Yamaha F-20 Service Manual

GUITAR AMPLIFIER

SERVICE MANUAL

This document is printed on chlorine free (ECF) paper with soy ink.

GA

011551

CONTENTS

SPECIFICATIONS .............................................................................. 3

PANEL LAYOUT.................................................................................. 3

CIRCUIT BOARD LAYOUT................................................................. 4

BLOCK DIAGRAM .............................................................................. 4

DISASSEMBLY PROCEDURE ........................................................... 5

CIRCUIT BOARDS ............................................................................. 7

IC BLOCK DIAGRAM.......................................................................... 8

INSPECTIONS.................................................................................... 8

P ARTS LIST
OVERALL CIRCUIT DIAGRAM

HAMAMATSU, JAPAN

1.338K-713 K Printed in Japan 2001.09

F-20

IMPORTANT NOTICE

This manual has been provided for the use of authorized Yamaha Retailers and their service personnel. It has been assumed that
basic service procedures inherent to the industry, and more specifically Yamaha Products, are already known and understood by the
users, and have therefore not been restated.

WARNING: Failure to follow appropriate service and safety procedures when servicing this product may result in per-

sonal injury, destruction of expensive components and failure of the product to per form as specified. For
these reasons, we advise all Yamaha product owners that all service required should be performed by an
authorized Yamaha Retailer or the appointed service representative.

IMPORTANT: This presentation or sale of this manual to any individual or firm does not constitute authorization, certifica-

tion, recognition of any applicable technical capabilities, or establish a principal-agent relationship of any
form.

The data provided is believed to be accurate and applicable to the unit(s) indicated on the cover. The research engineering, and
service departments of Yamaha are continually striving to improve Yamaha products. Modifications are, therefor, inevitable and
changes in specification are subject to change without notice or obligation to retrofit. Should any discrepancy appear to exist, please
contact the distributor's Service Division.

WARNING: Static discharges can destroy expensive components. Discharge any static electricity your body may have

accumulated by grounding yourself to the ground buss in the unit (heavy gauge black wires connect to this
buss).

IMPORTANT: Turn the unit OFF during disassembly and parts replacement. Recheck all work before you apply power to

the unit.

WARNING: CHEMICAL CONTENT NOTICE!

The solder used in the production of this product contains LEAD. In addition, other electrical / electronic and / or plastic (where
applicable) components may also contain traces of chemicals found by the California Health and Welfare Agency (and possibly other
entities) to cause cancer and / or birth defects or other reproductive harm.

DO NOT PLACE SOLDER, ELECTRICAL / ELECTRONIC OR PLASTIC COMPONENTS IN YOUR MOUTH FOR ANY REASON
WHAT SO EVER!

Avoid prolonged, unprotected contact between solder and your skin! When soldering, do not inhale solder fumes or expose eyes to
solder / flux vapor!

If you come in contact with solder or components located inside the enclosure of this product, wash your hands before handling food.

IMPORTANT NOTICE FOR THE UNITED KINGDOM

Connecting the Plug and Cord

WARNING: THIS APPARATUS MUST BE EARTHED
IMPORTANT: The wires in this main lead are coloured in accor-

dance with the following code:
GREEN-AND-YELLOW: EARTH
BLUE: NEUTRAL
BROWN: LIVE

As the colours of the wires in the main lead of this apparatus may not
correspond with the coloured markings identifying the terminals in your
plug, proceed as follows:
The GREEN-and-YELLOW wire must be connected to the terminal in the
plug that is marked with the letter E or the safety earth symbol (or coloured
GREEN or GREEN-and-YELLOW).
The BLUE wire must be connected to the terminal that is marked with the
letter N (or coloured BLACK).
The BROWN wire must be connected to the terminal that is marked with
the letter L (or coloured RED).

This applies only to products distributed by Yamaha Kemble Music (U. K.) Ltd.

WARNING

Components having special characteristics are marked and
must be replaced with parts having specification equal to those
originally installed.

