Operating instructions

5.3.9 Reel Control Board
PB-4QHB (PB9B266)
As a system, the Reel Control board, swing arm potentiometers, motor drive
amp output, and reel motors, all maintain constant the tape tension. In
addition, the Reel Control board senses whether the swing arms are in the
correct position to allow arming of the reel servos, or in a position that would
require disarming of the servos. (Refer to the upper left hand corner of the
schematic.) One half of IC8 and half of IC7 serve as comparators that receive
DC voltages from the center wipers of the Supply and Take-up swing arm
pots. IC8's reference voltage input comes from VR1 (the Take-up arm POS
trimmer), and IC7's reference voltage comes from VR3 (the Supply arm POS
trimmer). Test points TP2 and TP5 are for checking the Supply and Take-up
position voltages, respectively.
The other halves of IC8 and IC7 are variable-gain inverting amplifiers, whose
gain is set by trimmers VR2, 3, 4 for Tup and VR6, 7, 8 for Sup which thus
set the gain for the entire servo system. IC8's output proceeds to the base of
Q2 through a DC discrete component Op Amp, and differentially drives the
Take-up motor drive amp output stage after leaving the Reel Control board
via card edge connecter points AB13. R89 provides a negative feedback path
from the Take-up reel motor ground return. The supply motor drive system
is fed similarly from IC7, and output from AB18. R107 provides the negative
feed-back paths from the supply reel motor ground return.
The DC output from each swing arm pot center-wiper, in addition to providing
constant tension information, feeds a pair of comparators which are adjusted
to indicate when the swing arm is at either extreme of its travel. Specifically,
the Take-up swing arm output goes through inverting buffer IC6B to
comparators IC6å and IC5A. VR9 provides the reference voltage to IC6A, and
is trimmed so that D27 lights up when the swing arm is fully up (toward the
rear of the deck); VR10 adjusts the reference voltage to IC5A so that D28
lights with the arm fully down. The supply swing circuit operates similarly,
with VR12 adjusting the reference voltage to IC4A for the upper limit (D30),
and VR11 adjusting the reference to IC3A for the lower limit (D29). the same
voltage which lights the LEDs is fed through a hex inverter IC2 to the AND
gates in IC1. When both upper-limit LEDs are on, then the upper AND gate's
output goes negative, allowing the servos to be energized if the STOP button
is pressed. Conversely, if both lower-limit LEDs are on, then the lower AND
gate's output will go negative and shuts off the servo system for safety.
Section 5 Maintenance
5 - 12
October 1990
MTR-90III Operation and Maintenance Manual