Ramsey Electronics PR20 Bedienungsanleitung

PR20—1

220 MHz

LOW NOISE

PREAMP

KIT

•16 db Gain, 1 db Noise Figure

•Runs on 9 to 15 VDC

•Hi-Q tuned input to reduce

overload

•Small size to fit in any rig

•The most popular preamp kit in

the USA!

•Easy 15 minute assembly

Perk up your reception on 220 MHz with this easy-to-build and install kit.

Works well with any radio - especially effective in repeaters. Rugged design

using a microwave transistor outperforms units costing triple the price!

Kit No. PR20

PR20—2

INTRODUCTION TO THE PR20 LOW NOISE PREAMPLIFIER

A “preamplifier” gives an initial boost, measured in decibels, to the level of a signal before it

is further processed by tuned circuits, filters, amplifiers, mixers, detectors, or other circuit

stages. Preamps may be designed for either RF or audio use and may be either tuned or

broadbanded.

The Ramsey Low Noise PR10, 20, and 40 Preamplifiers for 144, 220, and 440 MHz,

respectively, differ from our popular SA7 and PR2 broadband preamplifiers in that these

models include high-Q tuned input circuits to pre-select the desired frequency range,

restricting the strength of other out-of-band signals. Also, the microwave transistor is biased

for best low noise performance.

PR20—3

CIRCUIT DESCRIPTION

L1 and C2 form a tuned input filter for the preamp to reject out-of-band signals. The 3 db

bandwidth of this filter is 6MHz. L1 is adjustable for peak performance. The 2SC2498

transistor is a bipolar silicon NPN low noise UHF/VHF amplifier in a TO-92 case suitable for

service up to 5 GHz, with maximum device dissipation of 600 milliwatts. Q1 has a very low

noise figure, determined by the bias established by R1 and R2. The actual circuit is about

as simple as they come. Maximum gain and freedom from oscillation are accomplished by

the quality of workmanship in parts installation and I/O connector wiring

PR20—4

PARTS SUPPLIED WITH PR20 KIT

CAPACITORS

2 2 or 2.2pF disc capacitor (C1,2)

1 10pF disc capacitor (C3)

1 100pF disc capacitor [marked 100 or 101] (C4)

1 .01uF disc capacitor [may be marked .01, 103, or 10nF] (C5)

RESISTORS

1 470 ohm [yellow-violet-brown] (R1)

1 100K ohm [brown-black-yellow] (R2)

OTHER COMPONENTS

1 PRX printed circuit board

1 shielded adjustable coil (L1) [marked 359-7787]

1 2SC2498 or 2570 microwave bipolar transistor (Q1)

REQUIRED, NOT SUPPLIED

9 - 15 VDC power source

RF input and output connectors (your choice of type)

PR20—5

RAMSEY PR20 PRE-AMP PARTS LAYOUT

NOTE FOR PR-20:

Disregard C6

PR20—6

PR20 PRE-AMP ASSEMBLY PROCEDURE

Soldering nine parts to a PC board is not a big job, but good performance of the PR20

requires attention to keeping all leads as short and neat as possible. The order of parts

installation is not critical for this and other small kits, so the purpose of the following

suggested order of assembly is just to make it as easy as possible for you to get the parts

placed correctly on the first try. For your convenience, the PC board is considerably

enlarged. While working with a small PC board such as the PR20, a miniature vise plus a

magnifier may prove very helpful.

In all steps below, the word install means to insert the component (oriented correctly) into

the right holes, solder all wires, and cut or nip away excess wire flush with the solder

connection. IMPORTANT: Keep ALL component leads as short as possible!

1. Install R1, 470 ohm (yellow-violet-brown). Notice from the schematic that R1

determines Q1’s collector voltage and therefore the bias voltage fed by R2.

2. Install R2, 100K ohms (brown-black-yellow).

PR20—7

3. Install C4, 100pF (marked 100 or 101).

4. Install C5, .01uF (marked .01 or 103 or 10nF). Disc capacitor C5 is the bypass

capacitor for the DC voltage supply points on the PC board.

5. Install Q1, 2SC2498. Press Q1 into its respective location on the PC board, which

should be quite obvious by the placement of the previous parts. Orient the flat side

as shown on the PC board illustration. Press the transistor in place firmly with

finger pressure so that a minimum amount of wire leads are exposed above the PC

board. In soldering, do not be afraid to apply enough heat to make good, clean, solid

connections.

6. Install C3, 10pF (marked 10).

7. Install C2, 2 or 2.2pF (marked 2 or 2.2). C2 and L1 form the tuned input filter.

8. Install C1, 2 or 2.2pF (marked 2 or 2.2). C1 is the antenna input coupling capacitor.

9. Install L1, the shielded coil. L1 fits only one way, with the three thin leads oriented

toward Q1. This coil has metal mounting tabs; use plenty of heat to solder these.

PR20—8

two mechanical points after making sure that L1 is pressed firmly against the

component side of the PC board.

PROGRESS SUMMARY: The wiring of your Ramsey PR series preamp PC board is now

complete. It will take you less than a minute to completely recheck every single soldering

connection as well as for the appropriate part installations.

Correct parts placement checked.

PC board soldering checked...touched up if needed.

INPUT AND OUTPUT CONNECTIONS

Use short pieces of miniature coax such as RG-174 to connect the PC board input and

output points to the RF connectors you wish to use. For the neatest installation, it is good

practice to pre-tin the coax ends before soldering to the PC board. Plan your probable

applications with as much foresight as possible so that you don’t spend more on cable

adapters than you did on the kit!

PR20—9

TESTING AND ADJUSTMENT

With your preamp installed between antenna and receiver, the only adjustment needed is the

obvious one: peaking L1 for maximum signal strength. This may require more patience than

most of us would think, because the adjustment should be made on a weak signal

transmitting long enough to permit the adjustment. Since this is a low noise preamp, don’t

look for an imaginary increase in noise level or waste time trying to make a strong repeater

signal seem stronger!

CAUTION

Under no circumstances should transmitter RF ever be applied to the input or output of this

preamplifier. Even though this sounds like unneeded advice and common-sense knowledge,

we should remember that most of today’s receivers are in transceiver configuration, and that

all it takes is one accidental push of a button out of habit, or an inadvertent bump of a keyer,

to destroy a preamplifier.

PR20—10

PR20 ON-OFF SWITCHING SUGGESTION

Depending on your particular application and intended frequency, you may wish to use a

DPDT power switch with one pole controlling voltage and the other pole connecting the

input and output of the PR20 when the DC supply voltage is in the “OFF” position. This

setup is very practical when you want to switch the preamp in and out of a receiver antenna

line. ALL RF connections to the switch should be as short as possible. Use good quality

miniature coaxial cable.

STRONG SIGNAL OVER-LOADING

If you live in a situation where there are very strong nearby stations, you may experience

overloading of the receiver to which the preamp is connected. This is especially true for

small hand-held radios - after all, what do you think they leave out to make the size small?

Front-end filtering components are large, but they serve a needed purpose in high RF

environments. Unfortunately, there is little you can do except to add the missing filtering.

Digi-Key (1-800-digi-key) sells the TOKO line of helical filters which are excellent. Expect to

pay about twice as much as you paid for this preamp!

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