Crispy
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Post by Crispy on Nov 23, 2017 21:05:30 GMT
Frans, I need your help again. I got home last night from work only to be confronted with "The TV is dead - can we have the old Panasonic Plasma back"  The TV is only about 18 months old, a Samsung Smart TV - again the missus said "Its not so smart now is it - can we have the Panasonic back"  Apparantley she was watching it when it just went POP - dead as a DoDo. I thought no problem it will be the fuse in the plug. Yes the fuse in the plug had blown, but there was still no power getting to the TV. Time to look inside to see if anything was obviously wrong like a leaking or bulging cap or burnt out resistor - nothing that I could see. The Capacitors all seem to be Samwha - don't know if they are any good or not? I used my MM and found the internal fuse had also blown, so I had a bright idea to by pass the fuse and see if it would power up - this time a really big BANG and looking inside again I found I had blown up a part NT801S (thermister)? I measured all the diodes and they seem OK? The board before:  The blown up thermister?:  Do you think the thermister was the original problem? Any help is greatly appreciated. Chris |
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Post by solderdude on Nov 23, 2017 21:57:05 GMT
It looks like the thermistor is there to limit the inrush current.
When it has shorted it can't do that any more which may cause a fuse to blow.
Could also be that it blew because something behind it is shorted.
Perhaps measure the bridge rectifier ?
See if it measures a short somewhere
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Crispy
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Post by Crispy on Nov 24, 2017 21:17:53 GMT
It looks like the thermistor is there to limit the inrush current. When it has shorted it can't do that any more which may cause a fuse to blow. Could also be that it blew because something behind it is shorted. Perhaps measure the bridge rectifier ? See if it measures a short somewhere Thanks Frans, I have measured the bridge rectifier if it is the component arrowed red below?  The measurements are below - I think definitely blown.  Is there anything else I should be checking? Thanks Chris |
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Post by solderdude on Nov 24, 2017 22:15:33 GMT
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Crispy
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Post by Crispy on Nov 25, 2017 17:07:42 GMT
Looks O.K. You could simply replace the thermistor and see what happens. Farnell order code: 9754172 The TV might work again. May work for only a short period of time. You suprise me every time Frans, I thought the BR was trash, but what do I know  Farnell code works well on CPC website, they had two left so I orderd both - just in case? CPC are pretty quick with delivery so expecting tues or wed nxt week. Any Idea what may have caused it to stop working in the first place, do you think the thermister got too hot or do you think something else may have gone capputt? Thanks again Chris |
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Post by solderdude on Nov 25, 2017 21:23:21 GMT
What a thermistor does is the following.
At room temperature this particular one has a resistance of around 5 Ohm.
This is effectively in series with the power supply.
That supply has to charge capacitors.
Through the 5 Ohm the current is limited.
When it gets hot the resistance lowers.
It should end up to around 0.15 Ohm or so and then keeps its temperature constant and the resistance high.
These parts simply 'wear' everytime a high current is flowing through it (turning TV on).
At some point in time they can 'short'.
When that happens the inrush current becomes very high and the fuse blows.
The question remains whether or not the wear is caused by underlying problems which may be causing excess current draw heating the Thermistor up or that everything works fine but the part simply broke down.
Lets hope for the latter.
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Post by ronzo56 on Nov 25, 2017 22:41:12 GMT
Somewhat unrelated to the TV Frans. So my apologies for the derail. But say a pair of Quad 2805 speakers had thermistors in the circuit board. And someone(s) when the owner wasn’t home, moved them by disconnecting them with the power amp on and both speakers on, then reconnected with power still on could that cause the fuse to blow in both speakers and a thin steam of smoke to come out of the speakers. And upon inspection the once green thermistor turning yellow with a little brown discoloration around the edges? Of course the speakers don’t function and the amp on the separate sub woofer just makes a 60 Hz hum with no signal. Just wondering. Totally hypothetical as my mind has never accepted the reality even to this day.  |
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Post by solderdude on Nov 26, 2017 9:01:15 GMT
I assume the Quad 2805 is the ESL63.
So the speaker was disconnected from the amp (which hypothetical amp) with the mains still connected to the ESL63 and amp and in on state but no sound playing.
Then the speaker was moved a bit and then the amp output was reconnected, while both still on but no music playing ? or was there there music playing (loudly ?) on the amp and then it was reconnected ?
There are a few revisions of the board inside.
None of them have a thermistor. There could be 1 or 2 VDR's (which may look the same) in the HV generator, depending on revision, but these could not be blown by this.
Perhaps it was capacitor (also are discs) in the HV circuit ?
Possibly R15 was the one that let out magic smoke.
It would do that when there was a high surge while connected (say an amp playing too loud suddenly connected)
That would trigger the spark protection which basically shorts (well loads the amp with 1 Ohm) the amplifiers output.
No problem for Quad amps... they are designed to deal with this.
Most other amps, however, will simply put too much current in (when powerfull enough) and burn out that resistor or destroy the amp and maybe even temporarily make the amp have a high DC on the output before the protection (if it had any) kick in.
