Daewoo DLV-42C7LMB sometimes starts after LONG period

[dkneyle]

The power supply was showing signs of bulged caps so I replaced all those physically damaged and a couple of others in the same area. There's quite a few 1uF around the board which looked OK and I left in place. (Circuit attached.) Thought this would bring it back to life, but no.
The power LED went through its start-up cycle and the stayed green as per working condition. However the control buttons including power were unresponsive. I left it sitting powered up on the bench when I heard a relay click in and suddenly it was working, picture on screen, buttons responsive. I turned it off with the power button and then again all the buttons became unresponsive. Couldn't restart it.
Suspecting a mainboard failure I swapped with one from the 32 inch Daewoo I've been repairing that share the same mainbaord just with different FLASH memory from the chip labels I saw. Exactly the same symptoms (and I know that mainboard works in the 32 inch Daewoo).

I've got the 5V standby supply all OK. Even though the power LED is steady I get no PS-ON signal. When I jumper PS-ON to earth I get relay click but none of the other voltages come up.

Got me a bit stumped as to the sequence of what's going on here? I thought by posting here someone might say "oh yes, of course, its........"

[heavymachines]

Re: Daewoo DLV-42C7LMB sometimes starts after LONG period

Check power button board ? just a quick guess with the symptoms.

[budm]

Re: Daewoo DLV-42C7LMB sometimes starts after LONG period

Per schematic, that PS-ON is active high to turn on the power relay which will then turn on the main power supply that supplies the 30V, 24V, and the 12V to run the rest of the TV.
So when the TV IS off the DCV on PS-ON should be <1VDC, when power switch is activated then you should have >2VDC and you should then see the 30V, 24V. and 12V present on the power supply connector.

[dskall]

Re: Daewoo DLV-42C7LMB sometimes starts after LONG period

If you have a remote try unplugging buttons and see if it works with remote.

[dkneyle]

Re: Daewoo DLV-42C7LMB sometimes starts after LONG period

Bugger! Caught out by the old reverse logic on the PS-On line! Well, at least that means it's the power supply I guess? Well, having the circuit diagram I'll get stuck in and do some tracing on the hot side. I'll replace most of those small electros as well and report back.

[dkneyle]

Re: Daewoo DLV-42C7LMB sometimes starts after LONG period

I've just spent a few hours on it to no avail.
The main power capacitor has ~405VDC across it, so I'm assuming the PFC circuit is working.
The driver IC has a VS of ~10VDC. I'm assuming this is about the right voltage for this IC. Someone correct me if it's wrong.
I replaced a couple of electros around the driver IC. C26 (47uF) and C30 (22uF). I was still assuming a capacitor fault because I've had it self start after a period on two occasions. I looked closely for dry joints using a mag glass but they all seem ok. If I get nowhere I'll probably run a hot iron over all those joints just to be sure.
In circuit the FET drivers check out OK.
The driver just doesn't appear to be oscillating.
Any suggestions of next steps?

[budm]

Re: Daewoo DLV-42C7LMB sometimes starts after LONG period

So the PS-ON is present and you do have 405VDC on the main filter cap which it is kind of high so you should verify the batteries in your meter.
And at this point you do not have the 30V/24V/12V, is that correct?

"The driver IC has a VS of ~10VDC." Which IC are you talking about? Since you have the diagram then you should provide the designator of the part so I can see what you are looking at.

[dkneyle]

Re: Daewoo DLV-42C7LMB sometimes starts after LONG period

I just checked the specs for L6598 (IC2) and 10.5VDC would appears to be right in the middle of the acceptable voltage range for Vs on this IC.
My Fluke 77 meter took a nasty knock and is probably out of calibration. I've ordered a new all-singing all-dancing meter but it will take a couple of weeks probably to arrive. So I'm getting by with the damaged meter and recognising it is probably at best giving approximate readings only.
So yes, I have PS-ON of about 3.3VDC. The power relay clicks in. But no significant voltages on any of the DC output lines. There's just a few 1uF caps here and there I haven't changed. I'm looking at the circuit and see C34, a 1uF that appears to be in some sort of feedback circuit. Not sure if that would be significant?
The 'intermittent' nature of the fault suggested maybe a mechanical failure. I've tapped and prodded, wiggled wiring looms, all to no effect.
I replaced C26, the 47uF filter cap for Vs of IC2, but looking at the supply voltage with the AC setting there appears to be a lot of AC ripple or noise. My unreliable meter seem to indicate almost 400mV and it was jumping around quite a bit.

[budm]

Re: Daewoo DLV-42C7LMB sometimes starts after LONG period

"But no significant voltages on any of the DC output lines."
So you are GETTING some DC on the output side, right.
Can we see some good clear pictures of this power supply board?
Check the capacitance of C15 (333J/630V, need to check it off the board.

[dskall]

Re: Daewoo DLV-42C7LMB sometimes starts after LONG period

Would that be a capacitive coupled transformer?

