PTC Resistor in Degaussing circuit

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.

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Paul Hovnanian     mailto:Paul@Hovnanian.com
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Paul Hovnanian P.E.
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My money is on a bad solder joint.

Reply to
hrhofmann

My money is on a bad solder joint.

A liitle of my money on that also, although it is quite common for the PTCs to fail intermittent internally by one of two mechanisms. First, the thermistor element(s) can crack, and second, they can go intermittent in the spring clips which hold them inside the case. If all is well for a couple of weeks, then probably it was just a bad joint. When they go intermittent internally, sometimes, you can see them arc inside through the case, if the room is dark.

Arfa

Reply to
Arfa Daily

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.

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Paul Hovnanian  paul@hovnanian.com
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Reply to
Paul Hovnanian P.E.

I've seen one fail shorted. But you should realize it long enough in time when it happens to shut the set off before it could damage other components

Reply to
Meat Plow

I Had some monkey once, who decided that when the PTC went faulty, and the screen had all funky colours, that he would manually short out the pins on the PTC (Which totally messed up the colours - Overmagnetised).

When I got the set after that, I replaced the PTC [hence fixing the degaus cct] the colours were still not sorted. Perhaps an external degauzzing may have helped, but did not have one, and the 'monkey' came and collected the set as is (probably to give to his mate to try fix).

P

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Reply to
Peter Kolbe

soldering definitely. check also the condition of the PTC; if the case is cracked or scorched it is probably best to change it.

-B

Reply to
b

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.

Reply to
f825_633

In article , Paul Hovnanian P.E. writes

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

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

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Mike Tomlinson
Reply to
Mike Tomlinson

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