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Asteriods Transformer Power brick Voltages

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    Asteriods Transformer Power brick Voltages

    Hi Guys,

    Does any one know how Atari labels the AC voltages in their circuit diagrams? Is it RMS or Vmax?

    I don't have a working Atari power brick to check, so I can't make a start on my transformer build.

    Below is a transformer from an Asteriods. I'm interested in the 36 volts secondary windings with the center tap, 36V-0-36V. Which would be points 6 and 7 on output plug J5.
    Click image for larger version  Name:	asteriods transformer.jpeg Views:	0 Size:	96.7 KB ID:	2152037

    I'm trying to work out why there is +22 volts at C87. I saw a guy on YouTube measure +25V.. "Close enough", they said... but still a bit confusing? The input voltages to the transformer are RMS for sure, but outputs could labeled Vmax I think as 36VAC x .636 = 22.9VDC, the closest calculation I can find?? Anyone? <- edit: I was confused here. Still confused but that's normal for me. lol

    Click image for larger version  Name:	onboard supply.jpeg Views:	0 Size:	59.4 KB ID:	2152036
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    Last edited by taito; 28 July 2020, 10:54 PM.

    #2
    You will notice its actually 36 across the outside terminals making it a 18-0-18 winding and they are using that winding for the psu you pictured, Of course the 36v AC is only going to be approximate, and given that its being rectified, you should have around 22 - 26v dc at that cap. 36 x 0.707 = ~25v. That voltage wont be all that critical as its then being regulated.
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      #3
      Thanks for your comment cwispy, much appreciated, I'm still unconvinced though. I still think 36VAC(Hi) is VMax.
      My Circuit analysis textbook says for Center Tapped and Bridged rectifier circuit use the following formula to calculate the DC from a know Vmax.

      Vdc = 0.636(Vmax - 2Tv) where Tv is the voltage drop for the diode in a full wave circuit.

      (Vmax is equal to Peak to Peak /2). To clarify, here is what I mean....

      Click image for larger version  Name:	20200729_134210.jpg Views:	0 Size:	116.6 KB ID:	2152248

      These are my steps...

      Vdc = 0.636(Vmax - 2Tv)

      = 0.6.36(36 - 1.4) ( assuming diode drops are 0.7)

      = 0.636(34.6)

      = 22.0056

      = 22Volts on the button!

      Which is what Atari states on their schematics. ( I don't know why the guy I saw on YouTube got 25 volts, but I'm not going to go with that anymore.)

      If anyone has an Asteriods game, and could unplug J5 and measure the voltage between the red wires, which is possition (6) and (7) on the plug, I would greatly appreciate it.

      My reasoning is... I would be a total waste of time to wind a transformer wrong, and potentially destroy a game too.

      Cheers all and thanks!
      Last edited by taito; 29 July 2020, 01:54 PM.

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        #4
        Ok, lets start at the beginning.

        Firstly most of these transformers are made for 220v AC primary and the ratings at the windings are peak to peak values usually with a regulation of anywhere from 10 - 20% (usually the higher side in older transformers). As a result, when connected to a higher primary source of 230 - 240v AC, the secondary voltages rise in proportion to the primary rise. So on the 36v rail for example:

        Assuming a 220v primary, we have a ratio of 6.1111 giving us
        At 230v we have an output of 37.63v
        At 240v in you have an output of 39.27v.

        As I understand it now from your last post you are looking to wind a new transformer, but I would have to ask why given that they almost never fail? Anyway given that you are wanting to wind a new transformer, you simply need to follow the voltages on that drawing. Which has these windings (from top to bottom) VA ratings are only approximate based on the fuse ratings.

        11v-0-11v @160VA
        18v-0-18v @200VA
        6.3v @18VA
        80v-65v-0v-65v-80v

        I would not place too much emphasis on the Youtube videos or even the circuits as the unregulated voltages are generally not overly critical to the operation of the circuit. When it is critical, it will be regulated to ensure smooth and consistent power.
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          #5
          Originally posted by cwispy View Post

          Firstly most of these transformers are made for 220v AC primary and the ratings at the windings are peak to peak values
          Ok...that's what I said from the get go as Vp-p/2 is Vmax, First post then second post.. kinda the long route but we got there.

          I love it when a plan comes together!

          Originally posted by cwispy View Post
          As I understand it now from your last post you are looking to wind a new transformer, but I would have to ask why given that they almost never fail?
          Awe, that's easy to answer, don't have one.

          Comment


            #6
            Been kicking this one around a bit more. For the circuit I've been describing, it looks like a 18v-0-18v is possibly the right choice here. But since I can't be sure at this stage, I wouldn't use any of the information here to hook up your own machine. Just saying. Further, it's quite possible that the 25Volts measured in the youtube video was such because the regs very little work. Again.. speculation at best.

            Last edited by taito; 11 August 2020, 09:30 AM.

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              #7
              I was going to respond with another long explanation again, but I would have just been reiterating exactly what I stated before, and now this thread is just annoying me.

              You are not going to get a 3v drop at the main filter cap because there is little to no load at the regulators!

              If you do not trust the schematic, then you can not trust ANY of those listed values, so go and buy a fully working machine and reverse engineer it to get the trusted voltages.
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                #8
                So anyone else who's interested... the question is/was...

                Does any one know how Atari labels the AC voltages in their transformer circuit diagrams? Is it RMS or Vmax?

                I now think it is most possibly RMS in all cases, primary or secondary.

                Anyone else care to comment?

                After much thought the 25volts I mentioned is because there was no real load on the 7815,... etc. The monitor was not deflecting at that stage... which is also on the 18v-0-18v rail. So it was floating at Vmax.

                18rms x 1.414 = 25.452V. Which is what was observed. A currious finding. But makes sence. The 22v on the schematic is probably where it sit when running the monitor too. The pcb xy circutry was not operating at that point. Also as it was earlier in the repair video.


                "Repairing a 1979 Atari Asteroids arcade PCB" on youtube.
                Channel: Joe's Classic Video Games

                If anyone is interested.


                So this is what I mean. One probe is on a cap with no load... flat.. the other an ac signal.
                Was done using 25 volt rms secondary.. a sine wave here. The resulting dc is 40v. {20v/div}

                Click image for larger version

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                Last edited by taito; 11 August 2020, 11:33 PM.

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