
Xerox Alto Restoration Part 2: Firing up the monitor [video] - andars
https://www.youtube.com/watch?v=MDKxOmVDapQ
======
epberry
I'm really loving following this and I admire the tenacity this must take. For
example...

Screen brightness too low? Okay, just whip out some ancient circuit diagrams,
trace them to find the capacitor at fault, find that capacitor on the board,
desolder it, test it, find that it's faulty, resolder a new one. Well that
helped but it's not much better _sigh_. Now examine the switch, CRT tube, etc.

And here I've been known to throw in the towel when having 'this' issues in
JS.

~~~
Animats
I do that kind of thing for antique Teletype machines.[1] The discouraging
projects are when you get all the way to a working machine and the typing
quality is still too bad to be usable. That machine had been dropped at some
point. I could do more work on it, but I could get a better one on eBay.[2]

[1]
[http://brassgoggles.co.uk/forum/index.php/topic,43672.0.html](http://brassgoggles.co.uk/forum/index.php/topic,43672.0.html)
[2]
[http://www.ebay.com/itm/272286062507](http://www.ebay.com/itm/272286062507)

~~~
userbinator
Teletypes are interesting because they're basically mechanical UARTs. Watching
the mechanisms in them operate still fascinates me.

~~~
dghughes
You may enjoy Bill Hammack's video "IBM Selectric Typewriter & its digital to
analogue converter"

[https://www.youtube.com/watch?v=bRCNenhcvpw](https://www.youtube.com/watch?v=bRCNenhcvpw)

~~~
Animats
The IBM Selectric is a magnificent piece of engineering, and close to being
the last complex piece of mass-manufactured mechanical logic . The only
electrical part is a motor.

But it's not the ancestor of the computer printer.

The Selectric's main ancestor is the Blickensderfer_typewriter [1], from 1892.
This used a curved type element rather than a typeball, but had a similar
method of turning key presses into typing element positions. As with the
Selectric, the font could easily be changed. There was a proportional-spacing
successor to the Blickensderfer, the Vari-Typer, and IBM imitated that, too,
with the IBM Selectric Composer.

The Selectric had a moving print head rather than moving paper, with cables on
moving idlers used to transmit position to the print head. That mechanism is
from the Teletype Model 28/35, from the early 1950s. Moving the print head
rather than the paper was a Teletype concept - it worked much better with roll
paper, and the machine was narrower.

The Selectric mechanism was not designed for electrical inputs and outputs.
The add-on mechanism for that was an afterthought, and kind of a hack. The
unit had to be built into a table, and the mechanism for the keyboard extended
below the tabletop. IBM and Remington had previously built typewriters with
electrical I/O, and those were used with some early mainframe computers.

Many computers, before and after the Selectric, used Teletype machines as I/O
devices. That lasted until daisy-wheel printers and cheap CRT terminals were
invented.

(Mechanical line printers have a completely separate history. They descend
from the Potter Flying Typewriter.[2])

[1]
[https://en.wikipedia.org/wiki/Blickensderfer_typewriter](https://en.wikipedia.org/wiki/Blickensderfer_typewriter)
[2]
[https://www.computer.org/csdl/proceedings/afips/1952/5041/00...](https://www.computer.org/csdl/proceedings/afips/1952/5041/00/50410106.pdf)

------
kens
Summary: we got the monitor working (using a signal generator from the Seattle
Living Computer Museum (thanks!)), but the display is very dim, probably due
to the old CRT. Does anyone in the bay area have experience with CRT
rejuvenation?

~~~
userbinator
There's other things that could cause a dim display, not necessarily just a
worn tube. Low anode voltages is another possibility that comes to mind. The
various television/electronics forums may have people willing to assist with
troubleshooting.

If it's really the tube, and it's a truly rare custom tube and not just
something you might be able to find a suitable replacement for, maybe these
guys can help:

[http://www.earlytelevision.org/crt_rebuild.html](http://www.earlytelevision.org/crt_rebuild.html)

~~~
Animats
I was about to post a link to them. Full CRT rebuilding is a reasonable option
for a B/W tube. (Color requires very precise electron gun alignment so the
shadow mask lines up with the dots, and that's hard to do as a repair.)

The last commercial CRT rebuilder for ordinary TVs in the US closed in
2010.[1] But there's still a company that does it for military display devices
that need to be kept going.[2] "We are your obsolescence solution now and in
the future."

It really is a custom tube. PARC built the first tubes in-house (PARC was also
a copier R&D facility, so they could build precision optoelectronics) but then
sent the job out for production.

[1] [http://www.tvtechnology.com/miscellaneous/0008/last-lone-
wol...](http://www.tvtechnology.com/miscellaneous/0008/last-lone-wolf-crt-
rebuilder-closing/206279) [2]
[http://www.thomaselectronics.com/](http://www.thomaselectronics.com/)

------
frik
Was the camera operator x-rayed by the high voltage arc in the tube? (7:47 in
the video)

At 7:35 he filmed in a close up the caution label on the tube backside with
x-ray warning fine print

    
    
      X-RAY WARNING: When picture tubes are operated above 16 
      kilovolts, and when personal exposure is prolong at close 
      range, special shielding precautions against X-ray 
      radiation may be needed.

------
protomyth
Is there some history of refresh rates around? I'm really curious why they
used a "weird" rate on the Alto.

~~~
kens
The Alto has a "weird" refresh rate because it uses a portrait-format display
with 875 scan lines, compared to a "normal" display with 525 lines. The higher
scan rate stresses the horizontal drive circuitry and can cause overheating
problems.

~~~
protomyth
So, they didn't do what portrait monitors do today? Interesting, thanks for
the information.

~~~
kens
"what portrait monitors do today" \- you mean just turn the monitor sideways?
That would have kept the scan rate lower, but the Alto would need to send the
pixels a column at a time rather than a row at a time. That would be tricky to
do (keeping in mind this was all done with TTL chips and microcode, so they
wanted to keep things simple).

~~~
hcs
> but the Alto would need to send the pixels a column at a time rather than a
> row at a time. That would be tricky to do

Couldn't the buffer just be column-major?

~~~
kens
The Alto could have used a column-major buffer and a sideways display, but
that makes software more complex. For instance, text is in rows so its easier
to render row-major. Using an 875 scanline display was easier - just change
some resistors and capacitors in the monitor. (The exact resistors and
capacitors are listed on page 36 of [http://bitsavers.informatik.uni-
stuttgart.de/pdf/xerox/alto/...](http://bitsavers.informatik.uni-
stuttgart.de/pdf/xerox/alto/5-017-1017B_Ball_TTL_Series_Service_Manual_Oct76.pdf))

~~~
FullyFunctional
I'm equally puzzled by this. The complexity argument is IMO bunk; it's no more
difficult.

There is however a possible efficiency argument: if you are more likely to
write horizontal rectangles of bitmaps, then you'd want the memory word to be
horizontal so you have fewer memory locations to update. Was that really the
case?

UPDATE: I'm was using 2016 thinking, my bad. In those days, compared to the
complexity of the rest of the system, making the CRT circuits custom was
trivial. If this could, even marginally, simplify the rest of the system (not
least the software), it would be the right choice.

------
amq
Reminds me the scene in Alien where they turn on the damaged Ash.

