two MINUTES on, then off-on, then black...

[808dude]

I've got a lay-around Sceptre X24WG that I would like to either take a short time to repair, give it the ol' heave-ho, or heave it gently to someone who'd use it after repairing, since I really don't need it. (it would be a Honolulu-pickup freebie, though...)

Symptoms:
From cold, it comes on and looks just slightly dim for a few minutes, but otherwise the image looks fine. After those first few minutes (not seconds), backlights start cycling off/on every second or so for a while. This dithers into plain old blackness by a few minutes later.

Only after a long OFF time can the above sequence be repeated, as if some component heats up and goes off the curve...

I am guessing that some, but not all, of the troubleshooting methods mentioned in the '2-seconds-to-black' sticky would apply, but not sure.

What data to provide? I've got most basic tooling/DMMs, a low-budget cap meter (not sure it qualifies as an ESR), and a willingness to keep stuff out of the landfill.

Mahalo... Dave

[budm]

Re: two MINUTES on, then off-on, then black...

Start by uploading good clear pictures to show all the boards first.

[808dude]

Re: two MINUTES on, then off-on, then black...

Thanks Bud...

If needed I can provide some daylight-photos, but weather's not conducive right now.

For now, I'm guessing the action's going to be on this board:


The only other for-real board is this:

[budm]

Re: two MINUTES on, then off-on, then black...

It may be the lamp problem, inverter problem , or power supply problem.
I would monitor the DCV between the two legs of that filter cap sitting next to the inverter transformer LTL22000703 to see if it fluctuating a lot or not.
You can also try heating the caps in the secondary side of the power supply section (bottom left corner of the board) with hair dryer before applying the AC power to the unit to see if it will stay on longer.

[808dude]

Re: two MINUTES on, then off-on, then black...

Thanks, Bud!

Clarifying, maybe - your last word "longer..." did you actually mean "for less time?" I'm thinking if it's heat-related, and I "fuel the fire" by adding heat to those caps as you suggest, and they're the problem, it would reach the breakdown point sooner vs. later. Maybe I missed your point, though.

--Dave

[budm]

Re: two MINUTES on, then off-on, then black...

The capacitors may be on the edge of failing, as the lamps start up they may no be puliing full current from the power supply until they are stabilized, but then the cap may not be able to supply full current demand due to high ESR. When cap is heated up the ESR will go down. So you should monitor the DCV on that filter cap by the inverter transformer to see what happens to the dcv when the circuit starts failing after a few minutes.
And if you put your ears close to top edge and the bottom edge of the LCD panel, do you hear any sizzling sound or smell Ozone.
The board has no sign of heat damage, the only thing that get hot fast in a few minutes are the lamps.
But I take it that you already did the good visual inspection of the board for bad connections.

[808dude]

Re: two MINUTES on, then off-on, then black...

I'm back on this after some distractions...

I soldered a twin-pair onto those cap leads (the one nearest the dark inverter-xfmr you called out on the right midriff of the image, it's 35V/470uF) and monitored as I think you suggested, Bud.

I had 23V right off, dropping to 17.75V after a few seconds, where it held while the image looked more or less normal (perhaps a bit dim, but livable). About two minutes in, when the CCFLs began going black (three seconds black, then a flash of normal illumination, repeating), I noted that the cap voltage was back to the original 23V for each three seconds of blackness, then would drop for each flash of normal picture (hard to say to what voltage it drops, since it's a DVM and the drop/flash is very brief, that is, well under a second).

What does this indicate?

Thanks - Dave

[budm]

Re: two MINUTES on, then off-on, then black...

OK, that is very large Voltage swing, we need to see if the 5V supply also has large Voltage swing or not too, check the DCV on the cap C810, C803.
Also need the good straight shot pictures of the whole bottom side of the power supply board.
I also wonder if the DCV on the primary side filter cap (big brown cap laying on its side) also has large Voltage swing due to bad cap.

[808dude]

Re: two MINUTES on, then off-on, then black...

Thanks Bud, I will repeat that check, on the two caps you mentioned (not the other similar ones?)

