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Inside the vintage Xerox Alto's display, a tiny lightbulb keeps it working

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Re: Inside the vintage Xerox Alto's display, a tiny lightbulb keeps it working

#21

Post by Theodores »

I like it how someone that really knows their subject can explain things so fluidly. I have a good idea of how CRTs work but in the many diagrams and lessons that I have endured I have not read such a fluid, easy to understand overview of how a CRT works.

Not everyone has this gift of being able to explain stuff in layman's terms even if they are expert on the subject. Carl Sagan was best at it, on the other extreme you have those that use jargon beyond the audience's knowledge, pretending to know but not understanding it really. This whole Alto series has had some great writing in it.

Re: Inside the vintage Xerox Alto's display, a tiny lightbulb keeps it working

#22

Post by theoh »

ben1040 wrote: post #18

Reminds me of an electric organ a friend had. It was totally analog and dated back to the 60s or 70s. The volume control knob abruptly stopped working; the organ was stuck at maximum volume. He opens it up to discover the volume control simply controls the brightness of a light bulb, which had burned out. The light bulb shone on a photo resistor, which governed the output volume.

Faders in DJ equipment are often optical -- apparently a potentiometer would be noisy.

http://djtechtools.com/2011/11/01/fader-technology-a-primer/

Re: Inside the vintage Xerox Alto's display, a tiny lightbulb keeps it working

#24

Post by jsmthrowaway »

theoh wrote: post #22
ben1040 wrote: post #18

Reminds me of an electric organ a friend had. It was totally analog and dated back to the 60s or 70s. The volume control knob abruptly stopped working; the organ was stuck at maximum volume. He opens it up to discover the volume control simply controls the brightness of a light bulb, which had burned out. The light bulb shone on a photo resistor, which governed the output volume.

Faders in DJ equipment are often optical -- apparently a potentiometer would be noisy. http://djtechtools.com/2011/11/01/fader-technology-a-primer/

That would make DJ gear the exception, then. There are slide pots for that purpose which do quite well with audio across them, they’re just in bigger mixers for sound reinforcement and radio gear. Digital obviously negated needing them to work the same way — it actually rearchitected how an entire mixer works — but analog pots got very good toward the end of their “run”. Lots of quality radio gear out there with beautiful 10Ks, much still in use. I wouldn’t call it “old” like that article does. Then again, DJs also beat the shit out of faders when they work, so it might be a good idea from a reliability perspective.

Coming from the reinforcement side, a lot of DJ gear baffles me. They look at audio much differently, which I suppose is understandable when you’re mixing two decks with an occasional bandpass versus a 50+ channel metal band with subgroups and racks of outboard effects.

Re: Inside the vintage Xerox Alto's display, a tiny lightbulb keeps it working

#25

Post by Johnythree »

WalterBright wrote: post #19
dfox wrote: post #6

Using various interface-ish components only for their electrical parameters is neat trick that is very common in certain applications. In this case the lightbulb is slightly weird but probably serves to shape the 600V pulses in some way and also to protect the switching transistor should the grid outputs get shorted to ground. Looking on the schematic there are also neon lamps across the outputs to protect the CRT fr…

My old Dodge instruments need 5 volts, so there's a mechanical device to convert 12V to 5V. It involves heating a bimetalic strip that opens and closes a circuit quickly, i.e. buzzing, which produces 5V. Some people have decided to fix this by using an electronic voltage circuit to get a clean 5V from the noisy 12V supply. But the instruments just wouldn't work right. It seems the instruments relied on the 5V supply…

Actually the reason is that the instrument itself also uses a bi-metal strip and it relies on the ratio between the two strips to cancel out voltage fluctuations.

It's very elegant, but if you take out one, it goes haywire.

Re: Inside the vintage Xerox Alto's display, a tiny lightbulb keeps it working

#26

Post by yardie »

Our boat has a Hitachi alternator on the engine. From reading the schematics one of the leads connects to a warning light. This warning light is to let you know the alternator isn't charging.

That warning light also has a second, and even more important, function. It induces a load on the alternator to energize the charging coils. A few hours spent removing what I thought was a defective alternator and $100 having it rebuilt. Only to find out it was a $1 bulb.

In this digital world analog circuits are like witchcraft to me. How you think things work is not at all how it works.

Re: Inside the vintage Xerox Alto's display, a tiny lightbulb keeps it working

#27

Post by Johnythree »

The classic example was the light globe used in a polaroid camera.

