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
#2Wow, 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.
Re: Inside the vintage Xerox Alto's display, a tiny lightbulb keeps it working
#3Wow, 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.
I recently came across a couple big boxes of ECL boards at Xerox PARC that I think are from a Dorado. But without the backplane, it would be pretty hard to make a Dorado out of the boards.
Some photos: https://photos.app.goo.gl/cn1CV9ts5sINLlwa2 You can see the 10181 ALU chips - these are ECL versions of the popular 74181 ALU.
Re: Inside the vintage Xerox Alto's display, a tiny lightbulb keeps it working
#4Really 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.
Re: Inside the vintage Xerox Alto's display, a tiny lightbulb keeps it working
#5Wow, 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.
I recently came across a couple big boxes of ECL boards at Xerox PARC that I think are from a Dorado. But without the backplane, it would be pretty hard to make a Dorado out of the boards. Some photos: https://photos.app.goo.gl/cn1CV9ts5sINLlwa2 You can see the 10181 ALU chips - these are ECL versions of the popular 74181 ALU.
Wow, they appear to be about the right size. And yes, it would be impossible to assemble without a backplane, and fabricating one would be challenging even with documentation.
Re: Inside the vintage Xerox Alto's display, a tiny lightbulb keeps it working
#6Using 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 from overvoltage (which you will find in essentially any modern CRT monitor).
Another such neat trick is in most simple photographic flashes: the neon bulb which indicates that flash is ready also provides feedback to the flyback charging supply by increasing it's load when correct voltage is reached.
And then there is typical CRT degaus circuit, which produces the required damped oscillation waveform by running mains voltage throught two thermally coupled thermistors.
Edit: Horizontal deflection circuits of CRT TVs and monitors are usually full of neat tricks: shifted hysteresis inductors, deriving various voltages from the flyback and output multiplier, synchronisation of main SMPS switching frequency to horizontal retrace... One can view CRT VGA monitor as one big switchmode power supply with video amplifier bolted onto it (and many CRT monitors are actually mechanically built this way, with RGB video going directly from the captive VGA cable to amplifier on the neck board)
Re: Inside the vintage Xerox Alto's display, a tiny lightbulb keeps it working
#7Using 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.
Re: Inside the vintage Xerox Alto's display, a tiny lightbulb keeps it working
#8Using 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…
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Re: Inside the vintage Xerox Alto's display, a tiny lightbulb keeps it working
#9Surprisingly, despite being exposed to 600 volt pulses, the problem with this 28 volt bulb wasn't that it had burnt out, but that one of its tiny leads had broken.
That's because the bulb doesn't have 600V across it. In a normal application as an actual glowing bulb, it's rated to have 28V across it, pass a certain current, and emit light at a certain brightness.
In this application, it's being used as a current limiter, and the high voltage supply to CRT grids is very low current (several milliamps at most) --- thus, the current through the bulb and the voltage across it will also be very low.
Re: Inside the vintage Xerox Alto's display, a tiny lightbulb keeps it working
#10Surprisingly, despite being exposed to 600 volt pulses, the problem with this 28 volt bulb wasn't that it had burnt out, but that one of its tiny leads had broken. That's because the bulb doesn't have 600V across it. In a normal application as an actual glowing bulb, it's rated to have 28V across it, pass a certain current, and emit light at a certain brightness. In this application, it's being used as a current limi…
It would have 600v if whatever else it was in series with shorted. It's not clear how much resistance the rest of the circuit has, so it's hard to say whether the pulses briefly exceed 28v across the bulb or not.
It's not uncommon to exceed forward constant max current specs for an LED with pulses (PWM), for example.