That ESL, however, even with a burned out R15 may still have played but with less bass afterwards on another amp.
When it did not work at all any more that suggests the HV circuit may have died but this is separate from the amp connections.
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Post by ronzo56 on Nov 26, 2017 9:54:17 GMT
Yes the 2805 is a version of the ESL63. Sounds like it was a capacitor. The hypothetical amp was a Luminance Audio KST-150. Solid state. It was on but didn’t have music playing. Both speakers were plugged in and on. Yes you got it right. The speaker cables were disconnected with power on everything. Then after moving speakers a few inches, the cables were reconnected. The failure happened when I got home and played some music. Smoke from one speaker and then both speakers fuses blew out. The main amp was fine. Oh and the REL subwoofer only hummed after that. I assume they could be fixed, but the nearest repair shop for Quad is about 5000 Km away. So they sit in a closet. When I saw the picture of the thermister I thought about what happened. Never have had that much money go up in smoke in less than a minute.
The speakers no longer work on any amp. Must be the HV circuits.
Hope the TV has a better fate.
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Post by solderdude on Nov 26, 2017 11:53:44 GMT
Looking at the damage I rather suspect something happened with the mains voltage rather than the actions that happened.
I suspect that replacing the primary transformers might bring these great speakers back to life.
Maybe the NE555 also needs to be replaced when a substantial power surge ruined it but perhaps it is still O.K.
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Post by ronzo56 on Nov 26, 2017 11:59:07 GMT
We may never know. I keep hoping for someone to open a Quad repair shop closer to me. So far no luck. Thanks for answering my post Frans.
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Crispy
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Post by Crispy on Nov 26, 2017 12:38:06 GMT
What a thermistor does is the following. At room temperature this particular one has a resistance of around 5 Ohm. This is effectively in series with the power supply. That supply has to charge capacitors. Through the 5 Ohm the current is limited. When it gets hot the resistance lowers. It should end up to around 0.15 Ohm or so and then keeps its temperature constant and the resistance high. These parts simply 'wear' everytime a high current is flowing through it (turning TV on). At some point in time they can 'short'. When that happens the inrush current becomes very high and the fuse blows. The question remains whether or not the wear is caused by underlying problems which may be causing excess current draw heating the Thermistor up or that everything works fine but the part simply broke down. Lets hope for the latter. Thanks for that excellent explanation Frans, I now have a lot better understanding of what may have happened. Fingers crossed that a 15p fuse and a £1 thermister will do the job? If it does not work I could always buy a new PS board - would be cheaper than a new TV. |
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Crispy
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Madrigal music is playing - Voices can faintly be heard, "Please leave this patient undisturbed."
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Post by Crispy on Nov 26, 2017 12:46:47 GMT
Yes the 2805 is a version of the ESL63. Sounds like it was a capacitor. The hypothetical amp was a Luminance Audio KST-150. Solid state. It was on but didn’t have music playing. Both speakers were plugged in and on. Yes you got it right. The speaker cables were disconnected with power on everything. Then after moving speakers a few inches, the cables were reconnected. The failure happened when I got home and played some music. Smoke from one speaker and then both speakers fuses blew out. The main amp was fine. Oh and the REL subwoofer only hummed after that. I assume they could be fixed, but the nearest repair shop for Quad is about 5000 Km away. So they sit in a closet. When I saw the picture of the thermister I thought about what happened. Never have had that much money go up in smoke in less than a minute. The speakers no longer work on any amp. Must be the HV circuits. Hope the TV has a better fate. You sound like you have my sort of luck Ron  It was not so long ago I mistakenly connected the speaker wires the wrong way around and blew up the bass cone on my speaker and trashed the amp. I am only a hairy arsed driving instructor with very little knowledge of electronics but with excellent guidance from frans I was able to mend them. So what I am saying is the equipment is trash sat in a closet, so why not have a go and try and mend them? Yeh lets hope my TV is a simple repair? although with my luck that will not happen |
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Post by solderdude on Nov 26, 2017 12:48:05 GMT
Could also be that the Thermistor or a circuit behind it went belly up after a short mains power loss.
In that case the Thermistor is still hot and when the power goes off and on in quick succession a high inrush current could destroy the Thermistor as a result.
A TV (as well as many other devices) are tested for this and should survive these tests but I wonder if older units may still pass such a test when the Thermistor has aged already and has been hot for thousands of hours.
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Crispy
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Madrigal music is playing - Voices can faintly be heard, "Please leave this patient undisturbed."
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Post by Crispy on Nov 26, 2017 12:59:59 GMT
Could also be that the Thermistor or a circuit behind it went belly up after a short mains power loss. In that case the Thermistor is still hot and when the power goes off and on in quick succession a high inrush current could destroy the Thermistor as a result. A TV (as well as many other devices) are tested for this and should survive these tests but I wonder if older units may still pass such a test when the Thermistor has aged already and has been hot for thousands of hours. I was thinking that when I solder the new thermister on the board I will try to angle it as upright as I can with only a small amount of hot melt glue to hold in it in place, then hopefully if it works it should get more air around it and keep it cooler? |
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