Grounding - how much of it, and where? (Mimeo, would also apply to Newton, Obtronix, and real Apple-1)

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Joined: Aug 3 2011
Posts: 33

So, I'm planning out a Mimeo build, and need to decide how much grounding to do, and where to connect everything up.

It looks like most people just use a 2-prong AC connector and mount everything on wood or plexi, so there's just a signal ground, no chassis ground to speak of, and no earth ground.

However, I'm thinking that's not the best bet, and proper grounding would be a good idea.

There's a few possibilities here, which could be combined as needed:

Connect motherboard power pin 6 directly to earth ground
Ground transformer cores
Mount everything on a conductive surface (with standoffs, obviously) - could set up ground loops, though, which would be Bad News(tm)

Thoughts?

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Joined: Oct 9 2011
Posts: 1045
Re: Grounding - how much of it, and where? (Mimeo, would ...

You should join the retro-restore yahoo group. I put up a doc on what to do.

Disclaimer. You will be working with high voltage electricity and vintage components. I take no responsibility for the accuracy of the instructions or for any loss due to the use or misuse of the document.

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Joined: Aug 3 2011
Posts: 33
Re: Grounding - how much of it, and where? (Mimeo, would ...

Just submitted a membership request there, thanks for the tip.

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Posts: 33
Re: Grounding - how much of it, and where? (Mimeo, would ...

For what it's worth, I've looked through the messages, files, and photos, and didn't see that document of yours - the only thing in the Apple-1 folder was Mike Willegal's 6502/Apple-1 cheat sheet.

What's the title, so I can better search for it?

Thanks,
Eric

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Joined: Sep 5 2009
Posts: 80
Re: Grounding - how much of it, and where? (Mimeo, would ...

I use a grounded receptacle and connect a 100K to 1Meg resistor between earth ground and the Apple 1 circuit ground. Since the Apple 1 board is isolated this prevents voltage buildup on the circuit board but prevents serious ground loops. I also earth ground the transformer shells so that a short cannot make them hot since they are usually exposed and could be a shock hazard. Cover all exposed 110V wiring to prevent shock and potential accidental shorts, this may seem obvious but some of the setups I have seen are accidents waiting to happen.

For power connection I use a power entry module Schurter DD11.0111.1110 with companion fuse holder 4301.1409. This provides a socket to use a standard PC power cable and a place for the fuse and spare fuse. These are in stock from Newark and Mouser.

wsander

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Joined: Oct 9 2011
Posts: 1045
Re: Grounding - how much of it, and where? (Mimeo, would ...

bhtooefr wrote:

For what it's worth, I've looked through the messages, files, and photos, and didn't see that document of yours - the only thing in the Apple-1 folder was Mike Willegal's 6502/Apple-1 cheat sheet.

What's the title, so I can better search for it?

Thanks,
Eric

My fault, I thought I uploaded it... It's there now... It includes the bleeder resistor that Wendell talks about (I got the idea from him originally).

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Joined: Jun 5 2008
Posts: 380
Re: Grounding - how much of it, and where? (Mimeo, would ...

One other comment. The operations manual shows the fuse on one side of AC input and the switch on the other. I recommend putting both the fuse and the switch on the "hot" side of the circuit. This will prevent a blown fuse from making the system appear off, when in fact the switch is on and the transformers are powered.

regards,
Mike W.

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Joined: Aug 3 2011
Posts: 33
Re: Grounding - how much of it, and where? (Mimeo, would ...

Yeah, I noticed that in the assembly guide. I'm guessing that Apple did it that way back then because of unpolarized plugs, and not knowing which would be hot or neutral?

I think I've got a pretty good idea of how I what to do the electrical system now. There'll be some slight personal touches I do that differ as far as component selection, but electrically, it'll basically be Wendell's design I think.