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From: Paul Hovnanian P.E. on 9 Jan 2010 20:55
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.

--
Paul Hovnanian mailto:Paul(a)Hovnanian.com
------------------------------------------------------------------
Just an armadillo on the shoulder of the information superhighway.
From: hr(bob) hofmann on 9 Jan 2010 23:42
On Jan 9, 7:55 pm, "Paul Hovnanian P.E." <P...(a)Hovnanian.com> 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.
>
> --
> Paul Hovnanian     mailto:P...(a)Hovnanian.com
> ------------------------------------------------------------------
> Just an armadillo on the shoulder of the information superhighway.

My money is on a bad solder joint.
From: Arfa Daily on 11 Jan 2010 03:29

"hr(bob) hofmann(a)att.net" <hrhofmann(a)att.net> wrote in message
news:56a7fa34-d20b-4bac-88c6-c28698a51fb3(a)r24g2000yqd.googlegroups.com...
On Jan 9, 7:55 pm, "Paul Hovnanian P.E." <P...(a)Hovnanian.com> 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.
>
> --
> Paul Hovnanian mailto:P...(a)Hovnanian.com
> ------------------------------------------------------------------
> Just an armadillo on the shoulder of the information superhighway.

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


From: Paul Hovnanian P.E. on 12 Jan 2010 14:34
Arfa Daily wrote:

>
> "hr(bob) hofmann(a)att.net" <hrhofmann(a)att.net> wrote in message
> news:56a7fa34-d20b-4bac-88c6-c28698a51fb3(a)r24g2000yqd.googlegroups.com...
> On Jan 9, 7:55 pm, "Paul Hovnanian P.E." <P...(a)Hovnanian.com> 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.
>>
>> --
>> Paul Hovnanian mailto:P...(a)Hovnanian.com
>> ------------------------------------------------------------------
>> Just an armadillo on the shoulder of the information superhighway.
>
> 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.

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.

--
Paul Hovnanian paul(a)hovnanian.com
----------------------------------------------------------------------
Have gnu, will travel.
From: Peter Kolbe on 12 Jan 2010 15:48
> 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

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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