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DETAILED CIRCUIT DESCRIPTIONS
The following paragraphs contain detailed descriptions of each of the circuit
boards, and the Power Supply Assembly.
NAB Record Amplifier (4)
(See
Figure 32)
The input signal to be recorded is applied at terminal 14, after it has passed
through the input transformer and level control.
C 1 couple s it to 01, which is
biased to the proper operating point by R 1 and R2.
After amplification the signal
is coupled by C3 to an equalizer network, producing a preemphasis characteristic
at the high and low frequency extremities of the spectrum in accordance with standard
NAB practice.
This is accomplished as follows.
Consider R6 and R7 as an attenuator network, with RB short circuited, as it
effectively is at high frequencies because of C4 and CS.
This attenuator network
reduces the voltage swing at the junction of R6 and R7 from what it was at collector
of 01.
However, if a bypass capacitor CIB or Cl9 is connected across R6 it permits
the high frequencies to suffer less attenuation.
The degree to which this is true
depends on the setting of the variable capacitor.
Practical values of C IB and C 19
make it necessary for the junction of R6 and R7 to appear always as a very high
impedance.
Current cannot be drawn from this point without upsetting the equal
ization characteristic.
Thus, the junction is used to drive a field effect transistor,
02.
C4 and C5 show increasing impedance as the signal frequency is lowered;
therefore, the voltage at the junction of R6 and R7 would continue to rise as the
frequency drops, except that RB provides a shelving off to prevent unnecessary
sensitivity to subaudible frequencies.
Relay Kl automatically changes the high frequency equalization when tape
speed is changed.
The output of 02 is applied to terminals into which may be
inserted values of
ca,
C20, R II, and R 12 to adjust for any requirement arising
for shelving equalization in addition to the normal NAB equalizers just described.
All four components may be eliminated in many instances, a jumper being furnished
across the RII or C20 terminals.
After this network the signal is fed to the base of 03 whose bias is set by
R13, RI4, and RlS.
This operates with 04 in a Darlington circuit to provide gain
and low impedance output at CII to drive the NAB record head.
RIB normally
provides a degree of degeneration, determined by the setting of R20, and R 19 in
series with C 12, and C21.
But the degree of degeneration can also be reduced by
conduction of 05 and 06 when S 1 is closed.
05 and 06 act to change the degeneration
with instantaneous signal amplitude, thus they tend to deform the signal whenever
its amplitude attains a value sufficient to overcome their contact potentiaL.
This
results in a distortion, inverse to that which overload of the tape normally intro
duces.
By employing 05 and 06 in this manner, and properly adjusting the overall
degeneration by means of R20, the LIN ADJ. control it is possible to introduce the
proper corrective distortion into the recording process so that the normal 3% tape
distortion point shows somewhat less than 1%.
M64 PAR 2-71
B9
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