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

Post by phire » Sun, Oct 15, 2017, 6:08 AM UTC

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…

Post by cjsuk » Sun, Oct 15, 2017, 9:50 AM UTC

icantdrive55 wrote: post #32

cjsuk wrote: post #20

Earlier quoted context omitted.

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?

Post by cjsuk » Sun, Oct 15, 2017, 9:56 AM UTC

analog31 wrote: post #30

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

Post by yardie » Sun, Oct 15, 2017, 11:02 AM UTC

kazinator wrote: post #29

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 a…

Post by Turing_Machine » Sun, Oct 15, 2017, 4:29 PM UTC

mianos wrote: post #40

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…