[Elecraft] K2 Arrives - WTCPT question, DMM Question.

[Paul Gates, KD3JF]

Glad to know Ron that you have a good soldering station I have the same one and use the 700o setting for my soldering.
 
 
Paul, KD3JF
EX: WA4JGI, WA8TER
Central Maryland
FM19qd (Map Grid Square)


----- Original Message ----
From: Ron D'Eau Claire <rondec at easystreet.com>
To: elecraft at mailman.qth.net
Sent: Friday, March 10, 2006 3:02:51 PM
Subject: RE: [Elecraft] K2 Arrives - WTCPT question, DMM Question.


DMM's

I've got few DMM's from expensive Flukes to a cheap-o that arrived free with
a parts order one time. 

Comparing them, electrically there are only two differences: speed and
number of digits. Cheap meters typically have 3-1/2 digit displays. That's
okay for some things, but I find myself wanting the resolution provided by a
full four digit display. Cheaper meters take longer to display the value -
up to 2 or 3 seconds compared to almost instantaneous display for the more
expensive units. That's a function of the chip used in the meter.

Accuracy of all of the meters is identical, within the resolution of their
displays. It's amazing how cheap and accurate the voltage divider networks
used for the range selectors are these days.

Also, unlike the 'days of yore' all modern DMMs have a rather high input
impedance, > 10 Megohms. That's essential to avoid false readings because
the meter loaded the circuit too much. We used to spend a relative fortune
on a Vacuum Tube Volt Meters to get the sort of high impedance one expects
from a DMM today. I mention it because there are some well-loved old relics
from the past sill around that use real "wiggly-needle" d'Arsonval
movements, such as the famous Simpson 260 Multimeters. Modern builders
sometimes pick up. They do not have the low input resistance of modern
DMM's. They are okay in circuits carrying significant current, but useless
in high-impedance circuits. Unless you know something about the impedance of
the circuit you're probing, they will lead you astray.

Mechanical ruggedness is another consideration. I've got a Radio Shack meter
that has taken a bit of a beating and still keeps on working just fine. But,
undeniably, the Flukes and other "high end" meters are more rugged
mechanically, especially when equipped with the various rubber boots and
other protective accessories one can buy for them. 

I do enjoy the various ancillary functions modern DMM's provide, and I'd
probably not consider a replacement that didn't include, as a minimum,
capacitance and transistor testing. One meter I have will identify the
transistor pinout and  give me the hFe (gain) measurement automatically, so
I can grab any strange unit from 'ye olde junque box' and stick it in the
meter to find out how to wire it and what it will do in most circuits. 


SOLDERING

700F (375C) is a good temperature for soldering to Elecraft pcbs most of the
time. I'd not go lower. If you are using desoldering braid or soldering to a
large pad or one connected to the ground plane, you may want more. When
hitting large pads or stuff on the ground plane, I jump the temperature to
800F (425C) or so. That's one of the great features of my Hakko 936 that I
like: change the temperature with a twist of the knob. The tip follows the
knob setting in just a couple of seconds. 

I real good clue to the proper soldering temperature is the "dwell" time.
You debond pads and traces and damage components as much by excessive dwell
time as by temperature, so a cool tip may be more dangerous than a hotter
one. About 2 seconds is the longest I'd ever stay on any normal pcb solder
pad: perhaps 3 or 4 seconds maximum on one attached to a large ground plane
or other large metal.

Ron AC7AC 

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