit, monitor won't power up

If you do have 5V standby voltage which is used for running the main/logic board, that means the main board is not working if you do not see any indicator on the front of the unit. Do you also get >3V at the PS_ON pin which is generated by the main board?

[johnwartjr]

Re: Syncmaster 2493 - LED not lit, monitor won't power up

I get 0 VDC, but 7.9 VAC when measured against the ground pin on the same connector. Not sure if this is a DC or an AC reading.

These readings are with only the PS sitting on my workbench.. do I need to connect the other boards back to it to check the PS_ON pin properly?

[budm]

Re: Syncmaster 2493 - LED not lit, monitor won't power up

PS_ON signal is DC, in the cold side of the circuits, the chassis is the ground ref point for the cold side. The negative leg of the large filter cap is the ground ref point of the hot (primary side) side of the circuit, not the chassis.
You have to have all the board connected since the main board needs the 5V STBY and the power supply needs the PS_ON from the main board to turn on the PFC and the 12v/24v power supply section.
Please read my guide below to understand how it works.

[johnwartjr]

Re: Syncmaster 2493 - LED not lit, monitor won't power up

Thanks. I'll get back to this tomorrow and post back with the results.

[johnwartjr]

Re: Syncmaster 2493 - LED not lit, monitor won't power up

Sorry for the delay, life got in the way

PS_on is 3.1 volts
I checked the 24 volt supply, and it's a solid 24 volts
I also checked the pins marked 5.3 volts - they measure 5.4 volts

[budm]

Re: Syncmaster 2493 - LED not lit, monitor won't power up

OK, PS_ON is OK otherwise you will not be getting the 24V for the inverter board.
So now I will need good clear pictures of the inverter board as a whole and also in 1/4 section so I can easily see the components and traces.
Same for the main/logic board.

[johnwartjr]

Re: Syncmaster 2493 - LED not lit, monitor won't power up

More pics