[Elecraft] K2 Resistor Question...

[dking at vcn.com]

Thanks to David, Don and Gary for making me feel better this morning  
about my resistors.

I have been testing each prior to installation but when came to that  
group, got lazy and decided to only check them AFTER  
installation...which then made me wish I hadn't gotten lazy!!!  Great  
object lesson here...don't take shortcuts!

It just didn't seem possible that measuring on the leads directly on  
either side of the component that   there would be that much  
resistance draw after mounting...but the fact that it was so uniform  
on both of them, plus the third one (68 ohm) being down that much as  
well made me curious!  I really appreciate the explanations...that's  
why I joined this reflector and am assembling the K2..to learn!

I'm already dreading the empty feeling that will come about when it's  
done and I don't have it to retreat to in the evenings!  But, I've got  
the SSB board setting in a box to assemble, and keep pouring over the  
options on the website, so as long as the XYL doesn't completely pull  
the purse strings shut, I'll be able to keep on building!

Thanks.

Quoting "David F. Reed" <davereed at w5sv.org>:

> David,
>
> I am not sure how best to put this...
>
> Imagine you put a resistor of infinite value in parallel with your 100
> ohm resistor and measured it at the ends of the 100 ohm resistor; of
> course, you would see the 100 ohms; what if the two resistors were 100
> ohms each? well, being in parallel, you would see 50 ohms; if you put a
> 0 ohm resistor in parallel with the 100 ohm, what would you see?
>
> so anything with some resistance to it, a resistor, a trace, a relay
> coil, will affect the resistance you measure.
>
> A nice mental exercise you can do with a handful of resistors is to
> tack solder them together into a  cube, and start measuring across
> different points; maybe even make it simple, and do just a square first
> (4 resistors); you might start to develop some intuition about how this
> works. try it with same value resistors first...
>
> I think you will find your resistors are fine; when you get to a test
> sequence in the manual, you will see they tell you what to look for in
> resistance that is quite different and that this intuition gained will
> help you feel more comfortable.
>
> Alternatively, if you are more accustomed to math, you might just learn
> some of the associated applications of Ohm's law, or do both what the
> heck...
>
> dking at vcn.com wrote:
>> R6 and R8 are both listed as 100 ohm resistors (BRN-BLK-BRN), but   
>> after installation, both show 61.6 ohms on three different   
>> DMM's..while R7 which is a 68 ohm resistor is showing 50.1
>>
>> I recognize all of them read slightly lower than their "listed"   
>> values...but these seemed at such wide variance I didn't want to   
>> continue until I knew whether I should be replacing them or not.
>>
>> These three resistors all form a U around relay K12...so I didn't   
>> know if the relay would be sucking down my readings...but I'm   
>> placing the probe on the resistor wire on either side of the body   
>> of the resistor...so I can't see how something else could be   
>> causing that great of a difference....
>
> the rest of the circuit that is in parallel with the resistor is
> responsible for that variation.
>
>> Another identical 100 ohm resister still in the paper ladder to be   
>> installed later in the sequence is giving me a 97 ohm reading...
>>
>
> And if you removed the ones you already soldered in, they would measure
> close to the same 100 ohms (or 97 in your case.
>
>> Any advice would be appreciated.
>>
>> David King
>> KE7EKA
>> K2 #6048
>
>
> 73 de Dave, W5SV



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