ELECRAFT K3 Upgrade Instructions

Introduction
While the K3's speaker and line outputs meet the original K3 specifications, we've found that further improvement in
performance is possible with minor changes. Both harmonic and intermodulation products can be reduced by 10 to 30 dB at
both outputs. These changes are already being incorporated into new K3s.
The K3AFUPKT modification kit contains the parts to make both speaker amplifier and line audio output modifications, but
you can choose to do either or both, as they are independent:

The line audio output modification is on page 8.

The speaker amplifier modification is below.

Tools Required

You will need a DMM for making resistance checks, No. 1 Phillips screwdriver, diagonal cutters, long nose pliers and a
temperature controlled ESD-safe soldering iron with rosin core small diameter solder. A grounded wrist strap and ESD
dissipating mat are recommended whenever you work inside your K3.
Observe ESD precautions when working inside your K3. Wear an ESD wrist strap or touch an unpainted,
metal ground frequently while working.
Speaker Amplifier Modification
This mod changes RFC 47, 100 µH radio frequency choke that supplies power to the K3 audio amplifier. The
original choke has a resistance of about 1.5 ohms. This resistance is high enough to cause some clipping of the audio
signal at moderate volume settings, resulting in distortion products about 40 dB down. Replacing this choke with a
physically larger unit with a resistance of about 0.2 ohms results in a 20 to 30 dB reduction in most distortion
products.
RFC47 is located on the main K3 main RF board on the right side (viewed from the front) near the front panel
shield. (See Figure 9). The replacement RFC is Elecraft part number E690017 shown in Figure 1 and Figure 10.
Choose Which Procedure You Want to Perform
Two procedures are provided for replacing the RFC. Do only one of them. Choose the procedure that best fits your
skills:
1. First is a short procedure that simply disconnects the RFC by cutting a pc board trace and adds the new
RFC across the open circuit. This is done entirely from the bottom of the K3. You need only remove the
forward part of the bottom cover.
2. The second, longer procedure avoids cutting a pc board trace by removing the original RFC. You'll need to
remove and replace the KRX3 RF module (if installed) and the right side panel to gain access to the
original RFC. A complete step-by-step procedure to do this is provided.
Some soldering is required but no work with SMDs is required by either procedure. In either procedure the new
RFC is mounted on the bottom of the RF board.
Elecraft K3 AF Stage Upgrade Instructions
(LINE OUT and Speaker Amplifier)
Revision B, November 20, 2008
Copyright © 2008, Elecraft, Inc. All Rights Reserved
Elecraft • www.elecraft.com • 831-662-8345