Tv issue

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.
 
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Cyber-Mav said:
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?
Click to expand...

The article I'd linked showed what their role is they take care to limit the current peak going into the transformer on startup, before the relay is switched in by a timing circuit,
so don't think there is any feedback from other side of the thermistors.

could still be that the cap that on that timing circuit has degraded so the thermistors had to take current for longer(always?) - caps seem to prime suspect on many home electronics power supplies.
one of those phone FLIR cameras would be useful to look at heat.
 
Hmm need to get a hold of a cheapo flir camera for my toolbox.

Also am i missing something here? The caps in this pic below are 47uf 450v. Why are they so long? I can get that size of cap in less than a third of the lenght?

IMG-20231107-110707.jpg
 
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Cyber-Mav said:
Hmm need to get a hold of a cheapo flir camera for my toolbox.

Also am i missing something here? The caps in this pic below are 47uf 450v. Why are they so long? I can get that size of cap in less than a third of the lenght?

IMG-20231107-110707.jpg
Click to expand...
Are the ones you can find shorter, or shorter and wider? My assumption would be that these slim ones allow for an overall lower board profile.
 
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LuckyBenski said:
Are the ones you can find shorter, or shorter and wider? My assumption would be that these slim ones allow for an overall lower board profile.
Click to expand...
the ones i can find are same thickness but around quarter of the length. i would have thought physically larger capacitor is better for handling higher ripple but this is strange that such a low capacitance is in such a large capacitor.
 
Cyber-Mav said:
the ones i can find are same thickness but around quarter of the length. i would have thought physically larger capacitor is better for handling higher ripple but this is strange that such a low capacitance is in such a large capacitor.
Click to expand...
Might come down to quality or as you say, specs. I imagine those are quite critical ones in the TV so would want to be decent ish.
 
I thought the big caps are likely to be a reserve of charge for driving the LED's , as the picture brightness has to change ... and probably, in parallel, have reliability/safety in numbers;
caps likely to control the thermistor/relay the blue tantalum ones ...
voltage rating on caps guess that is a reliability thing to avoid dielectric breakdown - because they are running at less than 30V I thought (if you had a ccfl backlight or plasma maybe a different story)
 
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yes I see what you are saying - that size maybe a result of voltage rating , and cv^2 power that could be stored, at that voltage.
the high frequency switching of the fets, intrinsic in a switch mode power supply, and the stress that introduces may dictate that voltage rating
 
jpaul said:
yes I see what you are saying - that size maybe a result of voltage rating , and cv^2 power that could be stored, at that voltage.
the high frequency switching of the fets, intrinsic in a switch mode power supply, and the stress that introduces may dictate that voltage rating
Click to expand...
Pretty sure those are on the high voltage side of the power board - see the 250V fuse ratings.
 
I must admit I've not read the 5 pages, did the original issue get solved?

I had a similar issue with a TV and it turned out the front panel had shorted, meaning all the buttons got pressed!
 
Cyber-Mav said:
its fixed!!!!

IMG-20231109-110722.jpg

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?
Click to expand...
Hi, do you have an Rs part number please. I’m having the same issue with my same 75” Toshiba power cycling on and off. Thanks
 
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