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ORDER NO. KM41304679CE

Digital Cordless Phone
Digital Cordless Answering System

B : Black Version

(for Kazakhstan)
(for Tajikistan)
(for Uzbekistan)
(for Turkmenistan)
(for Kyrgyzstan)

Model No

Base Unit

Handset

Charger Unit Expandable

1 (TG7851)
1 (TG7861)
1 (TG7861)

1 (TGA785)
1 (TGA785)
2 (TGA785)

1

Up to 6
Up to 6

KX-TG7851
KX-TG7861
KX-TG7862
KX-TGA785*

*KX-TGA785 is an optional accessory, which contains a 
handset and a charger.

1 (TGA785)

1

(Charger Unit)

KX-TGA785

(Handset)

KX-TG7851

(Base Unit)

KX-TG7861
(Base Unit)

2

This service information is designed for experienced repair technicians only and is not designed for use by the general 
public. It does not contain warnings or cautions to advise non-technical individuals of potential dangers in attempting 
to service a product. Products powered by electricity should be serviced or repaired only by experienced professional 
technicians. Any attempt to service or repair the product or products dealt with in this service information by anyone 
else could result in serious injury or death.

There are special components used in this equipment which are important for safety. These parts are marked by 
in the Schematic Diagrams, Circuit Board Diagrams, Exploded Views and Replacement Parts List. It is essential that 
these critical parts should be replaced with manufacturer's specified parts to prevent shock, fire or other hazards.
Do not modify the original design without permission of manufacturer.

If lead free solder was used in the manufacture of this product, the printed circuit boards will be marked PbF.
Standard leaded, (Pb), solder can be used as usual on boards without the PbF mark.
When this mark does appear, please read and follow the special instructions described in this manual on the 
use of PbF and how it might be permissible to use Pb solder during service and repair work.

L When you note the serial number, write down all 11 digits. The serial number may be found on the bottom of the unit. 
L The illustrations in this Service Manual may vary slightly from the actual product.

3

PAGE

PAGE

1 Safety Precautions----------------------------------------------- 5

1.1. For Service Technicians --------------------------------- 5

2 Warning -------------------------------------------------------------- 5

2.1. Battery Caution--------------------------------------------- 5
2.2. About Lead Free Solder (Pbf:Pb free)---------------- 5

2.2.1. Suggested PbF Solder ------------------------------ 6

2.3. Discarding of P.C. Board--------------------------------- 6

3 Specifications ----------------------------------------------------- 7
4 Technical Descriptions ----------------------------------------- 8

4.1. Block Diagram (Base Unit) ------------------------------ 8
4.2. Circuit Operation (Base Unit) --------------------------- 9

4.2.1. Outline --------------------------------------------------- 9

4.3. Block Diagram (Base Unit RF Part)------------------10
4.4. Circuit Operation (Base Unit) -------------------------- 11

4.4.1. BBIC (Base Band IC: IC501) --------------------- 11
4.4.2. Flash Memory (IC601) (for KX-TG7861

only) ----------------------------------------------------- 11

4.4.3. EEPROM (IC611) ------------------------------------ 11
4.4.4. Power Supply Circuit--------------------------------12
4.4.5. Telephone Line Interface---------------------------13
4.4.6. Transmitter/Receiver--------------------------------13

4.5. Block Diagram (Handset)-------------------------------14
4.6. Circuit Operation (Handset)----------------------------15

4.6.1. Outline --------------------------------------------------15
4.6.2. Power Supply Circuit/Reset Circuit--------------15
4.6.3. Charge Circuit ----------------------------------------15
4.6.4. Battery Low/Power Down Detector--------------15
4.6.5. Speakerphone ----------------------------------------15

4.7. Signal Route -----------------------------------------------16

5 Location of Controls and Components ------------------18
6 Installation Instructions ---------------------------------------18
7 Operating Instructions-----------------------------------------18

7.1. For Service Hint-------------------------------------------18

8 Service Mode -----------------------------------------------------19

8.1. Engineering Mode ----------------------------------------19

8.1.1. Base Unit ----------------------------------------------19
8.1.2. Handset ------------------------------------------------21

8.2. EEPROM LAYOUT (Handset)-------------------------23

8.2.1. Scope ---------------------------------------------------23
8.2.2. Introduction --------------------------------------------23
8.2.3. EEPROM contents ----------------------------------23

