[sdiy] Future SYNTH-DIY/breadboard

[Tim Daugard]

From: "The Old Crow" <oldcrow at oldcrows.net>
To: "Tony Clark" <clark at andrews.edu>
Cc: "Synth-DIY" <synth-diy at dropmix.xs4all.nl>
Sent: Wednesday, May 25, 2005 10:56 PM
Subject: Re: [sdiy] Future SYNTH-DIY/breadboard


>
>   This is called, in government contractor-speak, the "LRU" or
"least
> replaceable/repairable unit."  They specifically don't want
boards that

Actually, the LRU is the Line Replaceable Unit. It is a piece of
equipment that can be quickly changed on the flight line to get
an aircraft up and fighting again. In civilian life, the
equivelent would be the computer, the printer, the monitor, the
UPS, etc.

The next level of repair is the local back shop where smaller
parts are removed and replaced. This level of repair includes
SRUs - Shop Replaceable Units. These are circuit boards, RF
assemblies, power supply modules, etc. A great example of an SRU
is a harddrive.

The reason for the SRU is you don't want a flight line tech
replacing a part that hasn't been tested in the LRU. Imagine
someone puting a blank hard drive in a computer in your aircraft
and saying the plane is fixed.

The SRUs are sent to a depot or manufacture for further repair
or disposal.

Part of the problem is the discussion we have been having here.
SRUs maybe confomally coated to prevent arcing, moisture
inclusion, etc. The SRUs MUST function even at 20,000+ meters.
SRUs may contain exotic materials, hard to process parts, and
any other number of problems.

It doesn't make economic sense to buy expensive rework equipment
and train people for more than 10-20 locations. So all the
expensive equipment and training is located in one spot. Spares
are bought, so that the supply lines will be full enough, that
there will always (almost) be SRUs available.

It also makes stocking forward supply easier. You take an SRU or
better yet an LRU with you to war. Much better than carrying
200-300 parts to fix a circuit board.

> have repairs done at the component level.  Their rationale is
once a unit
> fails, it is more prone to failure in the future, even if the
specific
> fault that caused the failure is found and fixed.  I
personally think this
> is stupid, but they're the customer, they're paying for it,
and what they
> say, goes. They want to be able to swap in another board
entirely.  They
> will send back a failed unit for study, but it is in no way to
ever be put
> to field use again.

Depends on the use of the board. For space use this maybe true.
For aircraft use we get the same parts quite often. One location
I worked at started a bad actor program. After the same serial
numbered part failed the same way three times, we sent it to
depot with a nastygram saying your fix isn't working, try
something else.

>   For consumer stuff, the LRU is a labor issue.  It costs the
company less
> money to swap out a widget than it costs for a technician to
resolve the
> fault on a broken one.

Absolutely, I had a good technician who worked for me who went
to work part time repairing TVs for a chain. He got in trouble
for repairing boards and charging the customer for a transistor
and labor instead of changing out the whole board.

A brief overview from someone who has been involved in it all
from a start in tube and wire repair, to helping contractors
determine what parts should be SRUs vs depot repair. (Won some,
lost some decisions)

Tim Daugard
AG4GZ 30.4078N 86.6227W Alt: 3.7 M
http://home.sprintmail.com/~daugard/synth.htm