Strange issue with LCD TV

[nerdyfest]

Hi, I've had to replace a failed cap in a Bush LCD TV and that's all fine now.
The TV won't start though unless I touch my multimeter to the chassis earth or one of the incoming 240V inputs. The other one doesn't start it up.
There is a MOV between the chassis and the live and a MOV between the chassis and the neutral.
Could it be one of these MOVs that's causing the issue? They are blue and have CD221K written on them in very small writing.
I did desolder them both but couldn't get any readings off them.

Thanks for your help.

[wraper]

This is not MOV but 220pf Y1 safety rated capacitor. Solder it back, it won't cause the issue.

[nerdyfest]

Cool,

Any idea about the problem?

[T3sl4co1l]

Sounds more like the above cap is helping you, if anything.  It's just in the wrong place, or doesn't have enough value.

Ugh, poorly designed products with EMC issues...

Presumably, when you touch the chassis or AC cord, you're adding just enough capacitance that it's shunting some RF energy away, reducing the voltage that's disturbing whatever is forcing the thing off and allowing it to start.

It's very roughly possible that some ferrite beads thrown over most cables will fix it (one or a few on the line cord, on the cable(s) connecting to the power supply inside, between other boards, etc.).  Also possible they'll make things worse.  Same goes with those blue (usually) caps -- they're usually placed between a common connection (a ground or minus, but not the safety ground as such) and a nearby shield, or to safety ground (as such).  Examples are, mains to safety GND/shield; DC- to GND; DC- to output common/GND; output common to GND; between GNDs elsewhere in the circuit; etc.

Tim

[nerdyfest]

Cheers Tim,

So if they had different values when tested could that cause an issue? If I replaced them with new ones might that sort the issue possibly?

What's really odd is if I touch the meter's probe to the chassis of the screen it doesn't come on but when I touch the screw that the screen earth lead connects to it does.

And it does have some ferrite beads already in the design and it did work before the cap blew.

[wraper]

might that sort the issue possibly?

Unlikely. Fault definitely is somewhere else. Even if that helps somehow, you still won't fix the source of the issue. Those capacitors are just for filtering some noise. Describe the issue in more detail. What means doesn't start up? Are there power supply voltages present at he moment of it not starting? It doesn't start from the remote or is completely dead? What if you switch it off from the remote, does it power on again after that?
Did you change all electrolytic capacitors in the power supply? Didn't you use some crappy conductive flux when soldering. Post some photos, especially at the place where that miracle screw is located, and your soldering job. It is possible that you just soldered something unreliably but when touching that screw, bending something a little bit therefore some contact intermittently appears again.

[wraper]

Ugh, poorly designed products with EMC issues...

What makes you think so? It doesn't tell anything about design quality. Just that device is faulty after a repair of unknown quality.

[Negative-Bias]

I'd first get a schematic, then I'd check all the voltages on the cold side of the power supply and then the hot side, but be careful the hot side ,can kill you if you put your paws somewhere  you shouldn't ,and a reminder are you using a isolation transformer between the mains and TV,I still also tend to use the dim bulb technique first before investigating anywhere and a isolation transformer as a must.

[T3sl4co1l]

Ugh, poorly designed products with EMC issues...

What makes you think so? It doesn't tell anything about design quality. Just that device is faulty after a repair of unknown quality.

It's not apparent what the present problem is, or how it might've been impacted by its history (failed capacitor and repair? other faults unknown?), but the present symptoms do sound like an EMC problem, which would've been marginal before but is now over a threshold.

It being a marginal issue would be supported by the sensitivity to nearby conductors in various places.

Unfortunately there's very little I can say about it, without seeing the thing and getting some AC probes on it (scope or RF probes, namely).

Tim

[nerdyfest]

This is the psu part.

[T3sl4co1l]

Doesn't look like there's much in there for chassis or grounding... does any of that metal extend up under the PSU?  I see a screw with an eyelet thing, and a clear connection by the mains terminals: are either of these ground?

I'm impressed they have two common mode chokes (green toroids), but very little filtering otherwise (blue caps).  I wonder if the rest of the circuit has to have low ground capacitance or high isolation for some reason ("double insulated" type, A/V inputs..?).

