Section 9 - Specifications - Elenco Electronics AK-200 Assembly And Instruction Manual

SECTION 9 - SPECIFICATIONS

Tests 3 through 7 require a voltmeter. If you do not have a voltmeter, skip these tests and go on to test 8.
INPUT VOLTAGE RANGE:
TAPE SPEED:
AMPLIFIER IC SPECIFICATIONS:
CHARACTERISTIC
CONDITIONS
Gain
Distortion
Distortion
Maximum Output
SECTION 10 - THEORY OF OPERATION - MOTION CONTROL
The tape speed is determined by the voltage across
the motor. The purpose of the Motion Control
Section is a) to set the tape speed by adjusting the
motor voltage, and b) to keep the motor voltage
constant as the battery voltage drops. To do this,
the Motion Control Section contains a voltage
divider (R7, R8 and VR2) and a motor control IC
(AN6650). See the schematic diagram, Section 13.
The IC contains the following major parts:
1. Reference Voltage - This circuit together with the
current source feeding it maintains a constant
1.3V between pins 2 and 1 (and thus across the
voltage divider) until the supply voltage to the IC
drops to approximately 1.6V.
2. Op-amp - The op-amp amplifies the voltage
difference between the (+) and (–) inputs. If the
difference is positive (+ input more positive than
– input) the output goes positive. If the difference
is negative (+ input more negative than – input)
the output goes negative. The gain of the op-
amp is high. A small difference at the inputs
produces a large change in the output.
The op-amp circuit consisting of the op-amp, the
transistor and resistors RA and RB (see
schematic diagram, Section 13) employs
negative feedback. This means that the op-amp
output changes so as to reduce any voltage
difference at the inputs. For example, if the (–)
2.2V - 3.5V
1 7/8 IPS
(Ta = 25C, Vcc = 3V, fo = 1kHz, volume = 100% unless noted otherwise.
TEST
TYPICAL
Input = -75 dBm
Volume = 50%
Input = -70 dBm
Input = -60 dBm
Volume = 50%
Load = 32W
THD = 10%
-13-
VALUE
54dB
0.7%
0.5%
30mW
input goes negative, creating a positive
difference at the inputs, the output goes positive.
The transistor then turns on harder and the
collector voltage drops. This voltage drop is fed
back via RA to the (+) input which tends to
remove the original difference between the
inputs. Since the gain of the op-amp is very high,
the two inputs are kept at virtually the same
voltage.
The motor voltage is adjusted by VR2. Turning
the wiper of VR2 toward R8 lowers the voltage at
the (–) input of the op-amp. This, as in the
example above, lowers the transistor collector
voltage and increases the voltage on the motor.
Once the motor voltage is set, the motion control
section keeps the voltage constant as the battery
voltage drops. If, for example, the battery voltage
drops by 0.5V, point A will drop by 0.5V. Due to
the constant 1.3V across the voltage divider,
point C, and the (–) op-amp input also drop by
0.5V. As explained above, this drops the
transistor collector voltage (point B).
example, the collector voltage must drop 0.5V to
make the (+) op-amp input equal to the (–) input.
Since points A and B both drop 0.5V, the motor
voltage remains constant.
In our