Measuring AC voltage values from output of ISO transformer

[Arm123]

I am trying to get a old medium res monitor working...

I first need to know if im getting the correct power from the ISO xformer and i am a bit confused when taking measurements.

The xformer is rated a 3A, 250V and I have the correct 120 VAC coming from the wall to the xformer.

On the output i have 4 leads (2 pairs).  One pair measures 170 VAC on one wire and 60 VAC on the other.  Both pairs measure the same.  This adds up to about 240 VAC on each pair.  Is this right because there is no load ?  And would the power be distributed evenly if there was a load on one pair (120 VAC on each) ?

My understanding of AC voltage is limited and im assuming there should be a neutral / phase wire and an hot wire for each pair.

Where am i messing up ?

[Kevin Mullins]

On the output i have 4 leads (2 pairs).  One pair measures 170 VAC on one wire and 60 VAC on the other.  Both pairs measure the same. 

That statement is rather confusing.....

If you have a "pair" of wires, you should have one lead of your meter on one wire and the other lead of the meter on the second wire.

This isn't in a Midway cabinet by chance is it?

[MonMotha]

It sounds like you have input and output backwards, and the input has multiple taps for setting the input voltage for various regions.  As a general rule, input is on the bottom and output is on the top, but this is far from universal.  Transformers can be made in all sorts of ways.

(Note that "input" and "output" are somewhat nebulous on transformers as most designs can be satisfactorily operated either way; there's just conventions for in/out)

If you have a Midway cabinet, like Kevin mentioned, you will likely have a TON of different input taps, and at least two output windings which are isolated from each other.  Midway intended for one to be used for the monitor and the other for "everything else AC", though they frequently got that wrong.

[Arm123]

If you have a "pair" of wires, you should have one lead of your meter on one wire and the other lead of the meter on the second wire.

That helps me Kevin... I was measuring with one probe lead to the ground (on the xformer housing) and the other touching the wire...  Thats my problem and why im getting strange AC readings.

And yes, its a Midway cabinet and its pissing me off.  Cuz i already tested the monitor using isolated power from my test rig.  It did power up and display an image, so i know it works.   I just have to get this xformer to give the correct power.

It sounds like you have input and output backwards, and the input has multiple taps for setting the input voltage for various regions.  As a general rule, input is on the bottom and output is on the top, but this is far from universal.  Transformers can be made in all sorts of ways.

I dont have the inputs / outputs confused, but your right about the xformer having a ton of taps on the input and output sides.  Its very confusing and i wish i could find a wiring diagram for this thing.  Also, your right about the separate output taps / windings for the 'everything else AC'.  What a pain in the you know what.

However, given the advice for taking my readings...   I think i should take all my readings over again and come back here.

[Kevin Mullins]

Think "purple" , which they are calling VIOLET .... you'll have two pairs, each pair has one solid colored and one with a yellow stripe. (solid + striped) and (solid + striped)

Also note that they have both the monitor power and the light fixture both coming off the same isolation, it is not normally done like that. Typically the monitor is the only thing on the output of the isolation transformer.

[MonMotha]

I dont have the inputs / outputs confused, but your right about the xformer having a ton of taps on the input and output sides.  Its very confusing and i wish i could find a wiring diagram for this thing.  Also, your right about the separate output taps / windings for the 'everything else AC'.  What a pain in the you know what.

Yes, if you have a Midway transformer, then things are weird.  There are the two output windings (the purple/violet ones), and then there's all the input stuff.  Use the tap table Kevin posted, though there's unfortunately no 1:1 (120V in - 120V out) option listed.  About the closest you can come is 125V (115V input tap with 120V in) or 109V (132V input tap with 120V in).

The diagram also doesn't show the two outputs as being isolated windings but rather two connections to the same winding.  I thought I remembered them being isolated from each other, but I could be remembering wrong.

[Kevin Mullins]

Luckily I believe all he is really concerned with is the outputs.
Because yes, the input side of it can get a bit confusing.

The diagram also doesn't show the two outputs as being isolated windings but rather two connections to the same winding.  I thought I remembered them being isolated from each other, but I could be remembering wrong.

That's what is odd about some of the Midways, they are NOT separate. Where you would normally see the light fixture and such on it's own output somewhere, Midway has them tied together.
I just worked on a converted Midway cab the other day and it was the same way, the only isolated winding went to both the light fixture AND the monitor. i wanted to verify it myself before I wired up the monitor. There are four wires, but the are indeed tied together. (one isolated output split two ways)

[Arm123]

SSSWWWEEEEEEEETTT !!!

Thanks so much, this is confusing, but will help me out alot.

NARC shall live again !

[MonMotha]

That's what is odd about some of the Midways, they are NOT separate. Where you would normally see the light fixture and such on it's own output somewhere, Midway has them tied together.
I just worked on a converted Midway cab the other day and it was the same way, the only isolated winding went to both the light fixture AND the monitor. i wanted to verify it myself before I wired up the monitor. There are four wires, but the are indeed tied together. (one isolated output split two ways)

Strange then that they even bother having the two sets of output wires.  The wiring diagram calls for the monitor to be on its own set of wires, and the marquee light and DC supply to be fed from the others.  This would be what you'd expect for two isolated output windings.  However, most cabinets were not wired that way.  They usually put the monitor and marquee light together.  Of course, none of that matters if the outputs are actually just the same winding internally.

The reason this works is that the DC supply is also isolating, and the marquee fixture can just float to whatever it wants to.  This means there's no ground loop from the game to the monitor via DC ground.  The fact that the single output winding is still isolated from the AC mains means that there's generally no shock hazard.  Still not the way I'd do it.

Actually, strictly speaking, the isolation transformer for the monitor isn't necessary at all in a system with an isolating DC supply if you don't mind every single wire in the game being a major shock hazard.    Of course, it's very, very wise to include it precisely so that this isn't the case.  A trivial loose wire could result in your joystick or coin door shocking users (this is one reason why external metal surfaces are grounded on cabinets).

Once you reference the DC outputs to somewhere (e.g. by tying them to earth ground, which I think Midway usually did), you absolutely 100% NEED that isolation transformer, though (otherwise you have a ground loop via the monitor and AC neutral back to the panel).  Note that PCs do always tie their DC ground to earth ground, so this is always the case if you're using a PC.

Don't you just love the concept of "ground"?   

[Kevin Mullins]

Strange then that they even bother having the two sets of output wires.  The wiring diagram calls for the monitor to be on its own set of wires, and the marquee light and DC supply to be fed from the others. 

Dang.... now ya got me wanting to go back and look at that machine again.
I don't recall seeing where the power supply was tied into. But I see what you're saying according to that top diagram.
There was a four pin connector like in the diagram, but I only saw two other connectors up from that. (marquee and monitor) Maybe it split again further up the line into a "Y" or something to the power supply and I just didn't pay any mind to it. (I was installing a monitor that's all I cared about)

Of course, none of that matters if the outputs are actually just the same winding internally.

Yeah, the four violet outputs definitely come off the same winding, I metered it out when I was poking around that other cabinet.

Arm123 - Let us know if you need any further clarification, don't want any sparks to go flyin'.