Elecraft K2VCOSHLDKT User Manual

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Elecraft K 2 V CO S h ielding I n struct i o n s

Rev. E, Oct. 6, 2005

The K2’s VCO (voltage-controlled oscillator) may be susceptible to low-frequency modulation by nearby large power transformers,
such as those found in high-current linear power supplies and antenna rotator controllers. Since the magnetic field from a
transformer falls off rapidly with distance, such modulation can usually be eliminated by simply moving the equipment away from
the transceiver (or moving it to the right side of the transceiver, farther from the VCO circuitry). In difficult cases, magnetic
shielding can be used, alone or in combination with repositioning of equipment.

These instructions describe how magnetic shielding can be added to the K2. Toroidal transformer T5 must be replaced with a
smaller iron-powder core that can be shielded by a high-permeability ferrite sleeve. In addition, slug-tuned inductor L30 must be
electrically disabled since it, too, is affected by magnetic fields. With L30 out of the circuit, the VCO inductance can still be
adjusted, if necessary, by squeezing or spreading the turns of T5. A DMM (digital multimeter) is used during this adjustment.

Parts Inventory (Order #K2VCOSHLDKT)

(1) E680013 T37-6 toroid core (yellow)
(1) E680024 Ferrite toroidal sleeve (dark gray), 16 mm OD, 12 mm ID, 8 mm tall (0.63 x 0.50 x 0.32”)
(1) E500003 470 ohm, 1/4-watt resistor
(2) E700021 4-40 nylon hex nut
(1) E700107 4-40 x 5/8” (16 mm) nylon machine screw
(1) E700108 Nylon flat washer, 5/8" (16 mm) diameter
(1) E700109 Paper flat washer, 5/8" (16 mm) diameter
(2 ft.) E760002 #26 red enamel wire
(1 ft.) E760004 #26 green enamel wire

Modification Instructions

 Turn off the K2 (or K2/100) and disconnect all external equipment.

 Remove the top cover, disconnecting all cables.

 Remove the bottom cover (flat part only — not the heatsink/rear panel).

 Locate slug-tuned inductor L30, which is near the left side panel and adjacent to toroidal transformer T5. In the next step, L30

will be isolated from the VCO, so it can no longer be used to adjust the VCO frequency. The new method is explained later.

 Find L30’s pads on the bottom side of the RF board. Cut the trace identified in Figure 1 that connects L30 to pin 2 of T5. Use

the tip of an Xacto knife or other sharp tool. Inspect the trace using a magnifying glass to ensure that it is cut.

Figure 1

 Locate R10 (top side of the RF board, near L30). Remove R10, being careful not to damage the pads. Re-install R10 on the

bottom side of the RF board, using the same pads. (If R10 is damaged, use the 470-ohm resistor supplied with this kit.)

 Locate T5 (near L30). Remove the nylon hardware holding T5 to the RF board.

 Unsolder all four leads of T5 and remove it, being very careful not to damage the pads or traces. One method is to hold a lead

with long-nose pliers, heat the joint completely, then pull the lead out slowly.

 Clean T5’s four holes out completely using solder wick or a vacuum desoldering tool.

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 Locate the T37-6 core (yellow), which is smaller than the original T50-6 core. It will be used to wind a new T5. As shown in

Figure 2, T5 will have two windings, identified as leads 1-2 (red wire) and 3-4 (green wire).

Figure 2

Green,

4 turns

1

2

3

4

Red,

17 turns

 Cut a 14-inch (35 cm) length of red enamel wire, and an 8-inch (20 cm) length of green.

 For the 1-2 winding, use 17 turns of red enamel wire. (Each pass through the core counts as one turn.) Wind the wire tightly

against the core. Leave the leads at least 1” (2.5 cm) long, and do not strip the enamel insulation off yet.

 Examine the 1-2 winding closely. As shown in Figure 2, lead #2 must exit from below the core, and lead #1 from above the

core. If this is not the case, remove the wire and correct the winding sense.

 Distribute the 17 turns of the 1-2 winding evenly over about 90% of the core as shown.

 For the 3-4 winding, use 4 turns of green enamel wire, positioned as in Figure 2. Wind the wire tightly against the core. Leave

the leads about 1” (2.5 cm) long and do not strip them yet.

 Examine the 3-4 winding for correct sense as you did for the 1-2 winding. Lead #4 must exit from below the core.

 Trim all four leads of T5 to about 1 inch (2.5 cm) long.

 Referring to Figure 3, insert the 5/8” (16 mm) nylon screw through the hole at the center of T5 from the bottom side of the RF

board. Secure the screw to the RF board using a nylon hex nut on the top side.

Top side of RF board

Figure 3 Figure 4

 Install T5 temporarily (Figure 4), inserting the leads into their numbered pads on the RF board. The nylon screw should pass

through T5. Press down until T5 rests on the nylon nut.

 Pull the leads through on the bottom, leaving a small amount of slack on the top as shown in Figure 4. The reason for the slack

is to accommodate the paper insulating washer, which will be added in a later step (see Figure 5, next page).

 Using a black felt pen, mark the point on each lead where it enters its pad on the top side of the board. Then remove T5.

 Remove the insulation from T5’s leads up to approximately the marked locations, then tin the leads. The enamel wire is heat-

strippable, so this can be done using a soldering iron. Additional information on stripping toroid leads can be found in the
“Assembly, Part II” section of your K2 owner’s manual; see Stripping Toroid Leads (page 54 in the latest revision).

 Re-install T5 (see Figure 4).