A larger blue cap (2200 or 4700pF) from whatever the common pin is on that ribbon cable, straight over to chassis ground, might help.  But it might also make things worse, especially depending on what's connected externally.

Tim

[wraper]

Replace all capacitors marked red. If this does not help, replace green too. Also you need to use LOW ESR type capacitors (except big capacitor marked green and small capacitor nearby to it). For replacement you have used general purpose capacitor. It will fail within a few months maximum, therefore needs to be replaced with a proper type too.

[wraper]

I wonder if the rest of the circuit has to have low ground capacitance or high isolation for some reason ("double insulated" type, A/V inputs..?).

A larger blue cap (2200 or 4700pF) from whatever the common pin is on that ribbon cable, straight over to chassis ground, might help.  But it might also make things worse, especially depending on what's connected externally.

I wonder why you are suggesting strange and potentially dangerous things? For any person who makes repairs of such things once in a while it is obvious that there is a fault somewhere. Before checking all electrolytic capacitors (if ESR meter available) or just replacing them, it is a waste of time to try find a fault somewhere else.

[T3sl4co1l]

I wonder why you are suggesting strange and potentially dangerous things? For any person who makes repairs of such things once in a while it is obvious that there is a fault somewhere. Before checking all electrolytic capacitors (if ESR meter available) or just replacing them, it is a waste of time to try find a fault somewhere else.

Dangerous?

Like I said, there is apparently a marginal problem that's showing "spooky" sensitivity.  Can you explain that problem as a consequence of poor electrolytic capacitors alone?  I doubt it.  As a knock-on effect, certainly possible.  The observed problem certainly concerns EMC, which can be exacerbated by lots of things, including bad capacitors, but at its heart, can't be fixed purely by changing capacitors.

As a priority, capacitors are definitely first.  I was under the impression that this was already performed, however ("I've had to replace a failed cap in a Bush LCD TV and that's all fine now").

Tim

[wraper]

Dangerous?

You are suggesting playing with mains voltage to the person who don't seem to be experienced.  And IMO without any serious reason.

Like I said, there is apparently a marginal problem that's showing "spooky" sensitivity.  Can you explain that problem as a consequence of poor electrolytic capacitors alone?  I doubt it.  As a knock-on effect, certainly possible.  The observed problem certainly concerns EMC

EMC? When cap is faulty, there will be multiple volts of ripple on the rail -> this will certainly cause some electromagnetic emissions if you are concerned about them. Overall intermittent issues usually are caused by faulty electrolytic capacitors in TV/monitors/routers/basically anything with SMPS/Buck converter inside, with problem becoming worse over time. Also if you managed to switch it on, usually device will work until switched off again.
Again, if the circuit won't not be faulty, it won't be susceptible to such marginal thing as touching the ground.

[T3sl4co1l]

Dangerous?

You are suggesting playing with mains voltage to the person who don't seem to be experienced.  And IMO without any serious reason.

He replaced the cap (unspecified which one though, I think).

EMC? When cap is faulty, there will be multiple volts of ripple on the rail -> this will certainly cause some electromagnetic emissions if you are concerned about them.

He replaced the cap.

Overall intermittent issues usually are caused by faulty electrolytic capacitors in TV/monitors/routers/basically anything with SMPS/Buck converter inside, with problem becoming worse over time. Also if you managed to switch it on, usually device will work until switched off again.
Again, if the circuit won't not be faulty, it won't be susceptible to such marginal thing as touching the ground.

Faulty caps make more ripple, but that ripple is low impedance, and differential to the supply lines.  It doesn't come and go when you touch a completely separate probe to chassis ground.

Are you familiar with common mode voltages and currents?

Tim

[fubar.gr]

That brown capacitor at the bottom of the board doesn't look good. The board area beneath it is darkened, it has experienced quite some overheating. And is that gooey brown stuff some adhesive or did it spit its guts out?

The Y capacitors could be dead too. Film capacitors do lose their capacitance over time. I had a dead psu come back to life by simply replacing the X and Y caps. The old ones measured something like 100 times less capacitance than rated.

[wraper]

He replaced the cap (unspecified which one though, I think).

Yeah, replaced one out of six. As we all know crappy caps always fail only by one at a time  . He replaced brown SamYoung KMG.

