32" LCD Beko/Grundig Chassis LX no start - fix

[Th3_uN1Qu3]

Got this 32" LCD in today. Schematic here. Standby LED was on, but the set wouldn't react when trying to power on. No bad caps.

It did power on with the Tcon disconnected, and shut down immediately as the cable was plugged in. I noticed that 5v that was supposed to be going to the Tcon went away when the cable was plugged.

No shorts, so i applied 5v externally. No smoke, hit the power button and worked fine. Surprise surprise: Disconnect external 5v and TV STAYS on.

Fault: FDS9933A dual P-channel MOSFET with too high Rds(on) on the half that supplied 5v to the Tcon, the startup curent of the panel made its output dip too low, and the mainboard would shut it back down. It's marked as T314 on the schematic.

Don't have it/can't find it/want a quick fix? Well, i have one for you, coz i went for the quick n dirty approach as well. I pulled a single P-channel fet from the battery circuit of a dead laptop board (you will ALWAYS find p-fets near the battery connector), and simply soldered it on top of the 5v half of T314, in parallel with it. TV works great now, another happy customer.

If you want the long story, i'll share it as well.

[tom66]

Re: 32" LCD Beko/Grundig Chassis LX no start - fix

Nice fix! Any pictures? I have never seen a genuine Beko yet. Probably should avoid them, apparently they have lots of problems.

[Th3_uN1Qu3]

Re: 32" LCD Beko/Grundig Chassis LX no start - fix

Couldn't take pics - batteries were flat. The service manual i linked to includes full schematics AND PCB layouts, so you will have no problem finding the part.

Forgot to add, TV was about 3 years old. So yeah, to be avoided. Will crap themselves just after the warranty runs out, as do Samsungs.

[tom66]

Re: 32" LCD Beko/Grundig Chassis LX no start - fix

Couldn't take pics - batteries were flat. The service manual i linked to includes full schematics AND PCB layouts, so you will have no problem finding the part.

Forgot to add, TV was about 3 years old. So yeah, to be avoided. Will crap themselves just after the warranty runs out, as do Samsungs.

It just makes me more depressed as an engineer in training to see poorly designed sets.

I mean, a T-con will draw 2-3A at 5V. That's nothing for a good p-FET. Whoever underspecified that part is an idiot.

http://www.digikey.com/product-detai...3TR-ND/1978731

31 cents. Probably much cheaper in the quantities TVs are produced at.

Heh, at least it's not a Vestel. Although Vestel bought Beko's TV division I think...

[Th3_uN1Qu3]

Re: 32" LCD Beko/Grundig Chassis LX no start - fix

Beko bought Grundig, Beko and Vestel are both from Turkey... i wouldn't be surprised. I wouldn't say underspecified part - rather, oversensitive protection circuitry. Btw, i own three CRT TVs based on Vestel chassis, the oldest is almost 7 years old and the newest 5 years, and none have had any issues yet.

I have fixed a few of them tho - shorted HOT is a very common fault, right after bad caps. This is due to the "torch mode" settings they are set at in store displays to catch buyers' eyes, and most people don't bother adjusting them. I run the contrast on my sets on no more than 1/4, because any higher will bloom in bright areas (especially noticeable on subtitles), which is a clear indication of an overdriven CRT. So far, so good on all three of mine.

[tom66]

Re: 32" LCD Beko/Grundig Chassis LX no start - fix

Vestel are rather fond of having a power supply with loads of passive protections, relying on things like the Vbe of transistors being in very specific ranges at specific currents. A single component out of tolerance and it won't power up, and you're damned if you try and find the problem! Luckily the PSUs are cheap enough to get hold of, so I don't usually bother, though I may try next time since Farnell's trade counter is just a 2 mile walk from where I am.

[tom66]

Re: 32" LCD Beko/Grundig Chassis LX no start - fix

You mention that they kill the HOTs due to the torch settings... Well, that's weird because I got hold of a few current transformers (around 120 in a box, thrown out from my work place) and decided to measure the power consumption of the 14" CRT I keep in the shed as a test TV. It's one of the Orion chassis full of Rubycons with the built in VCR. I measured the power consumption and it's damn static. From complete blackness (and CRT can do true black if you turn down the screen voltage a little) to analog snow, it used within a couple percent the same current. I can't understand that. I always thought CRT power consumption was picture content variable.

The only time I could get the SMPS current to vary was either playing a VCR tape (motor current), loud sounds (though barely noticeable, only has a 3W mono speaker), or running the screen voltage far too high (completely white out picture with retrace lines visible.) I even turned screen down until no picture, no variation from normal screen voltage. Focus just made the picture out of focus if it was wrong with no power changed. Tweaked B+ to 150V (nominal 120V) and current went up a little, but again, not much... so I really dunno.

Maybe it's different for bigger tubes. I have no idea.

I played around with measuring the power consumption of the 50" NEC plasma too. At black, it used quite a significant current from the priming discharge (the screen glows quite a bit under black, it's an older generation.) At full red, green or blue it used more current, about twice as much as black, blue used a little more than red or green. Full white only used about 20% on top of having blue on the screen. Again, a mystery to me, though I did notice the screen dim a little compared to having a single white block, and the power supply start humming quite loudly. That's about 550W there, so I think maybe the control logic starts limiting the brightness so the power supply doesn't have to be extremely powerful.

