[Elecraft] [K2] RF Probe and Voltmeter

[Don Wilhelm]

There are no explicit instructions for the RF Probe, but the 3 
components needed are shown on the silkscreen of the board - install 
them as indicated on the board.
Let me try to give you some written instructions for the tip and the 
cables.  I do not have a photo handy, but maybe someone else can offer a 
photo.
You can use one of the heavy leads from the D10 diode for the probe 
tip.  If you discarded that, use a piece of stiff wire, #14 copper wire 
will give good stability. strip a 1 inch long piece out of a piece of 
Romex cable.  Bend the tip so it is parallel with the center of the board.
Cut a length of the stranded black wire about 8 inches long and solder 
it to the ground marking next to the tip.  Solder the Alligator clip on 
the other end of that wire.

Strip the end of the RG174 coax and comb the wires of the shield. Solder 
the center conductor and the shield where indicated on the RF Probe 
board and secure the coax with a Ty-Wrap.
The other end of the coax must be stripped and the banana plugs attached 
to connect it to your DMM.
I usually strip the coax braid back about 1.25 inch and comb the shield 
strands.  Take about half of the shield strands and solder a 6 inch 
length of the black wire to the twisted shield strands.  Fold the shield 
strands and black wire back along the length of the coax.  If you want 
to make it neat, put a bit of heatshrink tubing over the exposed shield 
and shrink it.  Now put the red top to the banana plug over the coax and 
heatshrink, then solder the center conductor to the banana plug.  Screw 
the red cap onto the plug.
At the end of the black wire, solder the black banana plug - be sure to 
slide the black top on before soldering.

You do not have to assemble the voltmeter probe.  It might come in handy 
if you are in the field without your DMM, but other than that, it has 
little use when you have a DMM available.

Since you are just getting familiar with your DMM I can give you some 
hints - Usually no more than 2 significant digits are useful, even if 
your DMM displays 3 1/2 or 4 1/2 digits (I even have one with 5 1/2 
digits).  On those rare occasions where you want precision measurements 
(such as when measuring a 1% resistor or the voltage out of a precision 
voltage regulator), the first 2 non-zero digits are enough.  Most 
measurements listed for the K2 have a 10% tolerance, so if the 
measurement is within 10%, it is good to go. Be aware of the scale.

Check what your DMM indicates for a resistance reading with the probes 
not touching anything - that is infinite resistance (or an open 
circuit).  Different DMMs display that differently.  Mine uses a 
leftmost digit "1", others insert a "+" sign to indicate overrange, 
while some will simply show a blank screen.

If you get strange readings or ones that bounce from one value to 
another, that might be caused by the autoranging mechanism - switch it 
to a manual range for a sanity check.

73,
Don W3FPR

On 3/16/2014 5:33 PM, Lane wrote:
> Hi all,
>
> Please forgive the less than amateur with these questions.
>
> I am at the point just after the testing to include power-up, encoder, and
> relays. All worked fine.
>
> I'm on pg. 44 and trying to do the RF probe assembly. So I go to pg. 9 of
> appendix E ... and find I'm not really sure what I'm doing.
>
> I was unable to find any online photos of the probe and was wondering if
> anyone might have a photo. That would go a long way in helping me confirm
> how I think this should go together. This part of the instructions aren't
> as detailed as the K2 itself.
>
> Also, I'd like to verify that I do not need to build the voltmeter probe
> assembly if I have a DMM, which is also on pg. 44. Or are there other
> benefits in simplicity to using this voltmeter probe for someone that is
> still learning how to operate a DMM?
>
>