3M DELTA Series - Solenoid Control; Record Amplifier; Bias Amplifier

Electrical
Degree “ Positive" I “ Negative" I
G. SOLENOID CONTROL
0 “ - 60°
A
B
60° - 120°
A C
120° - 180°
B C
180° - 240°
B A
240°
- 300°
C
A
300°
- 360°
C
B
Run voltage for solenoid operation is provided
by rectification of the mains AC voltage in a full
wave bridge: DI & Dll — CR501, 502, 503, 504; Dill
— CR701, 702, 703, 704, High level DC is fed via R501
(DI1I-R701, 708, 715) to the solenoid, then returns to
the collectors of QI and Q501 (DIII-Q703 & 704;
Q707 & 708; Q7U & 712), configured as Darlington
pairs. Solenoid control (a low-going signal, 5-volt
logic level) is originated by the Play Logic board.
The processor produces a logic “ 0" which turns on
U501 (DIII-U701, 702, 703) through inverter U205B.
allowing QI and Q501 (DIII-Q703 & 704; Q707 &
708; Q71 1 & 712) to saturate, pulling in the solenoid.
A logic ” 1", inverted by U205B, turns off U501 (DIII-
U701, 702, 703) and turns on Q502 (DIII-Q702, 706,
710), thereby shunting the base supply current for
QI and Q501 (DIII-Q703 & 704; Q707 & 708; Q71 1 &
712), causing them to turn off and the solenoid to
drop out.
H. RECORD AMPLIFIER
Recording amplifiers for left and right channels
are identical, and therefore only the left (mono)
recording amplifier will be discussed. Audio from
the rear panel XLR connector and transformer is
routed to differenetial input amplifier U1001. Nomi 
nal input level is set to +6 dBm, but may be set to -6
dBm by installing W1001 wire jumper. This jumper
changes the stage gain by adding a 3.3K resistor,
R1001, to the circuit. U1001 (third section) is a sum 
ming amplifier that provides differential input sum 
ming and acts as a buffer/driver for the front panel
level control. Audio is AC coupled to the front panel
level controls, and then to equalization amplifer
U1001 (fourth section). High frequency equalization
is done in the feedback circuit of this stage, and is
controlled by R1005. Cl 009 is factory loaded to pro 
vide 1964 NAB record equalization, but is shorted by
wire jumper W1003 for 1976 NAB equalization. Audio
output from this amplifier is routed to the Bias PC
card.
I. METER AMPLIFIER
Metering functions internal to the Delta Series
are monitored by an electronically switched ampli 
fier and rectifier located on the Record Amplifier PC
card. Monitored signals from various circuits are
routed to an analog switch U1003 via calibration
potentiometers R1015 through R1022. U1003 is a dual
4 input CMOS analog switch that is selectively con 
trolled by the two front panel meter switches and
the on-board slide switch SI 001.
The selected signal is outputted to the meter
amplifier U1004 and then to the full wave meter
bridge CR1001, 1003, 1005, and 1007. The resulting
signal is fed to the front panel meter for mechanical
display. The right channel circuit is similar and will
not be discussed.
J. BIAS AMPLIFIER
Program and cue record circuits utilize a com 
mon source of bias. The bias frequency originates as
a square wave from the Record microprocessor
clock and is divided down to 119.3 kHz on the
Record Logic PC board. The square waves require
filtering before they may be used as bias (sine
waves) at the record head. Two sections of U1101
comprise a low pass filter which sharply attenuates
the harmonic content of the square wave, so that the
output is a low distortion sine wave at 119.3 kHz. The
signal is then AC coupled to the bias level potenti 
ometers R1 107, R1 108, and R1 131 for use by the left,
right, and cue head drivers.
Cue tones from the Record Logic card are fed to
U1 104 by way of Cl 125 and R1 132. U1 104 is the cue
head driver, where bias and audio are summed.
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