[sdiy] Mystery Caps - zero ohm resistors & anecdote

[Scott Gravenhorst]

Years ago, I worked for Digital Equipment Corp. as a field engineer.  DEC PCBs
were often made with many configurable options and settings.  It was not
uncommon to find as many as 20 or more zero ohm resistors installed on each of
the many boards that comprised these minicomputers.  A handful of these zero
ohm resistors was less expensive and more reliable than a switch pack.

Another major advantage to the zero ohm resistor (and they were color coded as
such) is that they came in belts and could be fed into an automatic part
stuffer since they conformed to the dimensions of a standard resistor.

DEC boards were manufactured with all of the zero ohm resistors installed. 
Configuring the (new) board in the field meant simply cutting out the ones you
don't want.

But if the customer wanted something changed, it seemed to always require
putting new ones in where they had been cut out.  We were required to use these
(although that requirement was often ignored when you got called out at 2:00AM
to fix some bank's minicomputer).  

A major pain in the ass for me was to follow up after some brainless idiot who
used staples or paperclips instead of copper or tinned wire (I cheated using
wire wrap wire which solders very well) because the solder didn't stick well,
especially when this bonehead didn't know how to solder in the first place. 
This "technique" would almost surely lead to a system that was intermittent and
the symptom was usually something like once or twice a day, the system would
crash.  Often one could trace this to variations in computer room temperature
(the staple would change size with temperature just enough to disconnect or
reconnect).  I remember spending 2 weeks on one large system, removing every
single PCB (there were 26 just in the CPU, about 14"x28") and replace any of
these that looked even slightly suscpicious.  And BTW, I often both got stuck
fixing systems like this (because management knew it would work properly when I
was finished) and then I got yelled at for taking so long to fix it.  And yes,
I did explain why, and no it never mattered.  In many ways, I was quite glad to
see DEC die.

harrybissell <harrybissell at prodigy.net> wrote:
>The zero-ohm resistor has another advantage... It can be used on a double
>sided (pre PTH) board to allow traces to run under it without fear of short
>circuits...
>
>This was in the days before plated through holes, and soldermask was very
>uncommon as well.
>
>Its true the major reason was pick and place... but at that time machines were
>available
>to insert 22ga wire jumpers (also supplied on tale and reel)
>
>H^) harry
>
>Wayne Dubois wrote:
>
>> We used to see these used alot as the usual bypass cap (.1uf, marked '104').
>> Our guess was that the form-factor made it easier on the pick and place
>> mechanisms for auto-insertion.
>>
>> I still get a giggle when I see a 'zero ohm resistor' which, I presume, is
>> used for exactly the same reason.  (These will have the same axial pkg as a
>> standard %5 resistor, but with a single black band in the middle.)
>>
>>  - w
>> ----- Original Message -----
>> From: "Neil Johnson" <nej22 at hermes.cam.ac.uk>
>> To: "Michael Ruberto" <frankentron at hotmail.com>
>> Cc: <synth-diy at dropmix.xs4all.nl>
>> Sent: Sunday, March 16, 2003 10:16 AM
>> Subject: Re: [sdiy] Mystery Caps
>>
>> >
>> > > not with a value .47uf.
>> >
>> > Ah, right :-)
>> >
>> > > >But my guess is that they could be glass-encapsulted multi-layer
>> ceramic
>> > > >capacitors.  The "50V"  rating sounds like they might be these.
>> > >
>> > > hmmm ceramic, so I guess I can assume at least 10% tolerance? is there
>> any
>> > > benefit to glass encapsulation? good humidity resistance?
>> >
>> > They look nice?  Seriously though, certainly quite resistant to moisture,
>> > as long as the glass-metal seals are good.
>> >
>> > Neil
>> >
>> > --
>> > Neil Johnson :: Computer Laboratory :: University of Cambridge ::
>> > http://www.njohnson.co.uk          http://www.cl.cam.ac.uk/~nej22
>> > ----  IEE Cambridge Branch: http://www.iee-cambridge.org.uk  ----
>> >
>> >
>

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-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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