
Inside the vintage Xerox Alto's display, a tiny lightbulb keeps it working - darwhy
http://www.righto.com/2017/10/inside-vintage-xerox-altos-display-tiny.html
======
dfox
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 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)

~~~
WalterBright
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.

~~~
zkms
> 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)](https://en.wikipedia.org/wiki/Vibrator_\(electronic\))
that would be powered by a solenoid and not just a bimetallic strip?

~~~
WalterBright
I think you're right.

~~~
zkms
No.

You were actually initially right, I searched for that actual part and it is
indeed a bimetallic strip along with a heater (that turns off when the strip
moves away from a contact) and not a device with a solenoid/relay.

Apparently whatever it supplies is tolerant of that sort of interruptions in
its power source -- I imagine that it would operate on a quite lower frequency
than a "vibrator"-type power supply where magnetic force moves the contacts
(and elasticity returns them).

~~~
WalterBright
I read about this probably 10 years ago, my memory was clearly a bit faulty on
it. I didn't want to take the device on my Dodge apart just to verify it :-).

The thing is, the instruments _require_ the noisy 5V.

You can buy kits with an electronic 5V source, and also to replace the old
guts of the instruments with modern ones. That retains the original look, with
modern accuracy and performance.

------
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.

~~~
cjsuk
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!

~~~
icantdrive55
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?

~~~
jacquesm
You can buy used ones from army surplus stores at incredibly low prices. I got
mine that way and the only thing that it needed to be in spec again was a
minor recalibration. Super nice gear.

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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.

~~~
theoh
Faders in DJ equipment are often optical -- apparently a potentiometer would
be noisy.

[http://djtechtools.com/2011/11/01/fader-technology-a-
primer/](http://djtechtools.com/2011/11/01/fader-technology-a-primer/)

~~~
jsmthrowaway
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 beautiful 10Ks, much still in use. I
wouldn’t call it “old” like that article does. Then again, DJs also beat the
shit out of faders when they work, so it might be a good idea from a
reliability perspective.

Coming from the reinforcement side, a lot of DJ gear baffles me. They look at
audio much differently, which I suppose is understandable when you’re mixing
two decks with an occasional bandpass versus a 50+ channel metal band with
subgroups and racks of outboard effects.

~~~
Bromskloss
> 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?

~~~
rzzzt
Some knobs use rotary encoders. A nice side-effect is that they are "infinite"
in either direction, unlike helical or single-turn pots.

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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](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.

~~~
analog31
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](https://en.wikipedia.org/wiki/HP200A)

~~~
cjsuk
Indeed. Works better with tubes. I built a slightly more modern version with
an op amp and a small 28v bulb in the feedback circuit and it didn’t quite
perform as well as I’d hoped. Was interesting however.

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ChuckMcM
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.

~~~
kens
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](https://photos.app.goo.gl/cn1CV9ts5sINLlwa2)
You can see the 10181 ALU chips - these are ECL versions of the popular 74181
ALU.

~~~
ChuckMcM
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.

------
userbinator
_Surprisingly, 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.

~~~
tyingq
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.

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flavio81
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.

~~~
notlob
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](https://en.m.wikipedia.org/wiki/HP200A)

------
Theodores
I like it how someone that really knows their subject can explain things so
fluidly. I have a good idea of how CRTs work but in the many diagrams and
lessons that I have endured I have not read such a fluid, easy to understand
overview of how a CRT works.

Not everyone has this gift of being able to explain stuff in layman's terms
even if they are expert on the subject. Carl Sagan was best at it, on the
other extreme you have those that use jargon beyond the audience's knowledge,
pretending to know but not understanding it really. This whole Alto series has
had some great writing in it.

------
yardie
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
rebuilt. Only to find out it was a $1 bulb.

In this digital world analog circuits are like witchcraft to me. How you think
things work is not at all how it works.

~~~
kazinator
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 arrangement of diodes cuts off the battery field coil circuit and
then the light goes out.

When the alternator is self-energizing, there is a feedback mechanism then at
play which controls the field current, keeping the alternator voltage level:
without it, the voltage would rise uncontrollably: more voltage from
alternator -> more current through field coil -> greater induction of voltage.
Before the alternator is self-energizing, what regulates the field current is
that bulb.

So that is to say, if you see the light _on_ all the time when the alternator
is spinning, that warns you that it's not actually working. It is not it's not
putting out enough voltage (perhaps none at all) to overpower the battery
circuit and become self-energizing. This could be because the rotor coils are
burned out, or other problem: wiring or whatever.

Since that bulb is off most of the time, we don't expect to have to change it
very often.

If an alternator bulb has burned out, and the unit is not very old, that could
indicate a problem.

~~~
yardie
Right on the money! We actually did have alternator problems as this was a 12
year old boat. The bulb was on constantly, had built up a layer of soot in the
glass, and couldn’t be seen in the sun.

Going back and forth with alternator mechanic he asked me to pull the
instrument panel and inspect the bulb.

------
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.

~~~
Turing_Machine
See also: "Muntzing".

[https://en.wikipedia.org/wiki/Muntzing](https://en.wikipedia.org/wiki/Muntzing)

------
jacquesm
Lightbulbs can function as a PTC, as the bulb heats up the resistance goes up,
and current drops until it stabilizes. This allows you to use a lightbulb as a
way to stabilize a voltage or a current.

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2sk21
In the model train world, we use car brake light bulbs as current limiters for
a reason similar to why they used one in this monitor.
[http://cs.trains.com/mrr/f/744/t/215408.aspx](http://cs.trains.com/mrr/f/744/t/215408.aspx)

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pjc50
Nice little touch there on measuring the 17kV signal from a distance of two
inches, without touching it at all, thereby avoiding lots of safety issues.

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Johnythree
The classic example was the light globe used in a polaroid camera.

The globe was powered from a mercury cell, and because it's voltage was so
constant, the light stayed at constant brightness.

Then a simple plastic lens overlayed alternate stripes from the image and the
globe. The user adjusted the aperture until the stripes vanished, which
guaranteed that the exposure was correct.

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swimfar
Cool. That reminded me of a very simple tube amp design I had seen that used a
light bulb as a current source.

[http://www.diyaudio.com/forums/diyaudio-com-
articles/160464-...](http://www.diyaudio.com/forums/diyaudio-com-
articles/160464-de-lite-amplifier.html)