8.3. How to Clear User Setting------------------------------24

8.3.1. Resetting both base unit and handset----------24
8.3.2. Resetting only handset -----------------------------24

9 Troubleshooting Guide ----------------------------------------25

9.1. Troubleshooting Flowchart -----------------------------25

9.1.1. Check Power------------------------------------------26
9.1.2. Check Battery Charge ------------------------------27
9.1.3. Check Link---------------------------------------------28
9.1.4. Check the RF part -----------------------------------31
9.1.5. Check Handset Transmission --------------------34
9.1.6. Check Handset Reception-------------------------34
9.1.7. Check Caller ID --------------------------------------34
9.1.8. Bell Reception ----------------------------------------35
9.1.9. Check TAM Operation (for KX-TG7861

only) -----------------------------------------------------35

9.2. Troubleshooting by Symptom (Base Unit and

Charger Unit) ----------------------------------------------36

9.2.1. Check Point (Base Unit) --------------------------- 36

9.3. Troubleshooting by Symptom (Handset) ----------- 38

9.3.1. Check Point (Handset) ----------------------------- 38
9.3.2. Troubleshooting for Speakerphone ------------- 41

10 Disassembly and Assembly Instructions--------------- 42

10.1. Disassembly Instructions------------------------------- 42

10.1.1. Base Unit ---------------------------------------------- 42
10.1.2. Handset ------------------------------------------------ 45
10.1.3. Charger Unit ------------------------------------------ 46

10.2. How to Replace the Handset LCD------------------- 47

11 Measurements and Adjustments -------------------------- 48

11.1. Equipment Required ------------------------------------ 48
11.2. The Setting Method of JIG (Base Unit) ------------- 48

11.2.1. Connections ------------------------------------------ 48
11.2.2. How to install Batch file into P.C. ---------------- 49
11.2.3. Commands-------------------------------------------- 49

11.3. Adjustment Standard (Base Unit) -------------------- 50

11.3.1. Bottom View ------------------------------------------ 50

11.4. The Setting Method of JIG (Handset)--------------- 51

11.4.1. Connections ------------------------------------------ 51
11.4.2. How to install Batch file into P.C. ---------------- 52
11.4.3. Commands-------------------------------------------- 52

11.5. Adjustment Standard (Handset) ---------------------- 53

11.5.1. Component View ------------------------------------ 53

11.6. Things to Do after Replacing IC or X'tal ------------ 54

11.6.1. How to download the data ------------------------ 54

11.7. RF Specification ------------------------------------------ 56

11.7.1. Base Unit ---------------------------------------------- 56
11.7.2. Handset ------------------------------------------------ 56

11.8. How to Check the Handset Speaker or

Receiver ---------------------------------------------------- 57

11.9. Frequency Table (MHz)--------------------------------- 57

12 Miscellaneous---------------------------------------------------- 58

12.1. How to Replace the Flat Package IC --------------- 58

12.1.1. Preparation-------------------------------------------- 58
12.1.2. How to Remove the IC ----------------------------- 58
12.1.3. How to Install the IC -------------------------------- 59
12.1.4. How to Remove a Solder Bridge ---------------- 59

12.2. How to Replace the Shield Case--------------------- 60

12.2.1. Preparation-------------------------------------------- 60
12.2.2. Caution------------------------------------------------- 60
12.2.3. How to Remove the Shield Case---------------- 60
12.2.4. How to Install the Shield Case ------------------- 61

12.3. Terminal Guide of the ICs, Transistors, Diodes

and Electrolytic Capacitors ---------------------------- 62

12.3.1. Base Unit ---------------------------------------------- 62
12.3.2. Handset ------------------------------------------------ 62

13 Schematic Diagram -------------------------------------------- 63

13.1. For Schematic Diagram -------------------------------- 63

13.1.1. Base Unit (Base Unit (Main)) --------------------- 63
13.1.2. Handset (Handset) ---------------------------------- 63

13.2. Base Unit (Main) ----------------------------------------- 64

13.2.1. KX-TG7851CA --------------------------------------- 64
13.2.2. KX-TG7861CA --------------------------------------- 66

13.3. Base Unit (Operation) (KX-TG7861 only)---------- 68
13.4. Handset----------------------------------------------------- 70

14 Printed Circuit Board ------------------------------------------ 73

14.1. Base Unit (Main) ----------------------------------------- 73

4

14.1.1. Component View ------------------------------------ 73
14.1.2. Bottom View ------------------------------------------ 74

14.2. Base Unit (Operation) (KX-TG7861 only) ---------- 75

14.2.1. Component View  ----------------------------------- 75

14.3. Handset----------------------------------------------------- 77

14.3.1. Component View ------------------------------------ 77
14.3.2. Bottom View ------------------------------------------ 78

15 Exploded View and Replacement Parts List ----------- 79

15.1. Cabinet and Electrical Parts (Base Unit) ----------- 79

15.1.1. KX-TG7851 ------------------------------------------- 79
15.1.2. KX-TG7861 ------------------------------------------- 80

15.2. Cabinet and Electrical Parts (Handset) ------------- 81
15.3. Cabinet and Electrical Parts (Charger Unit) ------- 82
15.4. Accessories------------------------------------------------ 83
15.5. Replacement Part List ---------------------------------- 84

15.5.1. Base Unit ---------------------------------------------- 84
15.5.2. Handset ------------------------------------------------ 87
15.5.3. Charger Unit ------------------------------------------ 88
15.5.4. Accessories ------------------------------------------- 88
15.5.5. Screws ------------------------------------------------- 88
15.5.6. Fixtures and Tools----------------------------------- 88