Dying VaxStation 4000 VLC - help?

From: Peter C. Wallace <pcw_at_mesanet.com>
Date: Mon Nov 25 09:44:01 2002

On Mon, 25 Nov 2002, Mark Tapley wrote:

> >Excellent. If you can use the right tools, do so. But it is _possible_ to
> >do SMD rework using a normal (good) soldering iron.
>
> I'll ask. But now I'm convinced the board has some hope of living, I
> hesitate to address it myself...
>
> >Are there any marking at all. You mentioned an orange band(s). How many?
>
> One. I shouldn't have said "band", it's really the whole outside of the
> component except the contacts at either end. It looks way too wide to be a
> color-code band.
>
> >Wait a second. The meter is 20kOhm/V right. I have no idea what range
> >you're using, but lets say it's a 10V range. So the meter's resistance is
> >200k.
>
> Yep, 0-15V range.
>
> >Now, it's a consequence of Thevenin's (?spel) theorem that the output
> >impedance of a potential divider is the given by the 2 resistors in
> >parallel. Here you'vr got 11500 Ohms and 61900 ohms. The equivalent
> >resistance of those in paralle is 9.7k. So the meter is 20 times as high
> >an impedance. That would give approximately a 5% error. Not that bad.
>
> Well, true if I'm right about the resistor values being 11.5k/61.9k. If
> they are 10 times higher, then I'm sunk.
>
> >I wouldn't. For one thing, desoldering SMD parts risks damaging the PCB
> >and even more so the component (most manufacturers recomend against
> >reusing desoldered SMD parts). After desoldering the value might not he
> >right anyway. For another, you don't know that all the capacitors on that
> >board should be the same value.
>
> True even with the proper (hot-air) desoldering tools? But suits my
> inclination anyway, except that I *would* like to know what that cap. is
> supposed to be.
> - Mark
>
>
>

        Actually a modified soldering iron is less harmful to PCBs (for small
parts) than a hot air tool (which may melt adjacant connectors etc) What I
have done for removing SMD caps is to take a Weller controlled temp iron tip,
hacksaw off the tip, leaving a blunt copper end. Next I tap the end of the tip
with a 4-40 tap. Now I take some 1/32 copper sheet, and bend it into a 'U'
shaped piece that fits loosely over the SMD part, contacting both solder
lands. With this tip in the Weller iron, and adequate flux and solder to make
good thermal contact with the part and card, its easy to remove the part.
        The most important thing to remember is to put _0_ force on the
circuit card when hot, this is what lifts pads (causing bad words to be said).
Both ends of the solder holding the part should be _completely_ melted before
attempting to remove the part...

        A (good) tantalum capacitor will withstand soldering temperatures
(the SMD ones can even be wave soldered) but I wouldn't reuse SMD parts on
general principles.

        BTW of the millions of SMD parts that we have used in the last few
years, I dont think we have ever seen a field failure of a resistor ( other
than mechanical damage from rough cicuit card handling or elecrical overload
from abuse or other component failure) Passives much more likely to fail are:

1. Aluminum electrolytic capacitors (in your power supply...)
2. Tantalum electrolytic capacitors
3. Ceramic high value capacitors (bypass)




Peter Wallace
Received on Mon Nov 25 2002 - 09:44:01 GMT