[wraper]

The Y capacitors could be dead too. Film capacitors do lose their capacitance over time.

Blue Y caps are ceramic.

[SeanB]

Brown cap is over the LCD backlight area, and there is probably a very hot driver underneath it. I would shotgun and replace all the secondary side ones first, then look further.

[GandalfDerGraue]

As already stated by "wraper" first step would be to check and maybe replace the marked capacitors.

Replace all capacitors marked red. If this does not help, replace green too. Also you need to use LOW ESR type capacitors (except big capacitor marked green and small capacitor nearby to it). For replacement you have used general purpose capacitor. It will fail within a few months maximum, therefore needs to be replaced with a proper type too.

If that doesn't help check the resistors.   Sometimes there are burned "bootstrap" resistors that fail to provide the starting power to the circuit that is needed for the power supply starting to oscillate. In such cases sometimes the small amount of additional ripple/ noise injection produced by touching some ground plates may be enough to get the whole thing oscillating. But never ever change the circuit! No different Capacitors nor any other rating/ type of the devices or you'll risk danger of fire!

And, of course, as always when repairing mains connected circuits be very careful. These "beasts" won't just "bite" a bit, they may kill!

If all that won't help just google for "AIP4802011B-G". This Power supplies seem to be cheapies used in a couple of TV sets and you can buy them for maybe 30$.

[dgc188]

I appreciate this is an old topic but like nerdyfest back in 2015 I am also having exactly the same problem whereby the tv fails to go into standby following a mains failure and touching the heatsink brings it back into life - from then the tv works normally until the next mains failure.

I've tried PM'ing nerdyfest to see if he came up with a result (fix) with the various capacitor replacements but, alas, no response has been forthcoming.

I've tried measuring the capacitors with my LRC DVM and the 82uF appears to check out, as does the small 22uF but not at their working voltages, especially the 82uF. Unfortunately I can't check the 3 x 680uF capacitors other than to say the meter indicates an out of range error so that might in turn indicate they have values in excess of 200uF.

I wonder whether anyone has any idea of, despite my DVM meter readings, whether either of, or all of these capacitors might be faulty and causing the problem. (The 680uF near the darkened area at the bottom of the board in the photo might appear to be for the backlighting and be of no issue to the fault).

Any thoughts guys (or gals)?
Dave

[stj]

nice, as a repair guy my first thought is what the hell burned the board around the backlight smoothing cap.
flip that psu over and take a foto of the underside.
and check all the soldering for microcracks too.

[dgc188]

Thanks stj

I've been over the board with a soldering iron checking for dry/fluxed encrusted joints and there's no micro cracks that I can spot with a strong magnifier. I've check all resistors for value (including the high value 910k SMDs normally associated with failure to start problems). I've not checked the SMD capacitors as there is no value printed on them; so without a circuit......

No idea what has discoloured the backlight smoother - nothing seems to have gone through to the other side and my board is exactly the same in as nerdyfest's photo - just some discolouring. But I don't think this has anything to do with the non starting problem; once the PSU has been kicked into life, the tv is good/no problems.

I'm just a bit reluctant to throw a bunch of caps at it if they're unlikely to fix the problem. So, any advice would be welcome.

Cheers
Dave

[dgc188]

To add comment to the unresolved fault originally posted by nerdyfest back in 2015 and for which I have been struggling with for a few days or so, I can post a resolution.

At last! After throwing 5 capacitors at this PSU (as has been suggested above), regrettably they didn't fix the problem. i.e. it still just sat there without the 'standby' LED on until a prod with a multimeter lead.

The only other part within the set that could possibly cause the problem was the Power Controller chip - CR6842S.

Initially, I had no idea what the chip was, what its function was, or anything else about it. Well done Google and a datasheet soon popped up which provided a host of information - what a complex little chip it is!

And such a small compact device; difficult for my shaky hands and fingers to solder back into circuit, but it got replaced and everything sprung back into normality.

A good resolution here and one I'll not forget in a long time!

Again, the chip CR6842S.

Thanks to all for reading and with a bit of luck, we will all have learned something from my experience it this Bush 22" LCD.

Regards
Dave

[Rasz]

CR6842S

is that the one market U101? thats what under the charred pcb