Fender Acoustasonic Junior Service Manual - page 3

ACOUSTASONIC JUNIOR

THEORY OF OPERATION

INSTRUMENT CHANNEL (cont)

The 

String Dynamics 

function (Fig. 2) is the summation of three signals:

1)

The original audio signal from U2B-7,

2)

The bandpass amp, U4A-1, and

3)

The notch control circuit comprised of operational transconductance amplifier, U5A,

op. amps, U6A&B, and potentiometer, R40

Σ

Audio In

Define

Frequency

amplitude control

String

Dynamics

Audio

R36

R37

R38

U4B

U4A

U6A-B, U5A

(Fixed bandpass)

Invert bandpass & add

Fig. 2     String Dynamics function

c)

A 

Mono-Effects Loop 

allows effects to be used on the instrument channel without affecting the 

signal from ch. 2.

MICROPHONE CHANNEL

The 

Microphone Channel 

(ch. 2) can also be used as a second instrument or line-input channel.  It, too, is designed

with operational amplifiers.  Phantom power is present at the mic jack.  The phase of channel 2 can be reversed with a

push-button switch.

REVERB

The 

Reverb 

circuit is typical of solid-state reverb circuits, except that the Reverb send from the 

Microphone Channel

has a separate control allowing individual adjustment for that channel.

CHORUS

The 

Chorus 

circuit is typical of 

Fender's 

chorus circuits in which the raw signal from U12-1 is shifted down a bucket-

brigade, U13, by pulses from a shift-register, U15.

The chorus is turned on and off by a master chorus in/out button.  This button is completely non-functional if the

Footswitch 

is plugged in (the one-and-only function of the 

Footswitch 

is master chorus off/on.)

ACOUSTASONIC JUNIOR

THEORY OF OPERATION

SWITCHING LOGIC

The 

Switching Logic 

accomplishes several functions:

a)

It switches the chorused signal to the right power amp, while leaving the clean signal on the left 

power amp;

b)

It allows for the separate selection of reverb for channel 1;

c)

It allows for separate selection of chorus for channel 2.

STEREO BUFFERS

The 

Stereo Buffers 

(left: U10A, right: U10B) route the L & R signals to the 

Stereo EFX Loop. 

Following the loop is

S5, which will switch the phase of both signals 180

°

.

POWER AMPS

The 

Power Amps 

are a traditional 

Fender 

design with no bias and/or offset adjustments.  The input is made through a

differential pair, Q11/Q12 L (Q21/Q22 R).  An interesting addition is the pwr up/dwn quieting circuit (Q10) which cuts

off the differential pairs through their emitter circuit while power is first turned on, or turned off.

POWER SUPPLY

The 

Power Supply 

is a bridge rectifier type which supplies +/- 39 V for the power amp rails; this is dropped to

approximately 16 V and regulated with 7815 & 7915 regulators to provide the +/- 15 V for the IC's.  An LED on the front

panel serves as a power indicator.

NOTE

Please notice that the output power measurement as stated on the schematic for this product is stated slightly different

than other products.  It says that for a given input (640 mV, 1 kHz) inserted at the 

stereo effects return

 (J6), the output

of each power amp should not be less than 40 Watts @ 0.2% THD into an 8

Ω

 resistive load, with both channels

driven.

The signal path through the amp still has signal (AC ) measurements shown in ovals to assist in determining relative

gains through the stages.

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