
Restoring YC's Xerox Alto, day 1: Power supplies and disk interface - kens
http://www.righto.com/2016/06/restoring-y-combinators-xerox-alto-day.html
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tdicola
Not sure it was mentioned in the last thread on this computer but if anyone
wants to actually use a real Alto and many other vintage computers check out
the Living Computer Museum in Seattle:
[http://www.livingcomputermuseum.org/](http://www.livingcomputermuseum.org/)
They have an Alto you can walk right up to and start hacking on--when I last
saw it it was running a pretty fun billiards game (worked well on its vertical
monitor). There are tons of other vintage computers you can use, including
even giant mainframes in a cold room like an IBM System 360, PDP-10, VAX
systems, and a lot more.

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kens
Keith at the Living Computer Museum has been very helpful with our Alto
restoration, providing information from their restoration experience, as well
as diagnostic hardware.

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walrus01
As cool as restoring the power supplies in their original condition is, it
might be a lot easier at least temporarily to replace them with modern 5V and
12V units. Also a great deal less damage risk to the electronics on the PCBs
which is what's truly rare and irreplaceable. Then you can repair and test the
'old' power supplies at your leisure and put them back into use if full
authenticity is required.

~~~
emeraldd
This might not be as easy or practical as it sounds. Replacing an existing
power supply that is known to match the requirements of the system with one
that was cobbled together from modern components is inherently risky. You're
either building something brand new and un-tested or hacking an existing
supply to fit a job it wasn't designed for. If the original is reasonably
intact and can be verified, it would be simpler and safer to repair it and
know the supply matches exactly.

~~~
walrus01
The original post shows that it needs +15V, -15V, +5V and -5V. This could be
implemented with four separate high quality active PFC modern power supplies
tuned to the right voltages. One thing that's unknown is what the load is for
the whole thing, so it'd be necessary to somewhat deliberately oversize the
power supplies, but it's not hard to find 700W rated power supplies that will
do 15V.

Any modern power supply will be safer and more reliable. If there's any doubt
about the pinout and cabling arrangement of the old power supplies, it's
totally possible to pull the old power supply, repair it as best as possible
with new capacitors, power it up on a test bench and probe it to see exactly
what its output is. Then match that on a new cable harness with a new power
supply.

It's not "hacking" to provide +15V or -15V , that's within the range of many
good quality tunable 12V power supplies.

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DanBC
You don't even need to tune a 12v supply. +15v -15v is a reasonably standard
part.

~~~
walrus01
True, though there's a much greater selection of really high quality (medical
grade, even!) active PFC 12V power supplies that will tune to 15V.

Some of them even come in specific 15V versions like a Meanwell RSP-750-15:

[http://www.meanwell.com/mw_search/RSP-750/RSP-750-spec.pdf](http://www.meanwell.com/mw_search/RSP-750/RSP-750-spec.pdf)

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GrumpyYoungMan
I find it interesting that they only had to replace three capacitors in one
power supply. Don't electrolytic capacitors (the most commonly used type) have
a limited lifespan because of electrolyte evaporation? Naively, I would have
expected that every single capacitor in the system would need to be replaced.

~~~
sbierwagen
The average lifespan of electrolytic capacitors between 1999 and 2007 was much
worse than usual, due to poor industrial espionage:
[https://en.wikipedia.org/wiki/Capacitor_plague](https://en.wikipedia.org/wiki/Capacitor_plague)

You could easily get the impression that electrolytics had really lousy
lifespans as a result of this.

~~~
nasalgoat
I work on machines that are usually 20-30 years old if not more and I pretty
much have to replace all the capacitors on them due to age. They tend to dry
out, crack, or balloon up.

Not a single one built after 1999.

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tcdent
Thoroughly enjoying following this. Please keep the videos and articles
coming.

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brudgers
Disk drive problems with an old computer reminded me of this discussion:
[https://news.ycombinator.com/item?id=11376711](https://news.ycombinator.com/item?id=11376711)

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xhrpost
Awesome, I actually have a platter disc that looks similar but just has a
permanent opening in the side, it doesn't pop up on top like this one.

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endlessvoid94
I can't wait to see an original version of smalltalk running on this!

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rbanffy
When the hardware is fully documented, I'd love to see on Kickstarter a small
board with some USB ports, HDMI output and an SD card slot emulating an Alto
(or a Star). Extra points if it came with a keyboard with the same layout as
the original (so you don't need to use a PC keyboard).

~~~
kens
The Alto hardware is fully documented [1], source code is available [2] and a
simulator is available [3], so there's not really anything stopping someone
from making an Alto clone.

[1] Complete schematics at
[http://bitsavers.org/pdf/xerox/alto/schematics](http://bitsavers.org/pdf/xerox/alto/schematics)

[2] Source code at [http://www.computerhistory.org/atchm/xerox-alto-source-
code/](http://www.computerhistory.org/atchm/xerox-alto-source-code/)

[3] Simulator at
[https://github.com/brainsqueezer/salto_simulator](https://github.com/brainsqueezer/salto_simulator)

~~~
rbanffy
I don't think it'd be an easy endeavor. I'm almost inclined to think it'd be
easier to start from an emulator source towards a hardware implementation
abstracting away stuff that's done in microcode hoping the software doesn't
look too close.