2

SPECIFICATIONS

Rated Output Power: 20W
Speaker: 20cm x 1
Controls: VOLUME, GAIN, MASTER VOL., TREBLE, MIDDLE, BASS, CH SELECT switch (A/B)
Input/Output Terminals: INPUT, AUX IN, PHONES
Output Impedance: 100Ω (PHONES)
Power Consumption: 30W/100V (J)

25W/120V (UL,CSA)
25W/230V (E,BS)

Power Requirements: U.S. and Canadian models: 120V, 60Hz

General model: 230V, 50Hz
Dimensions (W x H x D): 357 x 330 x 197mm (14.1" x 13" x 7.8")
Weight: 6.5kg (14 lbs 5 oz)

PANEL LAYOUT

F-20

  

 INPUT
 CH SELECT A/B (Channel Select Switch A/B)

A Channel

 VOLUME

* No sound is produced when the VOLUME knob is

set to "0".

B Channel

 GAIN
 MASTER VOL.

* No sound is produced when the MASTER VOL.

knob is set to "0".



   

  

Tone Controls

 TREBLE

 MIDDLE

 BASS

 AUX IN

* Use the volume control on the audio device to

control its volume level.

 PHONES

* The speaker produces no sound when a pair of

headphones is connected to this jack.

 POWER

3

F-20

CIRCUIT BOARD LAYOUT



Top View

MAIN 1/2

MAIN 2/2

AC Cord



Side View

Power Transformer

MAIN 2/2

MAIN 1/2



Front View

BLOCK DIAGRAM

MAIN 1/2

OP AMP

CN1 2P

+12V

31

IC1

-12V

GAIN

VR2

OP AMP

31

INPUT

JK1

POWER SWITCH

SW1

T1

T2

U, C, S, BS

IC2

-12V

CH. A

CLEAN

VOLUME

VR1

+12V

CH. SELECT SW

SW2

MASTER

VR3

FUSE

FZ1

CN2 4P

POWER TRANSFORMER

CH. A

CLEAN

CH. B

DRIVE

+B

OP AMP

57

INDICATOR

CH. B

DRIVE

T1

G

T2

IC1

-12V

+12V

MAIN 1/2

IC1. IC2

PASSIVE TYPE

TONE CONTROL

VR4 VR5 VR6

TREBLE

POWER SUPPLY CIRCUIT

CN3 3P

MIDDLE

BASS

OP AMP

+12V

57

IC2

-12V

CN4 5P

+B
+12V

-12V

-B

MAIN 2/2

+B

14

IC3

-B

CN5 5P

POWER

AMP

V695450

JK3

T3
T4

JK2

AUX IN

4Ω Speaker

PHONES

4

DISASSEMBLY PROCEDURE

MAIN1/2 Circuit Board

Needle Nose Plier

[P40]

1. Pre-Main Unit (Time required: about 1 min)

1-1 Remove the speaker wires.
1-2 Remove the four (4) screws marked [30]. The pre-main unit can then be removed by sliding it backward. (Fig.1)

2. Speaker (Time required: about 1 min)

2-1 Remove the four (4) screws marked [A]. The back board can then be removed. (Fig.1)
2-2 Remove the four (4) screws marked [B]. The speaker can then be removed. (Fig.1)

3. MAIN 1/2 Circuit Board (Time required: about 3 min)

3-1 Remove the pre-main unit. (See procedure 1)
3-2 Remove the six (6) knobs marked [P100] and the six (6) hexagonal nuts marked [C]. (Fig.5)
3-3 Remove the hexagonal nuts marked [A120] and the hexagonal nuts marked [A140]. (Fig.5)
3-4 Push the hooks of the spacer support (at the arrow symbol area of illustration) with a needle-nose plier to unlock.

The Main 1/2 circuit board can then be removed. (Fig.2)

F-20

[30] x 4

[30]: Truss Head Screw 4.0X25 MFZN2BL (VN347600)

Pre-Main UnitPre-Main Unit

[B]

[A] Back Board

[A]: Bind Head Tapping Screw 4.0X25 FCRM3-BL (EI340256)
[B]: Bind Head Screw 4.0X20 ZMC2BL (VB403600)

Speaker

(Fig. 1)

Needle Nose Plier

Needle Nose Plier

[P40]

[P40]

MAIN1/2 Circuit Board

MAIN1/2 Circuit Board

(Fig. 2)

5