Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

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

righto.com

Post reply on HN
Print view View original

31–40 of 54 posts

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

#31

Post by dharma1 »

jsmthrowaway wrote: post #24
theoh wrote: post #22

Earlier quoted context omitted.

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 beau…

Same reason some guitar wah pedals use optical control. There are ones with mechanical pots but you have to keep replacing the pots every couple of years when they become "scratchy" as you move the pot back and forth quite a lot in use

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

#32

Post by icantdrive55 »

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…

Those older Tektronics are works of art. The boards were meant to be repaired. I still remember the feeling I got when I took the case off mine. Yes, complicated, but so much thought was put into the layout.

I guess those days are over?

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

#33

Post by zkms »

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.

Yeah, I have vague recollections of taking apart an analog electric organ (like https://en.wikipedia.org/wiki/Hammond_organ) and seeing an "optocoupler" with an incandescent bulb and an LDR in a little enclosure.

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

#34

Post by zkms »

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…

> 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.

Are you sure that this is not a https://en.wikipedia.org/wiki/Vibrator_(electronic) that would be powered by a solenoid and not just a bimetallic strip?

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

#36

Post by WalterBright »

zkms wrote: post #34
WalterBright wrote: post #19

Earlier quoted context omitted.

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…

> 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. Are you sure that this is not a https://en.wikipedia.org/wiki/Vibrator_(electronic) that would be powered by a solenoid and not just a bimetallic strip?

I think you're right.

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

#37

Post by Bromskloss »

jsmthrowaway wrote: post #24
theoh wrote: post #22

Earlier quoted context omitted.

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 beau…

> Digital obviously negated needing them to work the same way

So, how do faders and knobs work nowadays?

Oh, are they perhaps still potentiometers, with the difference that you can now low-pass filter them before reading off and digitising their values, since you don't have to pass the audio signal through them?

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

#38

Post by [deleted] »

Bromskloss wrote: post #37
jsmthrowaway wrote: post #24

Earlier quoted context omitted.

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 beau…

> Digital obviously negated needing them to work the same way So, how do faders and knobs work nowadays? Oh, are they perhaps still potentiometers, with the difference that you can now low-pass filter them before reading off and digitising their values, since you don't have to pass the audio signal through them?

[deleted]

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

#39

Post by jsmthrowaway »

dharma1 wrote: post #31
jsmthrowaway wrote: post #24

Earlier quoted context omitted.

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 beau…

Same reason some guitar wah pedals use optical control. There are ones with mechanical pots but you have to keep replacing the pots every couple of years when they become "scratchy" as you move the pot back and forth quite a lot in use

Yep. Definitely maintenance involved and a culture of care and feeding of faders on high-end gear.

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

#40

Post by mianos »

As part of my electronics training many years ago we did a subject on TV repair as a practical introduction to circuit design, at the same time we studied transistors from a more theoretical perspective at college. I will always remember the instructor saying 'there is the design on how you are meant to use transistors and there is how they are used in practice in a TV'. For a consumer TV there was just as much focus on optimising costs as theory. We studied many amazing individual tricks like where three transistors can be used to replace four by putting two backwards. Just assuming a junction voltage of .6V would make you assume everything is broken in a working TV. Many of the tricks rely on non-linear characteristics of the components, such as a light bulb filament.
Post reply on HN