SERVICING

 

 

 

 

 

 

 

 

 

 

U804

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

14.850 MHz

Synthesizer IC

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

TCXO

 

 

 

 

 

 

 

 

 

 

 

 

 

Buffer

 

 

 

 

 

 

 

 

 

 

 

 

fR

 

 

VCO

 

 

 

Amplifier

 

 

 

U806

 

 

 

 

 

 

Reference

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Counter

 

 

 

 

Control

VCO

 

 

 

 

To Rx

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Phase

 

 

 

 

Module

 

 

Q801

 

and Tx

 

 

 

 

 

 

 

 

 

 

 

 

fV

Detector

 

 

 

 

 

 

 

 

 

 

 

 

¸ 64/65

 

 

N

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Prescaler

 

 

Counter

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Prescaler

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Control

 

 

 

 

Lock

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Detect

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 4-1 Simplified Synthesizer Block Diagram

Tantalum SMD Capacitors (P.N. 510-26xx-xxx)

Tantalum SMD capacitor identification varies with vendor and physical size of the capacitor. The positive (+) end is usually indicated by a colored band or beveled edge. The value and voltage may be indicated by printing on the capacitor or by using a special code.

4.2.4 SMD INDUCTOR IDENTIFICATION

SMD inductors (P.N. 542-9000-xxx) use three colored dots to indicate the value. The two dots on the left side indicate the first and second digits of the value in nanohenries, and the single dot on the right side indicates the multiplier (see Table 4-2). For example, brown, black, and red dots indicate a value of 10 nH x 100 which is 1000 nH (1.0 µH). The last three digits of the part number are also the value and multiplier.

4.2.5 TRANSISTOR/DIODE IDENTIFICATION

Surface mounted transistors and diodes are identified by a special number. Refer to page 6-1 for more information.

4.3 SYNTHESIZER TROUBLESHOOTING

4.3.1 INTRODUCTION

When there is a synthesizer malfunction, the VCO is usually not locked on frequency. When the VCO is unlocked, the LOCK output on U804, pin 18,

Table 4-2 SMD Inductor Identification

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Color

1st Digit

2nd Digit

 

 

Multiplier

(Last PN Digit)

 

 

 

 

 

 

 

 

 

 

 

 

 

Black

0

 

0

 

1 (7)

 

 

Brown

1

 

1

 

10 (8)

 

Red

2

 

2

 

100 (9)

 

Orange

3

 

3

 

1000 (0)

 

Yellow

4

 

4

 

10,000 (1)

Green

5

 

5

 

100,000 (2)

Blue

6

 

6

 

---

 

 

Violet

7

 

7

 

---

 

 

Gray

8

 

8

 

---

 

 

White

9

 

9

 

0.1 (6)

 

is low. This is detected by the logic which then disables both the transmitter and receiver.

When the VCO is unlocked, the fR and fV inputs to the phase detector in U804 are usually not the same frequency (see Figure 4-1). The phase detector in U804 then causes the VCO control voltage to go to the high or low end of its operating range. This, in turn, causes the VCO to oscillate at the high or low end of its frequency range.

As shown in Figure 4-1, a loop is formed by the VCO and the prescaler, N counter, and phase detector in U804. Therefore, if any of these components mal- function, improper signals appear throughout the loop. However, correct operation of the counters can still be verified by measuring the input and output frequencies to check the divide number. Proceed as follows:

 

February 2001

4-3

Part No. 001-9800-001