[sdiy] Mystery Caps - zero ohm resistors & anecdote

[harrybissell]

I feel your pain   ;^P

H^) harry

Scott Gravenhorst wrote:

> 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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