Page 72 - Troubleshooting Analog Circuits

Make Good Connections 59

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Figure 5.4. By using Kelvin connections you can avoid measurement errors caused by IR drops in the
circuit that you’re trying to measure, and in its connections. In this circuit, there are (at least)
four pairs of Kelvin connections.

soldered joint as it is cooling, you won’t get a cold-soldered joint. But you should
know what a cold-soldered joint looks like and how much trouble one can cause in a
critical circuit. I feel sort of sad that today’s young people aren’t building kits for
electronic equipment. In the old days, you could learn all about cold-soldered joints
before you got into industry, by building a “Heathkit” or a “Knightkit.” I built several
of each. I made a few cold-soldered joints and I had to fix them. With modem wave-
soldering equipment, it’s fairly easy to avoid cold-soldered joints in your production
line. But on hand-soldered circuits, it’s always a possibility to have cold-soldered
joints, so, if you have a nasty problem, don’t forget the old solution: Re-solder every
joint. Once in a while you’ll find a joint that never got any solder at all!
If for some reason you have acid-core solder around-it’s mainly used for
plumbing and is not found in most electronics labs, for good reason-keep it strictly
labeled and segregated from ordinary rosin-core solder. Acid will badly corrode
conductors. Also, keep specialty solders such as high-temperature solder, low-tem-
perature solder, silver solder, and aluminum solder in a separate place, to avoid con-
fusion. There is also solder for stainless steel, which requires special flux.
Recently I have heard people promoting silver-solder as a kind of superior solder
for splicing speaker cables. The “Golden Ear” set claim that this solder makes the
audio sound better. However, I must caution you that silver-solder requires rather
high temperatures, such that you need a small torch, and some messy borax flux, and
I suspect that the high temperatures will do a lot more damage to the insulation and to
the copper wire (by oxidizing it excessively) than any advantage you might get from
a “superior soldered joint.”

Make Good Connections

Printed-circuit boards aren’t the only assembled component you’ll have to contend
with while trying to make circuits work. In Tracy Kidder’s Pulitzer-Prize-winning
book, The Soul ofa New Machine (Ref. 2). one of the crucial moments occurs when
the engineers explain to a management team that their new computer has a flaw that