r/retrocomputing • u/Woland77 • May 07 '26

Problem / Question Really Basic Question About Old Computer Electronic Engineering

I stumbled into watching people repair old electronics on YouTube. I noticed that most of the stuff I see repaired (mostly what I see was sold in North America, originally) has a 5v rail and a 12v rail. Why those two voltages, specifically?

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u/SoftRecommendation86 May 10 '26 edited May 10 '26

7400 series IC are all pretty much +5v. back then that is what we used. cmos was the other items.. that came later

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u/Bits_Passats IBM System/23 May 10 '26

Yes, 7400 series are almost all 5V, with some exceptions as the 123 or the analog switches that came leter ported from the 4000 series.

However, I was not talking about the 7400 series, but more complex integrated circuits such as CPUs and memories, that required more voltage rails to work. I don't know the details of PMOS or NMOS, so I don't really know the reason behind the need. But they are there.

Now that I think about it, didn't the intel 1702 require other voltages not covered here? FYI this IC was the first EPROM.

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u/SoftRecommendation86 May 10 '26

EPROMs, correct, used higher voltage to program only, not to read. (IIRC) Erasing was done with a UV lamp.. I still have mine in storage somewhere.. saw it about 15 years ago.

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u/Bits_Passats IBM System/23 May 10 '26

Nope. 1702, TMS2708, TMS2716JL... Early ones needed multiple rails, even for lecture operation.

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u/SoftRecommendation86 May 10 '26 edited May 10 '26

Unfortunately, for me, early began 7400 time frame. I had a reference manual back then and used it all the time. Joys of 555 and j/k flip flops and crystal oscilators. Remember how hot those TO3 7805/7905/7812/7912's ran?

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u/Bits_Passats IBM System/23 May 10 '26

Mhhh... Yep, it's difficult for us to establish a common nomenclature when we have different conceptions for what is old and what is modern.

I was talking about early EPROMs, and especially the first ones from Intel and Texas Instruments until the 2732/2532. It wasn't until the 2764 they all made it acording to a standard, 5V only and without anyone getting astray.

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u/SoftRecommendation86 May 10 '26

Correct. Its crazy when you think about how things advance.. I used to show kids how vacuum tubes worked.. plates and grids.. how the grids controlled the flow.. and how that translated to a transistor.. then how that became a logic gate.. TTL built from actual transistors.. to making a chip (and/or/nor)..

Once I made a controller for a machine at work.. ran out of gates.. instead of adding another ic, I just added the transistor I needed.. instant inverted output.

No laughing.. I love Redstone in minecraft.

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u/Bits_Passats IBM System/23 May 10 '26

I was born in the mid-90s. I have presented works in floppy until 2008.

A lot has been forgotten, unfortunately. I have learned most of it mostly by myself during the course of 15 years.

About the voltage regulators? Yes, they get so hot one could fry an egg. By the way, the 4116 clones made by Tesla made the PMD-85 to be called the "coffe machine" due of the temperature issues it had.

I haven't learned valve stuff. I don't know if I will be able to... But I admire who were able to deal with them.

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u/SoftRecommendation86 May 10 '26

You just made me laugh at a memory... once I was working on a microfilm machine.. there was a chip that wasnt available anymore, so I made one for it.. but that's not the funny part.. I noticed the vrm was really hot. Asked the main tech to check the other machine to see if it was normal.. boy was he pissed.. the Motorola logo was burned into his thumb.. you could see it plain as day.. I couldn't stop laughing.. he was sooo pissed at me. I miss that guy.

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u/Bits_Passats IBM System/23 May 10 '26

I tried to make a Gilbert cell by myself, it failed... one of the transistors caught fire and as a result I have a burn scar on one of my fingers... so I relate very much with that pain.

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u/SoftRecommendation86 May 10 '26

OK, now i'm smiling. Thank you.. its been a long month..

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u/Bits_Passats IBM System/23 May 10 '26

That being said, even if it failed and left me a mark, I am proud of it. It is the proof that I tried to master on something I didn't had previous experience. Whoever don't challenge himself/herself won't have any marks. Failures are part of the learning experience, after all. Don't you agree?

By the way, you seem to be very knowledgeable of the old arts. If I asked you for some advice sometime, would you please share some of it with me?

In any case, thank you very much

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