Operating instructions

5.3 Circuit Descriptions
Pertinent schematic diagrams in the drawing package accompanying this
manual are referred to after each subtitle alternate drawing numbers are also
given in parenthesis( ). Each circuit board has its own individual power
supply regulation to provide the necessary voltages.
5.3.1 Playback Head and Preamp
PB-16KOA (PB9A577)
The playback (repro) head has a high impedance output (2 kΩ at 1 kHz). Its
output is fed via a low capacitance audio cable into each channel's electronic
module. The signal is AC coupled to a set of matched transistors Q101.
These provide the initial gain stage. The output of this stage is fed to an
operational amplifier which provides a low impedance output to drive the EQ
and other stages (IC101).
5.3.2 Speed Selection and High Frequency Playback Equalization
PB-16KOA (PB9A577)
Equalization for 15 ips is switched on when FET Q104 is gated on by the logic
circuitry. Similarly, 30 ips EQ is switched on when FET Q105 is gated on by
the logic. Additionally, one has a choice between two different equalization
curves at each speed. Two miniature switches on the Bias Control master
circuit board allows the pre-selection of either a 35 µs time constant (IEC
curve) or 50 µs (NAB curve) at 15 ips. The shorter time constant is toggled
between 17.5 and 35 µs by FET Q102, and the longer is toggled between 30
µs and 50 µs by FET Q103.
5.3.3 Low Frequency Playback Equalization
PB-16KOA (PB9A577)
A standard low-frequency compensation is normally fixed at both 15 and 30
ips. Optionally, one may individually adjust the low frequency response of
each channel (using trimmers on each channel's electronics) by moving the
"LF COMP" switch on the Bias Control master board front panel from OFF to
ON position. (OFF position selects the fixed compensation.) When the LF
COMP switch is ON, FET Q107 is turned OFF, enabling trimmer VR104 to
adjust the amount of the channel's low frequency equalization (the knee of
the curve remains at 50 Hz).
5.3.4 Sync Playback
PB-16KOA (PB9A577)
Sync playback utilizes a circuit that is separate from, but identical to, normal
playback electronics. The circuitry operates in nearly the same manner, with
logic controlled selection of high frequency equalization, and the choice of
fixed or variable low frequency EQ.
Transformer T101 steps up the head's output signal and feeds the signal to a
set of matched transistors Q109 for preamplification. Second stage is
provided by half of IC102, and a low impedance output is given by the other
half of IC102.
15 ips Eq is switched ON by Q112, and 30 ips Eq is switched ON by Q113.
the 15 ips time constant is selected by Q111, Q114 and 30 ips time constant
is selected by Q110 and Q114. VR108 provides the adjustable low frequency
EQ which is switched ON by Q115.
MTR-90III Operation and Maintenance Manual
October 1990
Section 5 Maintenance
5 - 9