Akai GXC-570D

2.
PULSE GENERATOR CIRCUIT
CS. a ee
* output
EQUIVALENT
E CIRCUIT
FET PROTECTION
RESISTOR
COMMERCIAL
CURRENT
w
‘
4
ae DISCHARGE CURRENT
HUMAN BODY (SIGNAL SOURCE)
TOUCH BUTTON SYSTEM OPERATION
2-1. TOUCH BUTTON SYSTEM CIRCUIT
COMPONENT (Refer to Fig. 13)
As shown in Fig.3, generally divided, the touch
button system circuit is comprised of a pulse genera-
tor circuit, a rectifier circuit, and an integration
circuit and includes a malfunction prevention
capacitor and FET protection resistor, etc.
Because it is necessary for the malfunction prevention
capacitor shown by the dotted line to have a fairly
high voltage, a shielded wire distribution capacitor is
used with a capacitance of 50 to 60 PF.
As shown by the equivalent circuit in the diagram,
last stage transistor Q3 performs the exact same
function as a micro switch.
2-2. WHEN TOUCH BUTTON IS TOUCHED WITH
FINGER, OPERATION IS SAME AS MICRO
SWITCH SYSTEM USED TO DATE
(Refer to Figs. 4, 6)
Because of inductance in our bodies from commercial
power supply (50 Hz or 60 Hz), if we touch one of
the touch buttons with our finger, our body becomes
a signal source as-shown in Fig. 4 and the same thing
takes place as in the case of a micro switch system to
date. Therefore, when a button is touched with a
finger, a signal is supplied to FET Q1 gate as shown
in Fig. 4.
The charge at C3 when a signal is not supplied is only
equivalent to the drain voltage. Base bias is supplied
to Q2 from R7 and R8 and no signal is emitted at
Q2 collector. Consequently, Q3 base becomes
unbiased and Q3 does not function.
Fig. 4
The signal supplied to QI gate works to reduce the
impedance between FET drain and source, and the
electric charge charged at C3 passes Q1 > R6 > and
R8 and is discharged. ,
C3 discharge causes a large minus bias at Q2 base,
_ collector voltage increases, and this pulse is rectified
‘at DI and integrated at C10, R10 and supplied to Q3
as base. bias. Consequently, Q3 is turned ON and
functions in exactly the same way as a micro switch
has functioned to date.
2-3. WHEN A BUTTON IS TOUCHED WITH
A FINGER AND AT THE SAME TIME THE
DECK CHASSIS IS TOUCHED WITH ANOTHER
FINGER OR A HAND (See Fig. 5)
As previously explained our body acts as a signal
source to operate the touch buttons, but when
considering that in this case the high resistance of
the human body is utilized, the QI gate circuit
operation is different. As shown in Fig. 5, when a
button is touched with a finger and the chassis is
touched simultaneously, by means of the. resistor
shown in the circuit diagram, C2 electric charge
passes RI] > Ro > R4 and is discharged. From
looking at the circuit, it can be understood that
discharge is completed in an extremely short time.
In this very short discharge time period, FET is cut,
the drain voltage increases, and condenser C3 is
charged. When C2 discharge is complete and the
voltage of both terminals becomes equal, FET drain
current flows and the electric charge charged at C3 is
discharged. Consequently, Q2 base is minus biased
and ceases to opérate. Thus, Q3 functions in the same
way as a micro switch as outlined in item 2-2 above.
12
nls. 5,