12" Macintosh Monochrome CRT monitor

[Larry Offley]

no power I get nothing over 3v much past the plug. there was a small white cube that look very melted. no marking on it. seems to have infinite resitance.

[budm]

Re: 12" Macintosh Monochrome CRT monitor

Where is this part located in the monitor?

[stj]

Re: 12" Macintosh Monochrome CRT monitor

that looks like a degausing thermister.
if it got stuck on, it will take out the fuse.

is this an all-in-one or just a monitor? what's the exact model?

[Larry Offley]

Re: 12" Macintosh Monochrome CRT monitor

12" macintosh monochrome monitor



the part is right behind the power plug, the the Hot side of the plug first goes thru a large resistor then to this.

This is an old monitor the only reason I am trying to fix it is it is parts of a display for a museum and can't easily be replaced

the fuse is intact

[Larry Offley]

Re: 12" Macintosh Monochrome CRT monitor

I was able to source another 12" Monochrome Macintosh monitor that works. And got the Installation ready for opening. I would still like to fix the other one as a backup though. If anyone has a schematic for it that would be helpful.

Thanks

[stj]

Re: 12" Macintosh Monochrome CRT monitor

check the circuit boards for any name or model number that could be searched.
it's very unlikely that it was made by apple - probably made for them buy someone like NEC

[R_J]

Re: 12" Macintosh Monochrome CRT monitor

Its most likley a current inrush thermister NTC. Is it in line with the ac circuit, are there any board markings for it (ie ntc1 or th602) it be could be connected between the ac line in and the bridge rectifier ac in.

If you open the white plastic there should be a disc between two metal contacts, you could try measuring the the disc, it should be around 3-5 ohms cold.

[Larry Offley]

Re: 12" Macintosh Monochrome CRT monitor

check the circuit boards for any name or model number that could be searched.
it's very unlikely that it was made by apple - probably made for them buy someone like NEC

numbers on the board
630-0001-a
820-0258-e (that looks like the board numbers they still use)

it says apple computer systems on the board

[Larry Offley]

Re: 12" Macintosh Monochrome CRT monitor

Its most likley a current inrush thermister NTC. Is it in line with the ac circuit, are there any board markings for it (ie ntc1 or th602) it be could be connected between the ac line in and the bridge rectifier ac in.

If you open the white plastic there should be a disc between two metal contacts, you could try measuring the the disc, it should be around 3-5 ohms cold.

The ohm measurement is infinite. I popped the top off the component and there is 2 metal pads whatever was between them looks burnt to a crisp. like a lump of carbon

all of the resistors and caps have marking next to them but nothing next to this.

[R_J]

Re: 12" Macintosh Monochrome CRT monitor

This is a schematic I have of an apple color monitor that shows the th602 thermistor, I suspect the mono monitor has a silmilar ac input circuit.
If you ignore the degaussing circuit. you can see where your thermistor would be located, It that is it you could place a short (piece of wire) in its place just to check the monitor, but don't leave it that way as it would eventually blow the fuse

[budm]

Re: 12" Macintosh Monochrome CRT monitor

The inrush limiter burn up can be due to short circuit, I would check the rectifiers, check the resistance between the +/- legs of the main filter cap at least

[Larry Offley]

Re: 12" Macintosh Monochrome CRT monitor

The inrush limiter burn up can be due to short circuit, I would check the rectifiers, check the resistance between the +/- legs of the main filter cap at least

I will check that. The monitor was running inside a plexiglass box and looks like it was overheating.