[Elecraft] [K3] KRX3 Installation R91 mod

[Ron D'Eau Claire]

If you've got the right resistance across R91, you're good!

Golly Dave, never be embarrassed to stumble over a problem. To paraphrase an
old saying, "If you haven't damaged a trace, you haven't worked on enough pc
boards..." 

That pad was chosen because trying to solder directly to the IC with it's
very close spaced leads is much harder, so you're soldering skills are quite
redeemed for having succeeded! 

You bring to the group a good lesson though: work for performance, not
"pretty". When reworking boards it's quite easy to get all caught up in a
beautiful solder joint that looks like the original production soldering
(which was done by machines on perfect boards) and end up messing about too
much. In your case the only damage was minor to a trace. Too much heat can
damage I.C.s and even resistors like the one shown.

You don't always get really smooth flow all over the pad. All you care about
is good flow into the existing solder joint. Note the left end of the added
resistor. You can see that the solder there did not completely flow to the
edge of the pad although it flowed well into the existing solder. Leaving it
like that comes directly from experiences like yours :-)

Here are few things for everyone to be aware of when doing changes like
this:

1) If you notice that you're having more trouble than you recall in the past
getting good solder flow, check your iron. Many temperature-controlled irons
are actually measuring the temperature of the heating element and corrosion
building up where the tip connects can drastically reduce the amount of heat
reaching the tip. As you know, if it takes more than two or three seconds
for the solder to melt, the tip is not hot enough, no matter what the
temperature setting says. 

2) A tiny dab of solder added to the old joint and fresh "tinning" of the
new lead before trying to join them help a lot. You end up "sweating" the
two fresh solder joints together when you connect and heat them. You still
need a little fresh solder, but more for its rosin to deoxidize the surfaces
than for additional solder. 

3) Solder does not smoke. The puff of smoke that commonly rises when
soldering a joint is the rosin burning away. When that happens there's no
more rosin cleaning the surfaces. If the solder is molten, fine, but if not
there's nothing left to clean it until you add more solder and rosin. That's
why it's so easy to get too much solder on small joints and why it's so
important to use fine solder. I generally use 0.015" solder for this work,
and *never* anything bigger that 0.031" solder.  

3) A very small amount of rosin is very handy if you want to avoid adding
solder. Rosin pens that dispense a tiny drop are perfect for this but even a
bottle is fine if you use a tooth pick or bit of wire to add a drop just
before soldering. Rosin in pens and bottles are available from many on-line
resources and (for the lucky ones with one nearby) most electronics supply
houses. 

Ron AC7AC




-----Original Message-----
From: Dave Van Wallaghen [mailto:w8fgu at comcast.net] 
Sent: Saturday, January 24, 2009 6:14 AM
To: elecraft at mailman.qth.net
Cc: Ron D'Eau Claire
Subject: [K3] KRX3 Installation R91 mod

Hi all,
At first, I was only going to contact Ron with this as I was too 
embarrassed to reveal that I had maimed the RF board on my K3. But, I 
then thought maybe someone else could benefit from my experience and 
public humility ;-)

I have an early serial number #338 and needed to make the modification 
to R91 on the RF board as described on page 21 of the KRX3 Installation 
Manual. In doing so, I was having a hard time getting the 27 ohm 
resistor lead to tack onto the (looking at Figure 20) right pad of R91. 
I made multiple attempts and couldn't get a clean connection until I 
eventually lifted the pad. I was using a small but short soldering tip 
at 720 degrees. In my zeal to get a nicer connection, I simply played 
around with it too long and with too much heat, lifted the pad. Using a 
longer, narrower tip (as I am now) would probably have worked better.

After the horror and high emotion subsided, I sat down to figure out 
what could be done and hence a question as to my solution. Looking at 
the schematic for the RF board, it looks like the only connection to the 
right pad of R91 is pin 14 of U7. Does that seem correct to anyone else? 
I removed R91 and was able to solder a 22 ohm 1/8 watt resistor to the 
remaining trace of pin 14 of U7 and the pad next to R91 as outlined in 
the directions. DMM measurements between U7 pin 14 and the left pad of 
R91 show 22 ohms, so I'm pretty sure I was able to salvage the situation.

I will continue with the installation this morning, but would like some 
concurrence that what I've done looks correct to someone else's eyes 
before I power this beast back up.

BTW - Since I was going to install the KRX3, I decided to make the mods 
for the K3 Synthesizer ALS, Rear RS232 and Audio RF mods, K3 Amp Output 
Mod and the Front Panel Mic Mod. Having to somewhat disassemble the unit 
for the KRX3 anyway made doing these mods very simple. And as usual, 
Ron's documentation for these mods was nothing short of excellent, 
including some nice tips for lining things back up to guarantee all 
connectors and boards line up the way they are supposed to. As usual, my 
impatience leads me to push ahead of the text to get the job done, but 
if you follow everything Ron has written you will be guaranteed success.

Also, in my original K3 installation manual, the first KSYN3 board was 
not mounted with split lock washers on the non-component side of the 
board. Since I made the mod to it, I did remount it with the split lock 
washers as described in the KRX3 manual. I assume there is no problem 
with that?

Now, back to work...
Dave W8FGU