[dkneyle]

Re: Daewoo DLV-42C7LMB sometimes starts after LONG period

There's been some progress, but firstly let me answer those questions.
Yes, interestingly, there was about 700mV on the 12 VDC line, which falls away after mains is removed, so I'm confident some voltage is there (and not my faulty meter).
I too was wondering about the output capacitor? I haven''t got a facility to measure it at the moment, but I'll substitute another from a junked board.
I've grabbed a decent quality image of the board off the net. (Attached.)
So I ran a hot iron over all the pads around any component involved with IC2. It fired up and came to life. Controls were responsive. However the backlights were pulsing and the inverter boards (2 in this TV) were making a cycling ticking sound. I turned it on and off at the mains a few times and same symptoms. Then I used the side button control to shut it down and I couldn't bring it back to life. Switching on at the mains no longer works.
I might start physically removing a few components, check out of circuit and re-solder. I'm close and I think it's perhaps a mechanical fault somewhere rather than component fault.

[dskall]

Re: Daewoo DLV-42C7LMB sometimes starts after LONG period

Check Budm post 9 about C15. You might want to watch this video https://www.youtube.com/watch?v=L8nj3CpiEKY Different board same problem.

[dkneyle]

Re: Daewoo DLV-42C7LMB sometimes starts after LONG period

Thanks dskall. I reckon you and Bud have probably nailed it. That video even shows the exact same voltage I was getting on the 12 volt rail.
Unfortunately I don't have a suitable part from my junk supplies and will have to order one in to confirm. I suspect the cap is right on the edge; sometimes enough can get through to power the set, sometimes, as with the backlight flickering, it can't sustain full voltages, and usually it just won't let anything much through.
Will update this post when I have changed over that capacitor.

[dkneyle]

Re: Daewoo DLV-42C7LMB sometimes starts after LONG period

Damn. I got my all-singing all-dancing multi-meter and measured the value of that coupling capacitor. Spot on at 33nF. Still on the search for whatever is wrong.

[dkneyle]

Re: Daewoo DLV-42C7LMB sometimes starts after LONG period

Checked a few other polyester caps and re-assembled. Still fails to start. Decided to play over the board with hot air gun. Be damned if it didn't start up in that flicker mode. Was trying to measure some voltages when it shut down as it cooled off. Beginning to wonder about those FETs? It seemed to me it might have been as I heated up the heatsink for one that it came to life. Going to change them out and see what happens.

[dskall]

Re: Daewoo DLV-42C7LMB sometimes starts after LONG period

I would be checking Q2 and Q3 and everything around IC2

[dkneyle]

Re: Daewoo DLV-42C7LMB sometimes starts after LONG period

Well, it's working, but not before an almost disaster, and I'm still not really sure what the root cause was.
I went back and tested again with the heat gun. Heavy handed- too much heat! The coupling capacitor C15 split and C13 completely exploded almost. C26 electro (which had been replaced) bulged. I replaced C26, used a small mica (?) cap of correct value for C13, and scrounged a coupling capacitor which was only 22nF instead of the original 33nf. I then went over most pads on the cold side because I'd had a sense that was were heat had worked to bring some life back. Powered up and it started first time!
Ran for about 10 minute and the picture froze and non-response from the side buttons. All voltage checked out OK. Powered down, assumed a mainboard issue, re-seated all connectors. Been heat soaking and working for about 2 hours no probs.
I will probably order a proper 33nF cap but otherwise I think I might have her going again!

[dskall]

Re: Daewoo DLV-42C7LMB sometimes starts after LONG period

Well any repair you can walk away from is a good one I guess

[dkneyle]

Re: Daewoo DLV-42C7LMB sometimes starts after LONG period

Might not have quite yet got to the bottom of this. I've been running the TV, turning on and off, and then yesterday it started with the strobing backlight issue. "Great" I thought. "I'll take some voltage measurements." However no sooner did I get ready to do that than the problem vanished. This morning a couple of times at startup I could hear it begin to strobe but then it fired up OK. I wondering if it might actually be an inverter problem rather than power supply now I have that working OK. I still want to put in the proper 33nF coupling cap rather than the 22nF substitute I am currently using. I think this TV might have been in the weather so I'll also check the seating on cables around the inverters.

[dkneyle]

Re: Daewoo DLV-42C7LMB sometimes starts after LONG period

Replaced the coupling cap with a new encapsulated 33nF cap. Reseated a few connectors going to the inverters. No more signs of the backlight strobing problem at start up. Fingers crossed. Will be a free bee TV for a friend so I don't have a case of the guilts if it bounces.
As for what the true original problem was? I suspect a bad joint somewhere. Nearly all the electro caps were bulged as well but replacing them still didn't get it to start. And the strobing backlight at start up (once it would start)? Hopefully a poorly seated plug and socket.
And by the way, I have a fantastic new meter that does everything, even tells me the room temp and humidity while working. Mastech MS8229 for A$60 landed in Australia from China. The old Fluke meter has officially gone to God.