[Th3_uN1Qu3]

Re: 32" LCD Beko/Grundig Chassis LX no start - fix

A couple weeks back i changed the HOT on a 14" Teletech. I have one just like that and mine still works after 5 years (just that it's set properly as i explained above), so the theory of it not having constant consumption falls. There was one bad cap but in a totally unrelated area.

I don't know how this translates to power consumption, never bothered to check, but high brightness/contrast means higher EHT voltage, and bigger voltage spikes that the HOT now has to handle (due to leakage inductance of the flyback). You will notice as you crank up the brightness and contrast that the difference in picture size from dark to bright images increases, and the tube also goes out of focus slightly, which is a clear sign of high stress being put on the flyback, HOT and B+ rail.

[tom66]

Re: 32" LCD Beko/Grundig Chassis LX no start - fix

A couple weeks back i changed the HOT on a 14" Teletech. I have one just like that and mine still works after 5 years (just that it's set properly as i explained above), so the theory of it not having constant consumption falls. There was one bad cap but in a totally unrelated area.

I don't know how this translates to power consumption, never bothered to check, but high brightness/contrast means higher EHT voltage, and bigger voltage spikes that the HOT now has to handle (due to leakage inductance of the flyback). You will notice as you crank up the brightness and contrast that the difference in picture size from dark to bright images increases, and the tube also goes out of focus slightly, which is a clear sign of high stress being put on the flyback, HOT and B+ rail.

Well, in the Orion that didn't happen. My guess would be it always runs the flyback at a fixed high voltage. Do some other sets vary flyback voltage with brightness? I have noticed some CRTs shrink the picture slightly under higher brightness images. The Orion appeared to shrink it slightly, but it was less than a couple of percent variation. So it could be that there is variation (and tbh, I would be surprised if there is not) but it's so small it's not really significant.

There is this silly black pixel project from greenpeace: http://www.youtube.com/watch?v=QE_XAo9IHug Maybe it will turn out it is even more pointless if CRTs do not vary power consumption.

It's a waste of time, and energy. They would do better to reduce waste by encouraging RoHS to be repealed. (How many reflows have you done? I've done at least one PS3, but I don't usually bother with repairing games consoles that much.)

In a related note:-
I've got loads of those transformers, Taehwhatrans TS-10L's. Sold like 60 on eBay over about 3 months but still have another 60 left at home... so if anyone wants one, pay postage and you can have one. (Limit one per customer. Terms and conditions apply. Your home may be repossessed if you do not keep up repayments on your mortgage.) You need a scope to see the ac waveform, but you should be able to get away with a true RMS meter for basic measurements.

[Th3_uN1Qu3]

Re: 32" LCD Beko/Grundig Chassis LX no start - fix

Well, in the Orion that didn't happen. My guess would be it always runs the flyback at a fixed high voltage. Do some other sets vary flyback voltage with brightness? I have noticed some CRTs shrink the picture slightly under higher brightness images.

Higher EHT given that all other things remain constant = smaller picture. EHT is rarely regulated in TV sets due to the complexity of sensing the high voltage - maybe you have one where it is. EHT regulation is common in CRT monitors however.

There is this silly black pixel project from greenpeace: http://www.youtube.com/watch?v=QE_XAo9IHug Maybe it will turn out it is even more pointless if CRTs do not vary power consumption.

I have a better one: www.blackle.com I would have grabbed one of those CTs from you, but i've got a meter that is TrueRMS on AC amps too.

[tom66]

Re: 32" LCD Beko/Grundig Chassis LX no start - fix

Nah it's definitely not regulated (it's just a cheap CRT combi), there would be HV division resistors or similar, but just a HOT and flyback. I've not yet probed the HOT collector because... I'm scared of the 1kV or so that lives there in the form of transients. If I had a nice 10X probe which was rated to a higher voltage I might give it a go.

Blackle would waste more energy on most LCDs too and not save energy on CRTs it would seem. Maybe if you had a plasma monitor as a computer monitor. It would be great, you know, until your eyes burnt out and you got your electric bill back.

If you get the chance I would love to see quantitative measurements on CRT power consumption. *wink, wink* I'll try and do a plasma and/or LCD TV when I next get the chance to fix one.

[lti]

Re: 32" LCD Beko/Grundig Chassis LX no start - fix

This is due to the "torch mode" settings they are set at in store displays to catch buyers' eyes, and most people don't bother adjusting them.

A lot of people like having the screen that bright. I have seen many people using a computer in a perfectly dark room with the brightness and contrast turned up to 100%. If the monitor was off, the room would be dark enough to be used as a darkroom. If you turn the brightness down to 99%, they will complain that the screen is too dark.

[Th3_uN1Qu3]

Re: 32" LCD Beko/Grundig Chassis LX no start - fix

Owner called today and told me it failed again with exactly the same symptom. No idea what's wrong with it this time, but i guess we'll find out tomorrow. I just hope it's not a bad Tcon.

[tom66]

Re: 32" LCD Beko/Grundig Chassis LX no start - fix

I would guess that it is a power supply problem.

[Th3_uN1Qu3]

Re: 32" LCD Beko/Grundig Chassis LX no start - fix

I'm afraid it'll be exactly the same fault and i'll have to trace the *#!*)%@ protection...