Forum Rules and Etiquette

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This forum is part of our mission to promote the preservation of vintage computers through education and outreach. (In real life we also run events and have a museum.) We encourage you to join us, participate, share your knowledge, and enjoy.

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Rule 1: Remain civil and respectful

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To put things in engineering terms, we value a high signal to noise ratio. Coming here should not be a waste of time.
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Rule 4: "PM Sent!" messages (or, how to use the Private Message system)

This forum has a private message feature that we want people to use for messages that are not of general interest to other members.

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Why do we have this policy? Sending a "PM Sent!" type message basically wastes everybody else's time by making them having to scroll past a post in a thread that looks to be updated, when the update is not meaningful. And the person you are sending the PM to will be notified by the forum software that they have a message waiting for them. Look up at the top near the right edge where it says 'Notifications' ... if you have a PM waiting, it will tell you there.

Rule 5: Copyright and other legal issues

We are here to discuss vintage computing, so discussing software, books, and other intellectual property that is on-topic is fine. We don't want people using these forums to discuss or enable copyright violations or other things that are against the law; whether you agree with the law or not is irrelevant. Do not use our resources for something that is legally or morally questionable.

Our discussions here generally fall under "fair use." Telling people how to pirate a software title is an example of something that is not allowable here.

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New user moderation

New users are directly moderated so that we can weed spammers out early. This means that for your first 10 posts you will have some delay before they are seen. We understand this can be disruptive to the flow of conversation and we try to keep up with our new user moderation duties to avoid undue inconvenience. Please do not make duplicate posts, extra posts to bump your post count, or ask the moderators to expedite this process; 10 moderated posts will go by quickly.

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Why AT PSUs have 4 wires connecting to the on-off switch

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    Why AT PSUs have 4 wires connecting to the on-off switch

    I need to replace the regular AT PSU switch with a round 16mm latching switch, preferably illuminated (hole already there). From what I have seen those are single pole single throw. Why does the AT PSU has 4 terminals going to the switch? Will I have to concoct something with a relay?

    Well the other solution is to use an ATX PSU. Since the cables that convert one to the other take a SPST switch, but I would prefer to use an AT PSU. Thoughts?


    AT power switches are DPST because they switch both mains line and neutral


      Phase, your location is unknown, so we can't draw any conclusions about what your power distribution is. However, if you're in North America and have 120VAC power, then you need only switch the "hot" side of the line. Electrical codes in other countries (240V ones) may insist that both line connections be switched.
      Reach me: vcfblackhole _at_ protonmail dot com.


        Awesome. I’m int the US. So, if I understood you correctly, I need only to switch the “line” side of the wire. Correct?


          That's correct. There's no connect (other than via the line filter caps) between the neutral side and power/chassis ground in these switching PSUs, so they're effectively isolated.
          Reach me: vcfblackhole _at_ protonmail dot com.


            Perfect. Thank you very much!!


              I'll add that on my 240VAC shop machinery, both sides of the line are switched, since both sides are "hot" with respect to neutral/ground in North American power distribution. On 3 phase equipment, all three phase lines are switched, since all are "hot" with respect to neutral.
              Reach me: vcfblackhole _at_ protonmail dot com.


                Off topic perhaps but I owned a Thomson DVR at one point that was fused (internally) on the neutral. I thought that was a tremendously bad idea!


                  Originally posted by pearce_jj View Post
                  Off topic perhaps but I owned a Thomson DVR at one point that was fused (internally) on the neutral. I thought that was a tremendously bad idea!
                  Why? The fuse is triggered by overcurrent from a short. It doesn't matter where it sits. A fuse it not there to protect people from getting a shock!

                  In Germany and many other European countries, you can't even tell what neutral and what live is from the equipment side, as mains plugs are not keyed and also go in when rotated 180.
                  Last edited by Timo W.; September 27, 2021, 11:54 PM.


                    See, for example, the Schuko (CEE 7) receptacle and plug family. That one always baffled me--usually German-origin stuff is pretty decent with respect to safety.

                    Hence, my answer regarding non-North American systems. Although it's possible that a receptacle will be wired incorrectly (a code violation), it's less likely nowadays. As usual, a short story...

                    When we moved into our current home more than 30 years ago, I couldn't understand why a Toshiba microwave oven refused to power on in the kitchen. Plugged into other outlets, it was fine, but not in its intended spot. A quick check with a lamp indicated that power was indeed present at the subject outlet, but still the oven refused to operate. Eventually, I got out the multimeter and checked the outlet--backwards. Apparently the oven was smart enough to figure that one out (and no, the ground line wasn't being used as a power return). Checking around the house I found about 8 outlets that were similarly wired.

                    Back then, there was still a fair proportion of "hot chassis" equipment around. That could lead to unpleasant surprises down the line...

                    But Timo's response is why DPST power switches are common on most electronic gear today.
                    Last edited by Chuck(G); September 28, 2021, 07:45 AM.
                    Reach me: vcfblackhole _at_ protonmail dot com.


                      Originally posted by Chuck(G) View Post
                      See, for example, the Schuko (CEE 7) receptacle and plug family. That one always baffled me--usually German-origin stuff is pretty decent with respect to safety.
                      I believe it was because originally the German supply was centre tapped so it really didn't matter which way round you plugged it in

                      It must be said, the UK 13A plug is the most dangerous of all.

                      Not when in use, but when left on the floor. The pins inevitably point upwards and really really hurt when stood on with bare feet.
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                        Originally posted by Gary C View Post
                        Not when in use, but when left on the floor. The pins inevitably point upwards and really really hurt when stood on with bare feet.
                        Hah! I can believe that!
                        Reach me: vcfblackhole _at_ protonmail dot com.