Fisher MT-750 - Arm Position Sensing; Tonearm Leadin Operations; Tonearm Return Conditions; Direct Music Selection Logic

FUNCTIONAL DESCRIPTION Continue)
Arm Position and Microcomputer Ports
The arm position is detected by the three pairs of LED’s and
phototransistors which run across the guide angle, and a pair of
LED and a phototransistor mounted on the pickup. Illustrated
below is the hole arrangement of the three pairs of sensors
which run across the guide angle and the guide angle. See the
chart for the relation between each port.
TU
©@©®
Left Edge
Position of
Internal
Circumference
57 mm
30cm
Leadin Leadin Leadin
Position of Armrest
Internal (right edge)
Circumference
70mm
+12v +5V
Micro-
Qo1 computer
————— 4 LM6402
Internal Circum- | internal Circum- Armrest
Left
Edge
{Right
Edge)
“L”(Low Level)
“H"(High Level)
“H" (High Level)
ference 57mm | ference 70mm
“L”(Low Level) | “L”(Low Level) | “H”(High Level)
“L”(Low Level) | “H’(High Level) | “H”(High Level)
“L"(Low Level) | “L"(Low Level) | “L’{Low Level)
30 cm Leadin
The arm is shifted until PB1 is set in “H” level (at the end of the
square hole) if Plo is set in “L” level by the signals emitted from
the sensor on the pickup when PBo is in “H”, PB1 in “L’”, and PB2
in “H” levels.
When PB: is set in “H” level, the arm stops once (Port PG2: “H”)
and returns (Port PG3: “L”) for a period of time determined by
C22 (4.74F/25) and VRO1 (100K-B).
If the arm returns for a certain period of time, it stops (Port PG:
“H”) and lowers on the disc (Port PHo: “L”).
Note:
If the sensor on the pickup is located over the disc, the light is
reflected and Port Plo is set in “L” level. If a disc is not placed on
the turntable, the light is not reflected and Port Plo is set in “H”
level.
25 cm Leadin
As Plo is set in “H” level at the square hole for 30 cm, the arm
advances inwards until it reaches the square hole for 25 cm.
Then, the arm exercises the same action as those in 30 cm
leadin and lowers on the disc.
17 cm Leadin
When PBo is in “L”, PB1 in “L”, and PB2 in “H” levels, the arm is
shifted until PB1 is set in “H” level if Port Plo is set in “L” level by
the signals emitted from the sensor on the pickup. Then, the arm
exercises the same actions as those in 30 cm leadin.
Return Conditions
The return action is taken place when the following conditions
are met.
(1) when the stylus tip reaches the area of 57 mm or more away
from the internal circumference of the disc. (PBo: “L”, PB1:
“H”, PB2: “L”)
(2) when there is no music signal. (Pl2: “H”)
(3) The light is reflected to the sensor with the pickup in
“DOWN” mode. (Plo: “L”)
When the unit is operated without any programming and the
above described three conditions are met at the same time, the
return action is taken place.
When the reproduction of the last program is finished by the
programmed action, the return action is taken place if the above
described conditions (2) and (3) are met at the same time.
Conditions of Direct Music Selection
Direct music selection is taken place when the following
conditions are met at the same time. In other cases, select a
program after the tonearm has rested on the armrest once.
(1) when a program is finished at the area of moe than 70 mm
away from the internal circumference of the dsc.
(2) when the program to be selected next is not the first one.
(3) when music signals exist for more than 10 ser_
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