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From: f825_633 on 13 Jan 2010 13:06
Paul Hovnanian P.E. wrote:
> My Magnavox 27Y100-100AA TV sets colors started to go all wonky. It also
> lost that little "bzzt" noise it makes on power up. The degaussing coil
> ckt must be T.U.
>
> So I popped it open and discovered:
>
> The degaussing coil is connected through a relay and a "3 terminal PTC
> resistor" to the AC mains. The center lead pad on the circuit board was
> scorched. I soldered a jumper from the lead back to the relay, bypassing
> the burnt spot, and now all appears well.
>
> Question: I can imagine two possible failure modes.
>
> 1) Just a bad solder joint on the board finally went out.
>
> 2) The PTC has some intermittent failure which overheated the connection
> and will do so again at some point.
>
> In your experience, which is more likely (should I waste my time hunting
> for a new PTC resistor unit)?
>
> If its #2, I'd like to get the bad unit out before it burns out the next
> component in this circuit, but the intermittent failure seems unlikely.
> Usually when these kinds of things fail, they're gone. Its working fine
> now.
>

I'd support the general feeling and go with a poor joint
leading to the scorching, PTC's are only a few pounds change
it anyway if you're worried otherwise if it looks intact
then its probably fine. I'd be more concerned about
carbonisation on the PCB if its that bad, a bit of
tropicalised varnish wouldn't do any harm to re-seal and
insulate the PCB.

From: Mike Tomlinson on 15 Jan 2010 11:02
In article <qOGdnZ3SGas2U9HWnZ2dnUVZ_tti4p2d(a)posted.isomediainc>, Paul
Hovnanian P.E. <paul(a)hovnanian.com> writes

>An open circuit failure I can deal with. I'll just lose degauss again and
>have to watch weird colors. If this thing is failing shorted (or low
>resistance), it might take out other part$ of the board.

It won't. It'll blow the mains fuse.

Degaussing thermistors have no connection to other parts of the circuit.

--
Mike Tomlinson
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