K2 AF Gain Modification

I love to build kits, but I couldn't pass up the great deal I got on an assembled K2 from a previous owner.  Unfortunately, because I wasn't the builder, I had no idea what was inside or how to get to anything.  After a few hours of playing with my new K2, I noticed the AF Gain control didn't seem to work like it should.  I would rotate the knob from 0 - 100, but the audio would only change from 30 - 60.  I looked through the thorough documentation I got with K2, and discovered a packet entitled, "Application Notes:  Alternative Wiring for the K2's AF GAIN Control".  While the description of the problem didn't exactly match what I was observing, it was close enough to make me think I should implement this modification.  This mod only applies to K2's below a certain serial number ( I don't have that information on hand, but I'm sure it must be on Elecraft's website).  So be sure to check into that before starting on the mod.

Elecraft has an excellent write-up on how to make the necessary modifications, but - not having built the radio myself - I didn't even know which screws to take out to remove the top cover.  Of course, all the necessary information is in the manual, but I thought it might be useful to create a supplementary document with greater detail for some of the steps.  The step numbers I refer to are identical to those on the Elecraft Application Note to make it easier to stay in sync.

The following information is in addition to the information in the Elecraft Application Note...

PREPARATION

I'm a strong believer in static prevention, so my first comment is to wear a ground strap while working on your K2.

Step 1.  Remove the K2's top cover and disconnect all option cables and the speaker.

There are (6) screws.  Starting from the front of the radio, it's the screws in the left and right corners on the top of the cover, then the two screws on the sides about halfway back, and then the two screws on the back about halfway down on each side.  See the photo for more information. Now is as good a time as any to recommend that you get some containers to put the loose hardware in.  I used some plastic cups with masking tape labels on them to tell me where the hardware came from.  For example, for this step the label says, "Top Cover".  And the cup contains six screws that hold the top cover to the radio.

Step 2.  Remove the bottom cover (but NOT the heatsink).

This one is pretty easy.  The bottom cover is the flat plate on the bottom that covers 3/4 of the bottom of the radio.  There are (6) screws:  one in each corner of the bottom plate, plus two located along the front and back edge of the cover plate, about midway between the left and right sides of the radio.

Step 3.  Remove the KAF2 board and its standoff (if applicable) and the
screws holding the Control board to the Front Panel board.

There are two screws holding the Control Board to the Front Panel Board.  On the other side of the Control Board, there are lockwashers sandwiched between the Control Board and the standoffs which are mounted to the Front Panel Board.  Make sure you retrieve these before they fall into the hardware black hole.

Step 4.  Pry up the Control board using the long-handled Allen wrench supplied with the K2.  Unplug the board.

Unfortunately, I didn't have the Allen wrench that came with the K2 kit, so I had to find one in my bag of Allen wrenches that was long enough for the job.  I used a 3/16" Allen wrench which is in an L shape with dimensions 1" x 3".  (This wrench will be used later to remove the Tuning knob.)  If you look at the Control board to the right of center (as viewed from the back of the radio), you'll see the word "Lift" stenciled on the board.  If you rest the corner of the Allen wrench on the screw just behind that "lift hole", you can pry up the Control Board.  I also lifted on the left side of the board at the same time, and it came out easily.  Incidentally, you should store this board somewhere safe so it doesn't get damaged during the modification process.  I put mine in a pink polyethelene bag that a parts order came in.  I save those bags because they're anti-static, and are a safe place to store today's electronics goodies.  The nickel plated bags are even better, if you have any.  If you don't have one of those bags, you could make a loose-fitting envelope out of tinfoil.

Step 5.

Remove the four screws holding the Front Panel assembly to the chassis and unplug it.  In this step you will remove the front panel AND the Front Panel Board together.  The four screws are located at the four corners of the Front Panel:  two on top, and two on the bottom.

Step 6.  Remove the knobs and encoder hardware and separate the front panel from it PC board.

