Sharp LC-42D69U won't turn on, has 5V standby.

[kevinm34232]

Seemed like a main board issue. So I jumped PS-ON and BL-ON, backlight flickering. The 12V voltage comes up, but 24V is erratic, bouncing from 21-26V, every once in a while settling in at 24V. This happens with or without inverter board conncected.

So I think the main board is bad, and another problem on the PS.

Is there something common that would cause this unstable voltage, even with no load?

Also note the voltage at the large cap also fluctating when PS-ON, from 390-398V +-

Attached is SM, and you can find schematics starting on page 119. Any help would be apprecited! Thanks

[Andrew F. Ali]

Re: Sharp LC-42D69U won't turn on, has 5V standby.

So you have a Power Board issue......Why do you think you also have Main Board issue, though???

[CapLeaker]

Re: Sharp LC-42D69U won't turn on, has 5V standby.

Cuz he can't turn the tv on, so he forced the PSU.

[cts_casemod]

Re: Sharp LC-42D69U won't turn on, has 5V standby.

Generally a bad 24V supply will affect the backlight only, not the logic.

On your case it might be caused by a high ESR on capacitors, but as said earlier, this should not stop the main board and LCD panel from turning on.

[cts_casemod]

Re: Sharp LC-42D69U won't turn on, has 5V standby.

Also forgot.... It is normal for the HV capacitor voltage to oscilate as much as +/-15V, thats the Power factor correcting IC regulating to keep the input current sinosoidal (If the supply didnt had PFC you would see rectifid mains, at about 320-360V or half of that for 115V countries)

This is normal and not a fault.

[kevinm34232]

Re: Sharp LC-42D69U won't turn on, has 5V standby.

So I should start with the secondary caps? None are bulging, it's a somewhat newer TV so I didn't suspect them..

[CapLeaker]

Re: Sharp LC-42D69U won't turn on, has 5V standby.

To clear this up: do you get PSON and BLon from the main board?

[kevinm34232]

Re: Sharp LC-42D69U won't turn on, has 5V standby.

No. Just standby 5V+, power on comes from button controller board, but nothing else.

[budm]

Re: Sharp LC-42D69U won't turn on, has 5V standby.

Since you are not getting the PS-ON command from the main baord to turn on the power supply, then how about the BL-ON and the DIM command signals, are they present?
BTW, the switched 12V and the 24V are generated by the same power transformer, so

[kevinm34232]

Re: Sharp LC-42D69U won't turn on, has 5V standby.

The 12V is pretty stable. What does that mean? I'll have to double check, but I don't think I had any voltage except 5V standby.

[budm]

Re: Sharp LC-42D69U won't turn on, has 5V standby.

If the main board does not send the PS-ON then you are going to have any output at the switched 12V or the 24V, at least right now you do have main board problem.

[Andrew F. Ali]

Re: Sharp LC-42D69U won't turn on, has 5V standby.

Cuz he can't turn the tv on, so he forced the PSU.

OOPS!! Some how missed that

[kevinm34232]

Re: Sharp LC-42D69U won't turn on, has 5V standby.

I am jumping the PS-ON and BL-ON with 3.3V, and turning on the PS. This is when 12V and 24V come up, but I the backlights were flashing on and off. When I checked 24V line it was fluctuating.. I am just trying to see how I can narrow down where the problem on the power board is. I know the main board is bad too.. Thanks.

[CapLeaker]

Re: Sharp LC-42D69U won't turn on, has 5V standby.

have a look at the feedback circuit of the 24v supply. There is also an overvoltage protection circuit that needs to be checked.

[kevinm34232]

Re: Sharp LC-42D69U won't turn on, has 5V standby.

What would be a good starting point, I have not done much t/s'ing on these circuits?

[budm]

Re: Sharp LC-42D69U won't turn on, has 5V standby.

The dim command signal stable?

[kevinm34232]

Re: Sharp LC-42D69U won't turn on, has 5V standby.

Is DIM the BRIGHTNESS as in photo? I'll have to check again, but I think there was 0V. And remember, this is with the main board disconnected from the power board.

[budm]

Re: Sharp LC-42D69U won't turn on, has 5V standby.

the brightness is the same as DIM, 0VDC = NO BACKLIGHTS.

[budm]

Re: Sharp LC-42D69U won't turn on, has 5V standby.

BTW, what is the LCD panel model number?