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Looking for advice powering 100v CRT on US 120v AC

SamBushman

May 4, 2023
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I've recently purchased an old japanese computer via online auctions. While I wait for the computer to ship I've started thinking about what possible repairs I'll need to deal with. The computer itself will be receiving a power supply replacement, modifying it to run off USA home power (120v AC, 60hz).

The monitor is trickier though. It is rated for 100v 50/60hz, which tracks with standard Japanese power standards. My current thought is to take advantage of an AC power passthrough provided by the computer tower. After the tower's power supply replacement, that passthrough should output 120v 60hz. I found this an isolation transformer meant for arcade monitors: https://www.arcadepartsandrepair.co...es/monitor-isolation-transformer-100v-output/

Maybe I could wire that inline with the passthrough, making it output 100v? I'm no power supply expert, so IDK what the risks are or if this is a bad idea. Any advice?

Thanks!
 

Bluejets

Oct 5, 2014
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That should normally be ok as far as your voltage is concerned.
However, that transformer has a 100VA rating (Volt Amp) which means it's maximum output current would be 1 Amp @100V AC.
Question is, what does your monitor draw..??
If more than 1 Amp, then you are out of luck.
You could parrallel two of the transformers together though (for 2 Amp or less ) , so if it comes to that, come back and we'll show you what to do.
 

kellys_eye

Jun 25, 2010
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You need an auto-transformer. Some UPS devices that have line voltage monitoring/control have transformers with suitable taps that can cater for 100V in, 120V out (and 140V in, 120V out) that would do the job. Alterntively you can delve 'in' to the monitor as the 100V input is liekly converted to some fixed DC that would be easier to replicate.
 

SamBushman

May 4, 2023
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You need an auto-transformer. Some UPS devices that have line voltage monitoring/control have transformers with suitable taps that can cater for 100V in, 120V out (and 140V in, 120V out) that would do the job. Alterntively you can delve 'in' to the monitor as the 100V input is liekly converted to some fixed DC that would be easier to replicate.
Thank you all for the advice so far.

@kellys_eye I read a little on auto-transformers via Wikipedia. It sounds like the primary unique attributes of an auto-transformer are a lack of electrical isolation, potentially a smaller size, and possibly multiple taps for outputting different voltages. Could you elaborate a bit on what it is about an auto-transformer that motivates your recommendation?

Regarding bypassing the AC circuitry and wiring a DC power supply directly to the DC portion of the monitor, i have some reservations. Knowing what sorts of high voltages a CRT can operate at, I am uncertain I could drive the monitor off a common DC power supply. I also don't have any schematics, so I'd be left guessing. My current thinking is it may be simpler to supply AC power the monitor would be happy with
 

SamBushman

May 4, 2023
11
Joined
May 4, 2023
Messages
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That should normally be ok as far as your voltage is concerned.
However, that transformer has a 100VA rating (Volt Amp) which means it's maximum output current would be 1 Amp @100V AC.
Question is, what does your monitor draw..??
If more than 1 Amp, then you are out of luck.
You could parrallel two of the transformers together though (for 2 Amp or less ) , so if it comes to that, come back and we'll show you what to do.
I don't have the monitor in hand yet, so I don't know the wattage for certain. Images of similar monitors I've seen online indicate 86 watts, which I *think* would translate to 0.86 amps at 100v? If that's correct, then we may be in business.

Thanks @Bluejets for offering to help if the monitor draws too much. I'll have to take you up on that if we do go over 1 amp ;)
 

kellys_eye

Jun 25, 2010
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It sounds like the primary unique attributes of an auto-transformer are a lack of electrical isolation,
Yes, but it means 'nothing' in the sense of what you're trying to achieve and changes nothing about the original devices spec.

As for powering it from DC - many devices have built-in SMPS circuits to develop the internal voltages which themselves may all be derived from ONE single voltage source i.e. there may be a 12V DC SMPS intenally that is used to create all the internal (low voltage like 5V and high voltage i.e. 100's volts for the CRT etc). Find the 'common' 12V source and feed it from an external 12V source.

Schematics aren't always required so when you ge the equipment post some pics and we'll find you a solution.
 
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