Astro city with MS8-29FA - Shorted Degauss

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yosinlpet
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Astro city with MS8-29FA - Shorted Degauss

Post by yosinlpet »

Hi,

When taking the chassis (labeled MS8-29FA) out of an astro, i found a peculiar thing:
IMG_2384.jpg
The degauss pins are shorted and the degauss harness is just hanging freely in the cab.
Is that an issue? Should I just replace the framed blue wire shorting pins 5 and 7 with the manual degauss wires?


Thanks :-)
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grantspain
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Re: Astro city with MS8-29FA - Shorted Degauss

Post by grantspain »

well no because you can see that the chassis has been replaced at some point, the original was a ms9 hence the manual degauss cable- your chassis is configured to auto degauss and is a ms8 so therefore the original connector is not compatible
yosinlpet
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Re: Astro city with MS8-29FA - Shorted Degauss

Post by yosinlpet »

Thanks a lot :-)

I did not know about auto-degauss
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nem
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Re: Astro city with MS8-29FA - Shorted Degauss

Post by nem »

The connector is different, but the pins are the same. So you could take out the pins from the MS9 connector and put them in the MS8 connector.

Unless you're moving the cab constantly there's no need to have it on auto degauss.
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Re: Astro city with MS8-29FA - Shorted Degauss

Post by yosinlpet »

Thanks Nem, that's what I was talking about.

But your opposite answers raise a few questions:
Is auto-degauss harmful for the lifetime of the screen?
If not why should I bother with that detail?

Is there an other adjustment to make somewhere if i do connect the manual degauss?
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nem
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Re: Astro city with MS8-29FA - Shorted Degauss

Post by nem »

It will kill the PTC in the long term. So one day your screen won't properly degauss anymore.

You just need to insert the red wires (on the disconnected connector) in place of the black wires.
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Re: Astro city with MS8-29FA - Shorted Degauss

Post by yosinlpet »

Thanks, yes the fix was pretty straightforward.


What is the PTC responsible for? It might be very damaged already... Do you know where it is on the board and if there is a simple way to test it? This monitor struggles to turn on. Is it linked?
Last edited by yosinlpet on October 30th, 2020, 7:26 pm, edited 1 time in total.
mikejmoffitt
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Re: Astro city with MS8-29FA - Shorted Degauss

Post by mikejmoffitt »

FWIW that harness is what comes stock in the Pony Mk. III, for auto-degauss. What Nem says is correct - there is no timed mechanism to prevent it from burning out the PTC over time.

The PTC / thermistor's job is to increasingly limit current as it heats up. This is what makes a degauss a quick pulse that decays, instead of a constant (and very warm and power hungry) effect. As wired, it's as if someone is holding down the degauss button all the time.

On more sophisticated monitors that feature auto-degauss, this is usually gated by a relay and a timer circuit or microcontroller. That's why old PC CRT monitors tended to have a short delay, deguass, and then a relay click marks the end of the screen wobble.
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nem
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Re: Astro city with MS8-29FA - Shorted Degauss

Post by nem »

yosinlpet wrote:
October 30th, 2020, 7:16 pm
This monitor struggles to turn on. Is it linked?
If you can degauss your screen, the PTC is fine.

As for problems with turning on, capacitors are a likely culprit.
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