Preamplifier (7/9) - 3M Series 410 Instruction Manual

These circuits are activated by application of potential at terminal 12. In
order to prevent a recorded thump when the record button is depressed, R27 and
C20 are provided to permit the DC bia s on the ba se of 01 to rise slowly and C 15 is
provided across Rl7 for the same purpose on 05 and 06. The bias
env~lope
there ­
fore
g~ows
to .operating level in a matter of about 10 milliseconds. When the recording
mode IS deactIvated another thump or click is avoided by allowing reservoir capacitor
C2 to permit the bias waveform to decay to zero over a period of about 60 milli­
seconds. Rl allows C2 to charge at a reasonable rate when the circuits are activated
without causing a surge on the power supply, but CR 1 permits the capacitor to be
connected directly to the load during discharge. Ll and C 1 constitute a filter to stop
bias frequency ripple on the power bus connected to terminal 12.
The amount of drive applied to 05 and 06 is controlled by feedback resistor
R26, which also improves the waveform at the output by cancelling out internally
gene rated di stortion products.
The second winding of T1 feeds the 120 kHz signal to an almost identical
circuit except that the output transistors 03 and 04 have a higher power rating in
order to supply the erase head with sufficient drive to completely erase a saturated
tape. The description above applies except for a few points. Potentiometer R3 is
a variable resistor to allow setting the erase amplitude to the minimum that will
allow sufficient erasure (70 to 75 dB). Erase current is monitored across the
10 ohm resistor R29 between terminals 16 and ground. The bias current is
monitored from terminal 2, the record head return lead to ground, with R25
providing the meter calibration. This current is read on the meter when the
meter switch is in the BIAS position.
Meter calibration for the monitoring taken ac ros s R29 is accomplished by
potentiometersR3 on the 3N board. The reading is obtained by putting the meter
switch into the ERASE position.
Preamplifier (7/9)
(See
Figure 35)
This plug-in printed circuit board assembly installed in slot 7 for normal
playback provides the required signal amplification of the playback head signal to
drive the output line amplifier. In addition, the assembly performs the necessary
frequency equalization and phase correction for two tape speeds. W hen this PC
board is installed in slot 9, the signal from the record head is used as the input
to provide overdub.
The playback head is connected to the assembly through terminal 4 which
provides the signal path through CI and R2 to the base of 01, the first amplifier
stage. In the overdub mode (slot 9). the record head is connected via the
READY /SAFE/OVERDUB switch to terminals 14 and 17 and the primary of T 1.
The secondary of terminal 16 is connected externally to terminal 4 which provides
signal in the same manner as previously described for the playback head.
M64 PAR 2-71
91