More parts I need to replace on the VT100

[Dwight K. Elvey]

>From: ard at p850ug1.demon.co.uk
---snip---
>
>However, they seem to be a lot less use when it's a one-off unit which 
>you need to repair no matter what (like a classic computer). You're going 
>to have to do real fault diagnosis sometimes anyway, it doesn't make it 
>much more difficult to do it properly every time.
>

Hi
 Once many years ago while working for Intel, I was
called in to help fix a number of what we used to call
yellow dot boards ( failed but not repaired after third
rework and had a yellow dot placed on them ). The
yellow dot inventory had a sudden jump so someone
needed some additional help.
 The boards had a lot of analog circuits on them because
they had a discrete component PLL used for the disk
controller.
 I had the fellow from system test with me to take notes
on how to diagnose complicated failures on these boards.
As it turned out, about 90% were caused by a mix up
in the parts loaded on the boards. A low voltage zener
was placed in a number of locations that expected
signal diodes. This especially effected the balanced
mixer used in the phase detector.
 After some time of carefully showing the system test
fellow a number of scope signals and carefully explaining
the reasons I was looking at particular locations,
I happened to glance at his notes. It went something
like this:

D3, D5, D8, D20, D21

 There were no symptoms and nothing of what I showed him
about scope information. I soon realized that he was
just interested in a list of parts that I indicated as
causing problems and wasn't even paying attention to
anything else I was doing.
 The worst part was that since this was a one time
parts mix up, it probably would never happen again
so that his list was virtually worthless. I told
him this but that didn't seem to ring a bell with
him.
Dwight