[vcf-midatlantic] Old computer I can learn to repair?

[Herbert Johnson]

Caution: lecture. Reading time long, skip if impatient. a real question 
was asked. Sorry.

How to use an oscilloscope, is a tough question now  in 2024. It's an 
experience few people have today, that might have an interest in how 
some computer-thing works when broken (or not). So why do some people 
make it *hard*? ;)

Traditional responses to "how to use an oscilloscope", is 1) to learn 
the instrument thoroughly, 2) be sure it's accurate 3) use it for 
designated purposes like research, repair. 4) oops, better learn digital 
logic and microprocessor signals too. (whoa, that's so complicated).

That's because, computers and oscilloscopes were once expensive things, 
used by people with detailed knowledge of signals they show and how 
those signals are produced, and why. That's college-level tech, from the 
days of simple computers and simple electronics. That's my era.

But oscilloscopes now, are just a thing like other electronic things. 
Old and cheap, new and cheap, buy and try and figure it out. So, I don't 
think the old-school approach is gonna work. So here's my old-guy 
approach with try-it in mind.

YOu bought a 'scope. Grab anything you have that has a circuit board 
that's battery powered. Figure out where "ground" is, clip the ground 
lead there. Now use your scope probe, put it on parts of the circuit. 
Wadda see on the scope?

HOwever: learn what it means to "ground" things and avoid "AC shocks". 
That's why I suggest battery-powered stuff first.

If you don't see anything on your scope, look through whatever 
oscilloscope manual you can find. The manual for your scope will help 
BUT it's like "here's how all the knobs work" and not too useful for 
"why do that?".  But look for advice and how-tos. It would not hurt to 
find old "how to use your oscilloscope books", but there are PDF's of those.

also, find some info on whatever you are probing. Know what a schematic 
is? You'll find out! HOpe you find some for whatever you are messing with.

Do that awhile, on different things. Read, test, probe. Now you have 
experience. Get more information based on what you KNOW now, and then 
what you want to know.

If you want to repair something in particular (computers)? Grab one, any 
thrift store, your friend's closets and attics, whatever local. Or USB 
hubs or routers. *DOn't mess with CRT monitors or LCD's or TV's yet. YOu 
could hurt yourself seriously. Please avoid for now.* Look inside. Now 
what?

The chips are small, the leads smaller, and nobody tells you (or me) 
what those signals and chips mean. It's a dead end (for most of us) to 
use an oscilloscope on modern computers. Same applies for Arduinos and 
Rasp Pi's, other than looking at signals your program produces, where 
you know (or should) what signals look like and why. This is not your 
fault.

The old 8-bit computers?  The simplest old/new vintage computers are 
little single-boards based on 8-bit processors like those in the C128, 
Apple II, and other 1980 computers. There's lots of sub-$100 kits for 
those, today, if you like building kits. The old processor data sheets, 
describe the signals. Look them up, to get some clues. They use "big" 
chips, big pins, not the little "surface mount" chips on today's 
devices, too small to poke at.

See, you are learning stuff already, just reading this. I hope!

Still, at some point, you'll have to do some homework on how vintage 
computers work at the chip IC level. That's what the oscilloscope tells 
you about. It's not your fault or the oscilloscope's. that's what it's 
supposed to do, the computers do what they are supposed to do. If the 
computer fails and some signals fail - how would you know failed from 
good signals? You gotta do your homework eventually.

YOu'll break things, using your oscilloscope. Everyone does.

Plan B if "all this stuff is too hard"? Well, that "how to use an 
oscilloscope kit" amounts to a AC to DC power supply running a "five 
fifty five" or 555 chip oscillator/timer. That's a very famous chip, 
whole books (not hard to read) about them. Simpler than a processor!

A person, Forrest Mims, is an old guy who builds simple circuits, to do 
*real things*. He's done that for half a century! He's actually 
important for doing that. His circuits, are still simple. Look them up, 
see what you can do with computers or without, using simple circuits. 
Gotta start somewhere with circuits!

YOu used to be able to buy breadboard "build 99 circuits" kits with 
power and a breadboard ready for use - then put chips in it, hook up 
wires to do things. that's a great learning opportunity. Check Ebay for 
those old breadboards, repairing them is fun too.

https://www.retrotechnology.com/restore/breadbd.html

There's several "hackerspace" type companies that still sell learning 
how-to electronic kits, some that use non-microprocessor "analog ICs" or 
"digital IC's". try some of those things. It's all online now of course, 
no local shops.

There's actual hackerspaces, look for those near you if you are social 
and mobile. But the thing is, going with no knowledge is kind of 
intimidating. Some shops have beginner's days. Or go just to watch, not 
bug people with questions. Figure out the social scheme of the shop.

It's all a process. It takes time, attention, desire and interest. It's 
a real thing, not a ten-minute thing, that's how such things go. That's 
the world that an oscilloscope can explore, but you point the probe not 
the oscilloscope, so you gotta know.

I said, this was a real question.

Regards Herb Johnson

-- 
Herbert R. Johnson, New Jersey USA
https://www.retrotechnology.com OR .net
preserve, recover, restore 1970's computing
email: hjohnson AT retrotechnology DOT com
or try later herbjohnson AT comcast DOT net