Tv issue

[Cyber-Mav]

odd thing is when the thermistors blew why didnt the fuse blow? would this indicate thermistors blew due to over voltage or over current?
also looking form the back of the board it looks like those 2 thermistors are working in series.

 

[Cyber-Mav]

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any idea what type of thermistors i need? rough measurements with multimeter show around 8.3mm diameter of whats left and around 3mm thick.

 

[Cyber-Mav]

the SCK102 thermistor comes up with part number SCK08102MSY on here: https://www.lcsc.com/product-detail/NTC-Thermistors_TKS-SCK08102MSY_C211173.html description says its 10Ω 3A 2.4W ±20% Plugin,P=5mm NTC Thermistor.
wondering if i can get an alternative local like at rs components?

 

[Cyber-Mav]

relay could be unreliable/sooted-up (so thermistor got burned-up taking all the load) .. can you get the relay cover off to look , or put 12v across it to see,
or even listen closely when it's 'live' if board is exposed.

(I have previously cleaned blackened mains relay contacts with wet&dry paper on coffee machine 12v relay - that's 1.3kw though)

will do, will probably desolder it from board to check

 

[Cyber-Mav]

removed the relay and checked inside, it looks perfect. however removing was not a waste of time check the pics:

Spoiler: pics

got the part number of the old thermistor, looks like it is same as the other one on the board - SCK 102

also tested the 2 big bridge rectifiers part number D25XB80 and they both tested fine with multimeter, no shorts.

 

[Cyber-Mav]

At this point i guess i can start by replacing those thermistors and take things from there. Need to find. Some equivalent ones from rs components etc. Anyone know which ones to get?

 

[Cyber-Mav]

Is this it…

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no the one, bit of dodgey part numbering there too,
specs for it are here:

rs components does have 10ohm 2amp thermistors this one here seems to be closest match: https://uk.rs-online.com/web/p/thermistor-ics/1857867
but thermal time constant is 8 seconds less and its wattage rating is lower at 1.4w where as the SCK-102 is rated at 2.3w and capacitance is also different.
dunno if that means its no good.

 

[Cyber-Mav]

what is the resistance of the others .. can you derive spec.

isn't sck 102 fully documented has 20-25 ohm zero load options https://www.es.co.th/Schemetic/PDF/SCK.PDF

can get them on ebay ? cpc&rs threshold for postage a bit high.

rs components is close to me so i can drive down and get but they dont have the exact one. do you know if they got an alternative that will work? can buy like 20 of these from ali for 3 quid but will take a month to come. cant be without tv for that long.

 

[Cyber-Mav]

because you could measure it

lol

 

[Cyber-Mav]

ok twas a compliment that you may have used dimensions of undamaged thermistors to confirm most likely candidate, in addition to close resistance match,

current of just 2x2A is perhpas smaller that what relay suggests or power consumption of tv.

Thats what i thought, the fuse on the board is rated at 6.3a and didnt blow so not sure why those thermistors blew. Maybe 5 years of use was the limit for them?
Also plenty of space to use physical larger ones if need be. Just trying to find out if all i need to do is match the amp rating and ohms rating at 25c for it to be enough?

 

[Cyber-Mav]

Could you desolder a good one and take it to RS to compare/ask for some advice?

iv ordered the tdk ones from rs components. will collect them tomorrow. everything iv read up on says when replacing thermistors you just got to get the ohms, max current and thermal response time and physical size close enough. there is usually +or - 20% or so in their ratings. the cheapo china ones that it came with probably dont even meet the specs listed on the datasheets.

 

[Cyber-Mav]

@dlockers im bit surprised too, guess sparkys doing other stuff these days. most sparkys i speak to just say to swap the board for a new one. they all like halfords and quikfit these days, only good for swapping parts.

 

[Cyber-Mav]

Sounds good. Here’s hoping it all works for you and it’s just the case that the thermistors have failed, rather than an underlying issue. Would certainly be an incredibly cheap repair if so!

will find out tomorrow. parts come in a multi pack so 5 of them for £2.90ish.... if i get original ones from ali 3 quid gets me 20

 

[Cyber-Mav]

its fixed!!!!

Spoiler: pics

put 2 new thermistors in and now the tv turns off and stays off!!!! powers on and stays on too so im in full control now guys!!!! muhahahahaha

now the question is, the other thermistor thats on the board its working fine while the tv is still on the floor and opened up should i change that thermistor and put new one in too or leave it as it is?

 

[Cyber-Mav]

great news - let sleeping thermistors lie

almost strange you couldn't smell the heating/dilapidation of the predecessors ,
is the relay turn on noise, now imperceptible

yes the noise is a lot quieter.
tv is still on the floor and opened up while i was testing it since i need to wait for wife and kiddo to come back in evening for help putting tv back on the wall so i could just take the board out and swap the old thermistor for a new one? dunno its its my luck and the other old one goes next week lol.

 

[Cyber-Mav]

Swapped the last thermistor out too. Tested tv and all is working fine. Just need to get it back on the wall now.

Spoiler: Pic

 

[Cyber-Mav]

Its back on the wall and its happy days now. Got a lot of stuff to catch up on now. Watch some gamersnexus and jay2c later on

 

[Cyber-Mav]

Haha wife be happy cuz she be made.to pay for the fix. Haha

 

[Cyber-Mav]

I once had a friend buy a TV from me and it died. It was covid times so I helped him get the warranty voucher and upgrade to a newer model... He sent me a pic shortly saying "Is there a protective film on the TV? Wife tried to peel it"

And she had peeled about 3 inches off a corner... Of the polariser layer

He very much planned to make her pay if it wasn't repairable

dang thats bad, is that the same thing as those foldable phones where people peeled off something that was not supposed to be pealed? think it was on the samsung fold.

 

[Cyber-Mav]

Was looking at photos of the backside of the tv power board and think iv worked out what the thermistors do. They seem to feed current to the larger high voltage capacitor bank on the side of the board. Once a certain voltage is reached the relay next to the thermistors activates and shunts the thermistors bypassing them from the circuit and giving full power to the caps?. Some sort of soft start system that might be used to lower the strain on the bridge rectifiers?
Trying to work out why it would have failed.
The old working thermistor had 38% higher resistance at rest than the brand new ones.
So i wonder if the older thermistors have degraded with age and gave higher resistance therefore taking longer to charge the caps to the level where the relay takes over to bypass them. If thermistors are working at full tilt for too long they probably got mega hot and popped. I do believe they didnt pop at the same time since loud pops were heard on 2 occasions.
What i don't get though is why this sort of situation would prevent the tv from turning off. The situation makes sense for a non start or overload situation causing main fuse to pop, its strange.