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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

#11

Post by dfox »

kens wrote: post #7
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…

Nice examples. By the way, those symbols on the schematic aren't neon lamps, but spark gaps - simply pointed PCB traces separated by about a millimeter. Presumably these are to safely discharge any excessive voltage buildup.

In this case that probably suffices as there is single intended video source.

Probably all multisync monitors I've seen have actual neon lamps and I believe that these neon lamps are in fact crucial for the "multisyncness", because they provide time for the control electronics (which given the package and pinout seems to usually be 8051) to settle after mode change without blowing out the CRT.

For a long time I've considered all the warnings of the "wrong video mode may damage your display" kind as nonsense. Then about ten years ago I had blown two 12" POS monochrome CRT monitors by unintentionally running them at 1024x768@75Hz, there was correct image for about a second followed by loud bang.

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

#13

Post by notlob »

flavio81 wrote: post #4

Really cool pictures! Light bulbs are sometimes used for stabilizing oscillators. I think it works like an (audio) volume "compressor" would. For example the Thorens TD125 turntable uses a light bulb as part of the oscillator circuit that generates the AC signal that powers the motor.

A bulb was also a neat hack used in the 200A, the first product made by a small and relatively obscure garage startup in Palo Alto: https://en.m.wikipedia.org/wiki/HP200A

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

#14

Post by dfox »

dfox wrote: post #11
kens wrote: post #7

Earlier quoted context omitted.

Nice examples. By the way, those symbols on the schematic aren't neon lamps, but spark gaps - simply pointed PCB traces separated by about a millimeter. Presumably these are to safely discharge any excessive voltage buildup.

In this case that probably suffices as there is single intended video source. Probably all multisync monitors I've seen have actual neon lamps and I believe that these neon lamps are in fact crucial for the "multisyncness", because they provide time for the control electronics (which given the package and pinout seems to usually be 8051) to settle after mode change without blowing out the CRT. For a long time I've co…

By the way I was thinking about all the multisync monitors I disassembled and got the impression that in all of them horizontal deflection was driven by class D amplifiers (there was half-bridge with switching MOSFETs, IRF640 and such). Does anyone on HN know whether that is correct assumption?

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

#15

Post by cjsuk »

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 used for amplitude stabilisation in feedback networks in the negative feedback path.

Very common tricks and very useful little things.

Only problem is they get hot, eat a lot of current and snuff it occasionally. They were replaced by JFETs for typical feedback applications.

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

#16

Post by gumby »

ChuckMcM wrote: post #2

Wow, if you could get your hands on a Dorado that would be something. On one of my visits to PARC there was an office with the door closed and the name plate "E. Dorado" on it. The offices on either side had displays and keyboards but the machines for those offices were in this office. The display and keyboard cables being fed through the walls. Not a 'machine room' per se but a separate office.

The dorados and (some of the dolphins) were in machine rooms, not offices, as the ECL machines ran so hot. There were not a lot of dorados so you could connect to an available machine. One reason I used work at night was that a dorado would usually be available for me to connect to from my office.

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

#17

Post by SwellJoe »

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 tiny fraction of the former. (Though, I guess if we were to count transistors in an IC as individual parts, LCD trumps CRT.)

Then again, one could build a CRT without a billion dollar fabrication facility. The same can't be said of LCDs (AFAIK).

Regardless, I really enjoy these posts. I don't understand half of it (though I understood more of this one than usual, as my dad owned a TV shop when I was a kid and I got some hands-on experience with CRTs early in life), but they're very satisfying. I wish I could play with cranky old computers more often.

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

#18

Post by ben1040 »

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.

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

#19

Post by WalterBright »

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 being jittery. With a smooth voltage, the mechanical instruments would get stuck.

Watch a pilot in an older airplane. He'll tap the gauges, too, to get an accurate reading.

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

#20

Post by cjsuk »

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 are marvelous from just about every angle. Each box is a blend of physics, mathematics, mechanical and electrical engineering that is simply unsurpassed. The engineering is reminiscent of the space race. No expense spared. Everything covered in gold and full of precision machined parts.

My current favorite Tektronix unit (I have a few!), a 475, is 42 years old, works perfectly and is still classified as relatively high performance compared to modern equipment. And the knowledge gained from looking after it and understanding it is STILL valid and STILL 100% usable in 2017.

You don't get that value from a Javascript framework!

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