I accidentally did start measurements on the primary filter cap before you wrote the above, finding that it started at 165V, dropping about 10V in 30 seconds and holding there for a minute, at which point I realized I wasn't measuring the right voltage and stopped (screen hadn't begun doing the off-on thing yet.) Smoking gun?

[budm]

Re: two MINUTES on, then off-on, then black...

I am expecting the power supply Voltage to drop also as well if the main drive power supply is the problem since both Voltages (one for the main board and one for the Inverter circuit) are generated from the same power transformer.

[808dude]

Re: two MINUTES on, then off-on, then black...

I am expecting the power supply Voltage to drop also as well if the main drive power supply is the problem since both Voltages (one for the main board and one for the Inverter circuit) are generated from the same power transformer.

Bud,

I'm surfacing again with latest tests:
1) C810: started @ 5V, then over the course of a little less than 5 minutes, dropped steadily to about 3.5V - upon "black-screen," it returned to 5V.

2) C803: much the same - 5V to start with, only more quickly dropped to about 3.3V (under two minutes) at which point I got the black screen and return to 5V. (I take it the increasing speed of voltage-drop is a heat-up thing?)

3) Big main cap: I recalled it wrong in my previous post, or something's changed radically - rechecking, I got a CLIMBING voltage, not dropping, ranging from 156V to 165V over about 1.5 minutes, before it was back to the black screen.

What to do?

Thanks - Dave

[budm]

Re: two MINUTES on, then off-on, then black...

So both supplies (5V for the main and the supply for the inverter circuit) Voltage drop down, then it has to be something in common, I really do not expect the DCV on the main cap to change much my be 5V or so.
When the 5V feeding the main/logic board drop to 3.5v, that can cause the command signal to turn on the inverter backlights to drop out so the backlights go off then the power supply Voltages will rise back up again due to light load (backlights not on) then the ON command is present again and the load is present again and cause the Voltage to drop again so it is in circle.
the Voltage on the cap is = to 1.414 x Incomimg AC, 120VAC, the dc on the cap will be 120VAC x 1.414 = 169VDC (Vdrops of the two diodes are not significant).
I would check or try another main filter cap, so find out if you get constant AC to the unit or not, if you do have 120VAC, then I do not expect it to go down to 156VDC on the cap.

[808dude]

Re: two MINUTES on, then off-on, then black...

Thanks, Bud...that cap has a 400V/100uF rating...might I be OK with a lay-around of at least 400V, and perhaps exceeding the 100uF? I gotta go rummage. I'm thinking it might not be worth a purchase of identical value...and if this doesn't work, maybe it's a write-off?

[budm]

Re: two MINUTES on, then off-on, then black...

You can even test it with 250Vdc CAP, 100~120UF CAP.

[808dude]

Re: two MINUTES on, then off-on, then black...

Epilogue?

I was hoping this would be a simple "thanks-for-the-mentoring" email, but not quite.

I had no caps even close to the suspected-bad one, so I ordered a replacement from ebay out of China, which came a couple weeks ago, and I finally got around to installing it today. I was already declaring victory because the monitor went for 10 minutes without any recurrence of the lamps-out problem (that's more than twice as long as it previously took) but then at about 11 minutes, blackness...

Maybe this is a lost cause. Let me know - I don't have a great need for the monitor anyway, but still hate to throw away stuff that SOMEONE could use with simple repairs.

--Dave

[ReeceyBurger123]

Re: two MINUTES on, then off-on, then black...

Images of the bottom of the Psu ?

[808dude]

Re: two MINUTES on, then off-on, then black...

Sure, hope this helps...shots with and without flash. Orientation is same as component-side PCB shot posted originally.




Cheers - Dave

[sean0118]

Re: two MINUTES on, then off-on, then black...

Epilogue?
... so I ordered a replacement from ebay out of China, which came a couple weeks ago, and I finally got around to installing it today. I was already declaring victory because the monitor went for 10 minutes without any recurrence of the lamps-out problem (that's more than twice as long as it previously took) but then at about 11 minutes, blackness...

Good chance the new capacitor from ebay is also high ESR. There's a lot of fake capacitors on there.