You'll have to find the right size Allen wrench to loosen the setscrews on the knobs.  (There are TWO setscrews on each knob.)  Then remove the Tuning knob with the Allen wrench from Step 5.  Remove the felt washer from around the Tuning shaft, and then CAREFULLY loosen the hex nut holding the Tuning encoder to the Front Panel.  I say CAREFULLY because you don't want to slip and put a scratch in the Front Panel where you can see it every time you look at your beautiful K2.  The instructions don't mention this (until later), but you also need to remove that little flathead screw to the left of the encoder shaft.  Carefully remove the Front Panel Board, and store it safely in another anti-static bag.

Control Board

Step 2.  Cut the trace that connects pin 3 of U9 to pin 2 of connector P3.

Study the photos in the Application Note carefully before making any cuts.  Check and double-check that you're cutting the right trace.  There has been a discussion of various cutting methods on the QRP-L and Elecraft reflectors, but the Exacto knife method is pretty widely used.  Just take your time, and do the job with care, and you'll be fine.  I used a 3X visor during the surgery, and checked my work with a magnifying glass afterward using a small flashlight to illuminate the area from different angles to make sure I could see any residual metal traces.

Step 5.

This is a short run, and a bare resistor lead worked fine.  Just keep the wire up off the board between the two connection points.

RF Board

Step 3.  Solder one end of this wire to pin 3 of P1 as shown below.

Elecraft recommends using #24 AWG solid insulated wire.  I was able to use #22, but it would have been easier if I'd had #24 or #26 wire.  This is a longer run, and you definitely want to use insulated wire.

Step 6.  Cut pin 3's two ground traces at the indicated locations above and below the pad.

The traces you'll be cutting are VERY SHORT (maybe 1/16th of an inch long).  The traces run from pin 3 to the ground plane which surrounds the soldering pads of P1.  Once again, be sure you've got the right pad, and take your time making the cuts.  Inspect your work afterward with a magnifying glass, and verify per Step 7.

Front Panel Board

Step 3.  Cut the trace between R3's two left-most terminals.
Take your time, inspect your work, and verify per Step 4 (my measurement was 4.91k ohms).

Step 6.  Cut the ground connections to pin 3 as shown below.
This is another VERY SHORT pair of runners, similar to Step 6 on the RF Board.

Step 8.  No part of the wire should extend beyond the lower edge of the Front Panel board...

What they're saying here is that you want to keep your connecting wire "on the playing field".  Don't go "out of bounds", or the wire may short to the Front Panel when the board is installed.  See photo for further detail.

Checkout and Re-assembly

Step 2.  Install the hardware that secures the Control board to the Front Panel board...
The hardware consists of two screws that hold the Control Board to the Front Panel Board via two standoffs.  Don't forget the two lockwashers that go between the Control Board and the standoffs.  It's a little tricky to get them in.  I inserted one screw through the Control Board, and held the lockwasher with a pair of tweezers as I slid it over the screw in front of the standoff.  Get the screw started into the standoff, and then repeat this procedure with the other screw / lockwasher / standoff.  Then tighten both screws.

Step 4.  Connect a power supply, antenna, and a pair of headphones.
When I read this, I was concerned that I wouldn't be able to hear anything because my K2 has the KAT2 Antenna Tuner, and that's mounted to the top cover.  But Elecraft knows what they're doing... use the ANTENNA BNC connector located right next to the 12VDC power connector, and you will hear signals.  I did connect the cable from my SSB Board (mounted on the RF Board) to its mating connector on the Control Board thinking I needed that to get a display of my frequency.  I'm not positive this was necessary, but it doesn't hurt anything, and you'll have to connect it sometime during reassembly.

Step 6.  Verify that the AF GAIN control is functional.
I'm happy to report that my AF Gain control now works the way it should, so this was a very worthwhile modification to the K2.  The rest of the steps involve putting stuff back together, and there shouldn't be any confusion here.  Just refer to the earlier steps to see what goes where.  This is where those individual cups of screws will come in handy.

Step 15. 
In case you forgot, the two long screws go into the sides.

I hope this has been useful.  I know it would have been helpful for me because I wasn't familiar with the assembly / disassembly of the K2.  If you find any errors in these supplementary directions, please send me an email so I can make the correction.

Tnx es 73,
ed - k9ew