The globe was powered from a mercury cell, and because it's voltage was so constant, the light stayed at constant brightness.

Then a simple plastic lens overlayed alternate stripes from the image and the globe. The user adjusted the aperture until the stripes vanished, which guaranteed that the exposure was correct.

Re: Inside the vintage Xerox Alto's display, a tiny lightbulb keeps it working

#28

Post by SwellJoe »

cjsuk wrote: post #20
SwellJoe wrote: post #17

I never cease to be amazed at the sheer complexity and precision required to make analog electronics work. There are so many parts to a CRT, and about half of them seem like magic (directing electron beams with electromagnets!), and the first was built in 1911. LCDs seem downright simplistic in contrast. It's kinda the same as the move from internal combustion engines to electric. The discrete parts count drops to a…

It's all pretty simple when you know how. It's getting there that's the hard bit. It's arguably more challenging than software but once you've learned the abstractions and techniques then the knowledge is useful forever so you don't have to wipe the slate clean again when the emperor's new clothes are delivered. The pinnacle of analogue electronics and precision is the line of Tektronix analogue oscilloscopes. They a…

I'm hoping my dad kept his old scope. I haven't gone through his tools (he passed away a few years ago, and I didn't have the heart at the time, since fixing stuff was how I'd always bonded with my dad and every tool has associated memories), but plan to visit my mom in a few weeks. I'll see if it's still around. He owned a TV shop through the 70s and early 80s. His equipment was usually top notch stuff from the peak of the analog electronics era, but he got rid of some of it over the years, especially during moves.

Someday, I'll have more time to play with electronics. It's something I enjoy, and I have most of the tools for it (except a scope), but I always seem to have too many other things requiring my time and attention.

I worked with high-end 90s-era Tektronix scopes when I worked in television. They definitely weren't pure analog by that time, as they had memory and a variety of modes providing both digital and analog data, but they were still pretty cool.

Re: Inside the vintage Xerox Alto's display, a tiny lightbulb keeps it working

#29

Post by kazinator »

yardie wrote: post #26

Our boat has a Hitachi alternator on the engine. From reading the schematics one of the leads connects to a warning light. This warning light is to let you know the alternator isn't charging. That warning light also has a second, and even more important, function. It induces a load on the alternator to energize the charging coils. A few hours spent removing what I thought was a defective alternator and $100 having it…

The bulb limits the current draw from the battery. When the alternator is starting up, the field coil current comes from the battery. Simply connecting the battery across that coil with no additional resistance would draw to much current from the battery.

Alternators have a circuit such that when the battery current is no longer needed (the alternator has spun up and is putting out more voltage than the battery) an arrangement of diodes cuts off the battery field coil circuit and then the light goes out.

When the alternator is self-energizing, there is a feedback mechanism then at play which controls the field current, keeping the alternator voltage level: without it, the voltage would rise uncontrollably: more voltage from alternator -> more current through field coil -> greater induction of voltage. Before the alternator is self-energizing, what regulates the field current is that bulb.

So that is to say, if you see the light on all the time when the alternator is spinning, that warns you that it's not actually working. It is not it's not putting out enough voltage (perhaps none at all) to overpower the battery circuit and become self-energizing. This could be because the rotor coils are burned out, or other problem: wiring or whatever.

Since that bulb is off most of the time, we don't expect to have to change it very often.

If an alternator bulb has burned out, and the unit is not very old, that could indicate a problem.

Re: Inside the vintage Xerox Alto's display, a tiny lightbulb keeps it working

#30

Post by analog31 »

cjsuk wrote: post #15

This is a very common little device. The V-I characteristics are non-ohmic i.e. non-linear which make it very useful for feedback. Some notable uses I've seen it for floating around: 1. One of my DMM's, a Fluke 8600A, has a light bulb in it as part of the charge circuit: https://i.imgur.com/TMvRmKF.jpg 2. My Tektronix DM44 strapped to the top of one of my scopes has one as part of the ohmmeter input circuit. 3. Also…

The famous HP 200A oscillator, which was their first product, used a light bulb in the feedback loop of a Wien bridge oscillator to stabilize its amplitude. Not only was it non-ohmic, but it had a nice dynamic response: Fast enough to keep up with minor amplitude drift, but not so fast as to cause harmonic distortion.

https://en.wikipedia.org/wiki/HP200A

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