Xerox Alto CRTs Needed a Tiny Lightbulb to Function

In the real world, components don’t work like we imagine they do. Wires have resistance, resistors have inductance, and capacitors have resistance. However, some designers like to take advantage of those imperfections, something our old friend [Ken Shirriff] noted when he was restoring the CRT of a Xerox Alto.

[Ken] tried to connect a Xerox monitor to the Alto and — since it was almost as old as the Alto — he wasn’t surprised that it didn’t work. What did surprise him, though, is that when he turned the monitor off, a perfect picture appeared for just a split second as the unit powered off. What could that mean?

Keep in mind this is a CRT device. So a perfect picture means you have vertical and horizontal sweep all at the right frequency. It also means you have high voltage and drive on the electron guns. If you are too young to remember all that, [Ken] covers the details in his post.

He found that the CRT grid voltage wasn’t present during operation. The voltage derived from the high voltage supply but, mysteriously, the high voltage was fine. There was a small lightbulb in the grid voltage circuit. A 28V device about like a flashlight bulb. It measured open and that turned out to be due to a broken lead. Repairing the broken lead to the bulb put the monitor back in operation.

On paper, a light bulb lights up when you put current through it. In real life, it is a bit more complicated. An incandescent filament starts off as almost a dead short and draws a lot of current for a very brief time. As the current flows, the filament gets hot and the resistance goes up. That reduces the current draw. This effect — known as inrush current — is the scourge of designers trying to turn on light bulbs with transistors or other electronic switches.

However, the unknown Xerox power supply designer used that effect as a current limiter. The short 600V pulses would hardly notice the light bulb but if too much current or time elapsed, the resistance of the bulb would rise preventing too much current from flowing for too long. With the bulb open, the negative brightness grid provided an impassible barrier to the electrons. Apparently, the brightness grid lost power a bit earlier than the rest of the circuit and with it out of the way — or perhaps, partially out of the way — the picture was fine until the rest of the circuit also lost power.

We looked at [Ken’s] efforts on this machine earlier this year. Light bulbs, by the way, aren’t the only thing that changes resistance in response to some stimulus. You might enjoy the 1972 commercial from Xerox touting the Alto’s ability to do advanced tasks like e-mail and printing.

Continue reading “Xerox Alto CRTs Needed a Tiny Lightbulb to Function”

Retrotechtacular: The Incredibly Difficult Task of Copying a Document

It seems hard to imagine, but in the early part of the 20th century, there weren’t a lot of great options for creating copies of documents. The most common method was to use carbon paper to create multiple copies at once from a typewriter or a line printer. All that changed with a company called Haloid. Never heard of them? They later became the Xerox company.

The underlying technology dates back to 1938 (invented by a physicist who was also a lawyer). In 1944, they produced a practical copier and shortly thereafter sold the rights to Haloid. The Haloid company originally made photographic copy machines that used wet chemistry.

In 1959, the Xerox 914 (so called because it could copy a 9″ x 14″ document) came on the scene (that’s it, below). The 650 pound copier could make seven copies per minute and came with a fire extinguisher because it had a tendency to burst into flames. If you didn’t want to spend the $27,500 price tag, you could rent for only $25/month (keep in mind that in 1959, $25 would buy about 25 pounds of T-bone steaks). You can see a commercial for the 914 in the video below.


In the commercial, you’ll see them make a big deal out of the fact that the print was dry. That’s because a lot of previous machines used actual photographic processes with wet chemistry. Obviously, that also took special paper.

Even Further Back

If the copier didn’t exist until recently, how did people make copies before? Turns out there were lots of ways to make copies of varying degrees of bad quality or extreme trouble. In some sense, the best copies were made by scribes just writing down a second copy of things. There were a variety of machines that would capture what you wrote and make a copy by mechanical or other means. A polygraph (not the lie detecting kind) allowed Thomas Jefferson to write letters and make a copy. The machine moved a pen to match the movements of the author’s pen, thus making a near perfect copy. With a few adjustments, this became the pantograph which not only does the same job, but also can shrink or enlarge the copy. Carbon paper was widely used to make multiple copies of handwritten and typewritten documents.

Continue reading “Retrotechtacular: The Incredibly Difficult Task of Copying a Document”

Restoring The Groundbreaking Xerox Alto

The Xerox Alto was a minicomputer that had a lot of firsts to its name: first GUI, first Ethernet connection, and first computer to use a laser printer. This is the computer that inspired Steve Jobs to build the Lisa. And this was built all back in 1973! So when [Ken Shirriff] and a team of other old-computer aficionados got their hands on one, you know they’d get to work.

[Ken]’s blog describes the start of what’s sure to be a long journey. It mostly describes the Alto system and locates its place in computer history, but there are some interesting sidelines as well — like how [Alan Kay] also basically outlined all of the functionality of the modern laptop / tablet along the way to the Alto; it was supposed to be an interim Dynabook.

Work on this grandfather-of-modern-computers is just getting started, and [Ken] and crew are dusting off the power supplies and cataloguing memory boards. You can be sure that we’ll follow along with this restoration project, and keep you informed.

This Post Will Self Destruct in 10 Seconds

Ah yes, the classic Mission Impossible ultimate message security — after verification and playing the message, poof — it’s gone. You could design explosives into your electronics to have the same effect… or you can use Xerox PARC’s new chip, which features a self-destruct mode.

Wait, what? Yup — some engineer at Xerox decided to develop a chip that can literally self-destruct. It was developed for DARPA’s vanishing programmable resources project, and well, it sounds pretty promising for the future of high-security applications. Continue reading “This Post Will Self Destruct in 10 Seconds”

Content Centric Networking and a tour of (Xerox) PARC

You may be used to seeing rack mounted equipment with wires going everywhere. But there’s nothing ordinary about what’s going on here. [Elecia White] and [Dick Sillman] are posing with the backbone servers they’ve been designing to take networking into the era that surpasses IPv6. That’s right, this is the stuff of the future, a concept called Content Centric Networking.

Join me after the break for more about CCN, and also a recap of my tour of PARC. This is the legendary Palo Alto Research Company campus where a multitude of inventions (like the computer mouse, Ethernet, you know… small stuff) sprang into being.

Continue reading “Content Centric Networking and a tour of (Xerox) PARC”

Xerox ink will print circuits


Xerox has announced a breakthrough in printable circuits. They’ve developed a conductive ink called “silver bullet” that can be printed on many different types of substrate to create circuits. The key part of the new ink is its lower melting point. Plastic film substrate melts at 150 degrees Celsius but the ink is liquid when ten degrees cooler to avoid damaging the film. This begs the question: how do you then solder components to the circuit?

The benefits of printable circuitry are obvious. Aside from cheaper and easier RFID, disposable circuits like greeting cards, and fabric-based electronics, we’re hoping this will facilitate more environmentally friendly PCB fabrication. That really depends on the ink’s production process and the resilience of the resulting circuitry.

[via Gizmodo]