
Fixing the Ethernet Board from a Vintage Xerox Alto - mkeeter
http://www.righto.com/2017/11/fixing-ethernet-board-from-vintage.html
======
linker3000
Noted that Ken had an issue when a 74S TTL part was replaced with a 74LS
because the characteristics are different.

Back in the day (mid 1980s), when I was going through an electronic
engineering apprenticeship with a flight simulator company and many boards
were end-to-end TTL, it was generally understood that power-hungry 74S series
TTL were used for a reason (generally speed) and should not be substituted
lest 'funny things' happen.

Mind you, there was one time when debugging a glitch led to one chip being
replaced with the same type from a different family - something like a 74LS
replacing a 74F - and the propagation delay difference was enough to fix an
edge-case timing issue. Due to project constraints, the root cause wasn't
tracked down and so the board bill of materials made it clear that the 'odd
one out' was correct and should not be changed.

PS: Ken - I think I still have a quantity of 74S TTL 'pulls' from the day, so
if all else fails sourcing a part, look me up!

~~~
Declanomous
Wouldn't replacing the 74S86 with a 74VHC86 have worked? I'm a hobbyist, so my
knowledge is pretty limited[1], but I was under the impression that the VHC
series chips were drop-in replacements for the S series in both timing and
logic levels.

[1] Read as 'I know just enough to be dangerous, but not enough to understand
why what I'm doing is dangerous.'

~~~
CodeWriter23
The way to confirm your assumption is to look at the different typical timing
waveform charts from the data sheet for each part.

I once traced hundreds of thousands of dollars worth of bunk boards back to
some RAM chips substituted on the basis of being a "drop in replacement" by
the procurement department. The timing diagrams revealed the replacements
possessed different timing characteristics. Enough so that the chips would
occasionally leave the bus floating when the memory was read from the host
side.

~~~
katastic
"It's not a hardware problem, it's a _human_ problem!"

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ChrisGammell
Had a chance to chat with Ken, really enjoyed hearing about the history of the
Alto and the process they take for troubleshooting what is effectively
priceless hardware (given the fact that so few are remaining):
[https://theamphour.com/361-an-interview-with-ken-
shirriff/](https://theamphour.com/361-an-interview-with-ken-shirriff/)

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sillysaurus3
_I started probing the Ethernet board 's input circuit with the oscilloscope.
The board was receiving the input okay, but a few gates later the signals
looked kind of sketchy, as you can see above._

Is it even possible to do this kind of debugging on modern hardware? Or is
this a lost art?

~~~
SAI_Peregrinus
It's possible, but harder. A modern Ethernet card would probably be doing this
step entirely on one chip. A quick search shows the Microchip ENC28J60 as a
pretty typical part. The result is that you'd be able to probe the input to
the chip and the output, and see the failure at the output, and replace the
chip. Effectively the same as what was done here, just with more components
built into the chip. Louis Rossmann[1] does macbook component-level logic
board repairs, though 99% of the time he doesn't need to break out the
oscilloscope to find the problem. It's almost always a power supply or liquid
damage problem, or something similar that can be found with nothing more than
a multimeter.

[1]
[https://www.youtube.com/playlist?list=PLkVbIsAWN2lsHdY7ldAAg...](https://www.youtube.com/playlist?list=PLkVbIsAWN2lsHdY7ldAAgtJug50pRNQv0)

~~~
sillysaurus3
[https://www.youtube.com/watch?v=dK9pfbGSE4A](https://www.youtube.com/watch?v=dK9pfbGSE4A)
is incredibly cool. Thanks for this series!

It's so neat to watch a pro in a completely different domain. I know some of
the basic concepts, but it's mesmerizing to watch him reason through voltage
levels and pinpoint a failed resistor. That's magical to me.

~~~
heywire
Agreed. I became aware of Louis' work after looking for information on the
infamous Mid-2010 Macbook Pro 15 GPU kernel panic. After watching this video
[1], I was able to replace the same capacitor for ~$3 and restore a previously
unusable Macbook.

[1]
[https://www.youtube.com/watch?v=DzcgT_fiVTA](https://www.youtube.com/watch?v=DzcgT_fiVTA)

~~~
sillysaurus3
[https://youtu.be/xRwfJpNvWI0?t=1247](https://youtu.be/xRwfJpNvWI0?t=1247)

Bad idea to eat cereal while watching his videos... That was hilarious.

I love that he leaves in all the little mistakes. Most videos are polished and
presented. Tutorials are nice, but you get so much more raw data from watching
this. Plus it feels like having a conversation.

Looks like Ken has some videos like this too:
[https://www.youtube.com/watch?v=adEr2aRwHnI](https://www.youtube.com/watch?v=adEr2aRwHnI)

~~~
heywire
> I love that he leaves in all the little mistakes

In fact, in the video I mentioned, someone who was watching his livestream
pointed out that he installed a polarized capacitor backwards...

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sgt
That is a very interesting use of a BeagleBone. He made an interface to the
Alto's 3Mbit/sec Ethernet using the BeagleBone's PRU (a programmable real-time
unit of which I believe it has two). Someone also used the PRU to build a
video card for a Macintosh SE: [https://hackaday.com/2014/02/05/the-30th-
anniversary-macinto...](https://hackaday.com/2014/02/05/the-30th-anniversary-
macintosh/)

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CodeWriter23
A word from experience: an extender card can introduce noise, which can lead
to observations of self-induced problems due to the introduced noise. I don't
think this is the case for your situation, but symptoms the runt edges you
observed could be caused by the extender.

Also, I have on occasion observed the magic of an extender card causing a
flaky card to function properly.

If you get stumped, you might want to depopulate some non-essential cards to
open up a space to get your hands and probes on the card under investigation,
while it is directly plugged into the slot. Excuse me, I mean get your hands
and probes in there while the system is powered off.

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purplezooey
What job can I get where people pay me to restore vintage gear like this? :)

(answer: "already be rich")

~~~
ChuckMcM
Actually if you have the skills people will seek you out. The trick is to
repair things and demonstrate said skills. Since most of the vintage gear is
easily covered by a 100Mhz oscilloscope, a DMM, and a 10Mhz signal generator
you can do a lot of that. Fixing things you pick up at thrift shops will help
you develop the mindset and 'bodging' skills, then work your way up to more
interesting things. You will be amazed at what you find in the bin at some
places.

