Cobra CT6 Specifications download pdf

CONSTRUCTION & INSTALLATION

EXPANDER 240 ELECTRONIC CRYSTAL SWITCH

INTRODUCTION

The EXPANDER 240 is a compact crystal switch which allows up to six synthesizer mixing crystals to be remotely

selected. Its purpose is to provide new injection signals for loop mixing in PLL synthesizer circuits, or additional

mixing frequencies for older 23-channel type CBs using crystal synthesizers. Each crystal position includes a

series trimmer capacitor for exact frequency adjustment if needed. The appropriate mixing crystal is removed

from the radio, and installed in the EXPANDER 240 PC board instead. Two wires from the EXPANDER 240 are

installed in the empty crystal hole in place of the removed crystal. Crystal selection is made electronically

through diodes. This eliminates the problem of stray wire capacitance often occurring in mechanical switching

devices. Because of this advantage, the crystal PC board can be placed physically close to the required circuit,

while the switching wires can be made long enough to reach the actual switch mounted elsewhere on the chassis.

(Often a relatively long distance from the PC board.)

Generally speaking, any PLL type radio using a loop mixing frequency can be expanded by 40 channels per mixing

crystal (or 23 channels per crystal, in some early PLL circuits), up to the limit of the CB’s tuning circuits. Thus a

typical American 40-channel CB can be made to work on 80, 120, or even 160 total consecutive channels in some

cases. For the older 23-channel crystal-synthesized radios, the extra coverage will be 20 channels, for a total of

43 consecutive channels.

CIRCUIT DESCRIPTION

The electrical operation is very simple. Refer to the Schematic Diagram, Page 9. Wires from holes “A” and “B”

are connected in place of the crystal that was removed from the radio. A regulated DC voltage of +7-10 VDC

from the radio is applied through the user’s switch. The associated diode (D1-D6) will find a path to ground via

R1 and L7 and conduct. R1 and L7 are common to all switching diodes and are used for RF isolation. In addition

each diode is further decoupled by C1-C6 and L1-L6. When the diode conducts, there will be RF continuity from

hole “B” through C7, the associated diode, the series capacitor CT1-CT6, the crystal, and finally to hole “A.”

NOTE: Either a series capacitor or a bare jumper wire must be installed in the holes marked

“CT1-CT6.” Otherwise there would be no RF continuity to ground, and the circuit

wouldn’t oscillate. Since the trimmer capacitors are the most expensive parts in this kit,

and since few American CBs can cover more than 120 channels anyway, only three

trimmers are provided. They often aren’t even needed, and in most cases a bare jumper

wire will work. If a crystal does need trimming, try a fixed ceramic disc capacitor of

about 10-68 pF in these holes first. (Extra trimmers are available from us at $1 each.)

Published by:

CBC INTERNATIONAL • P.O. BOX 30655 • TUCSON AZ 85751 U.S.A.

TEL/FAX: 888-I-FIX-CBs (1-888-434-9227), (520) 298-7980

Internet: www.cbcintl.com • Email: [email protected]

1 2 3 4 5 6 7 8 9 10

Summary of Contents

Page 1

CONSTRUCTION & INSTALLATION EXPANDER 240 ELECTRONIC CRYSTAL SWITCH INTRODUCTION The EXPANDER 240 is a compact crystal switch which allows up

Page 2 - ASSEMBLY INSTRUCTIONS

___________________________________________________________________________________________________ EXPANDER 240 PAGE 10

Page 3 - PARTS SUPPLIERS

ASSEMBLY INSTRUCTIONS PLEASE READ CAREFULLY BEFORE PROCEEDING! NOTICE This device will operate as advertised when properly constructed, installed,

Page 4 - INSTALLATION

CRYSTAL ORDERING The following specifications should be observed when ordering crystals. FREQUENCY: As desired. ACCURACY: .005% or better. HOLDE

Page 5

INSTALLATION NOTE: The radio’s tuned circuits may need realignment after installation of the EXPANDER 240. Much depends upon the desired center

Page 6

EXAMPLE A: Cobra 29 AM chassis. This uses the standard 23 MHz and 14 MHz mixers, with the 11.730 MHz RX and 11.275 MHz TX oscillators. There are s

Page 7 - Internet:

originals, plus six to fit in the EXPANDER 240. Obviously, you could choose anything from 20-43 channels, depending upon how much you’re willing

Page 8

TYPE 3 INSTALLATION — All PLL AM or AM/SSB with Direct Frequency Mixing See FIGURE 1, TYPE 3. The oscillator output is coupled through a small disc

Page 9

C1-C6 = .01 µF ceramic disc capacitor (Radio Shack #272-131 or #272-1065) C7 = .001 µF ceramic disc capacitor (Radio Shack #272-126)

Page 10 - EXPANDER 240 PAGE 10

___________________________________________________________________________________________________ EXPANDER 240 PAGE 9

Cobra CT6 Specifications

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Cobra CT6 Specifications

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