Daewoo DLV-42C7LMB sometimes starts after LONG period

[dskall]

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

So you recommend Mastech MS8229. Looking for a new meter.

[dkneyle]

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

Not fixed!
With all covers back on it went back into backlight strobing after a period of time. Even got worse and dropped into '2 seconds to off' mode and would re-start with same strobing. When cooled down it would be OK. Pulled covers off and played over various parts with a HAIRDRYER (not heat gun this time). The 24V rial dropped to about 20V with a large AC component I couldn't accurately measure. I identified FET Q3 (or something nearby) as a likely suspect. I can induce the fault with heat, and by playing cool air over the area it quickly returns to normal operation. There are some 1W resistors in that area that I also wonder about, especially R36 (0.5 Ohm). But I've decided to replace Q3 as process of elimination.
It is a P13NK60ZFP, which is a 600V 13A 35 W FET. In my spares I have a P9NK50ZFP, which is 500V 7.A. This is a 42inch CCFL TV. Could I risk using this lower rated substitute do you think?

[dskall]

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

That's quite a difference. Manufactures don't tend to put in bigger components when every cent they save goes in their pocket. Good you can induce fault with heat but heatgun/hairdryer makes it hard to narrow down to one component.

[dkneyle]

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

I thought exactly the same about saving costs etc and why therefore put in a higher spec pair of transistors? And it is a significant difference in current carrying capacity. I was waiting to hear from you and seeing if you thought the same. I can buy 10 from AliExpress in China, landed for less that A$5. Might be a few weeks before they arrive but I'd prefer to know I've got this TV sorted before it goes out of the workshop, even if it is a give-away to a friend. If the new FET doesn't change things I'll just move onto other components in that vicinity. But I'm pretty sure I'm close now.

[dskall]

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

Well I'm thinking capacitor possibly optocoupler. Maybe you can use something to shield other components from heat to narrow down. I have hot air station so can put on small nozzle to concentrate on a small area. Or use freeze spray on individual components.

[dkneyle]

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

Well, you might have been right re capacitor. When I looked real close I noticed one of the mylar capacitors had an expansion split in it. This was from my previous disasterous attempt to check for heat sensitive faults with a heat gun. It was a 1nF cap, C12 on the circuit, connecting to pin 9 of IC2, the oscillator driver. I replaced it with another 1nF mylar cap I had in my box of spares and put the orginal FET back in. Put all back together again and played the hairdryer over the previously heat sensitive area. All continued to work. Moved the hairdryer around a bit to get the whole board nice and warm; no problems. Reassembled completely, and it's been running for over an hour without any issues. That cap was in the target zone of the heat sensitivity. I'll continue to run it and see how things go.
Do mylar caps ('greencaps' as I used to know them) have a history of failing? I ask, because this power supply board had almost every electrolytic bulged and I thought replacing those would fix the problem, which it didn't. I then had this disaster with my overheating the board, and a few mylar caps, including the coupling cap burst open. I replaced those and ran a hot iron over most of the joints and to my surprise it came to life. Did my excessive use of the heatgun simply identify the mylar caps that were on the way out? Because not all of them swelled and opened up.

[dskall]

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

Electrolytics are the usual ones to fail. Once in awhile the others but usually only one. Not a group.

[dkneyle]

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

Well, the set bounced, and I'm determined to find the culprit. Anyone offering technical assistance might like to refer to the power supply circuit diagram on Post 1 of this thread.
It bounced because it would run for about 5 mins then shut down. I confirmed the 5V Standby supply was good in failed condition. The 12 volt rail dropped to about 10 volts in failed condition and was varying. I assumed the failure is in the hot section of the SMPS. I have good voltage at the output of the bridge rectifier BD1, varying slightly as it gets loaded down then comes back up when load is removed. (Loading explained next.)
Voltage measured at the RTH thermistor (output of the PFC circuit) shows it comes up to about 350 volts, then drops off to zero, and cycles like this about every half second.
At this point I'm out of my depth with SMPS theory. Does this suggest the PFC circuit is working, but when it senses a failure in the following driver stage it shuts down?
I've replaced Q2, and Q3, ALL electros have been replaced as well as poly caps, and I've run a hot iron over all components associated with the higher voltage supply.
It's now gone into the failed state permanently, but previous investigations seemed to suggest a heat sensitive fault, hence my use of the hot iron on the solder pads.
I have ordered replacement IC1, IC2 and Q1 semiconductor components because they were very cheap. But I hate being a replace-and-hope technician; far more interested in seeing if I can identify the failure point.
Any SMPS gurus want to suggest where I should be looking?