Westrex RA-1712G User manual

RA-1712G
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INTRODUCTION
The Westrex RA-1712G is an integrated optical sound recording electronics package for
use with galvanometer recorders. The system takes line level balanced or single sided
input. It provides a front panel input level adjustment range of 20 dB. High and low
pass filters are front panel selectable for 16-mm, 35-mm or 8-mm sound track recording.
High frequency pre-emphasis to compensate for printer or other loss is front panel
selectable. Percent track modulation and average galvanometer drive are monitored on
a front panel meter. The noise reduction and drive signals are internally combined and
their relative levels set for a 2 dB margin. The margin is independent of external control
settings and is factory set. The output level of the galvanometer is controlled by a
single front panel ten-turn DRIVE control. The system can be used to record positive or
negative optical sound tracks.
INSTALLATION AND OPERATION
The RA-1712G accepts either a single sided or balanced line high impedance input. If
the input is single sided, the input terminal marked "-" should be tied to the center GND
terminal and the input applied between that terminal and the "+" input terminal. It is
important to follow good grounding practice in the installation of the RA-1712G. If
balanced line input is available (i.e., transformer input) the center terminal of the input to
the RA-1712G may be left open with the balanced input applied between the "+" and "-"
inputs terminals. In that instance the RA-1712G may be grounded either to power line
ground via its 3 terminal power plug or the system ground may be connected to the
center GND input terminal. If a single sided input is used, the system ground in
necessarily connected to the GND input terminal and the grounding terminal of the
power plug must be lifted from power ground with an isolation plug
For operation of the RA-1712G without the RA-1713 auxiliary electronics package,
terminals A and B on the other three terminal Jones connector on the back panel should
be shorted together with a 2" length of wire. (These terminals are used by the RA-1713
to access the filtered and level set signal of the RA-1712G and as a signal injection
point for signals from the RA-1713. The short loops the signal through.
The modulator output on the two terminal Jones strip of the back of the RA-1712G is
connected directly to the two input terminals of the galvanometer using 14 GA wire.
WARNING! NEITHER OF THESE TERMINALS SHOULD EVER BE CONNECTED
TO GROUND!
Before turning the RA-1712G on the first time, the drive control should be reduced to 0
and the polarity switch set to the standby middle position. IMPORTANT! Always have
the polarity switch in the standby position when turning the RA-1712G on or off. An
input signal source capable of supplying an audio input at 0 dB should be connected to
the input of the RA-1712G. The program switch should be turned off.

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Turn on the RA-1712G and toggle the polarity switch to the plus position. Observe the
modulator while the drive control is advanced. If the galvanometer begins to deflect in
the direction of increased track width as the drive is increased, return the polarity switch
to the standby position and reverse the modulator leads on the back of the RA-1712G.
Return the polarity switch to the plus position and continue to increase the drive control
until normal, .002" bias line(s) are achieved.
It is desirable during the setup of the RA-1712G to have a photocell or visual monitor of
the actual modulation of the galvanometer
Turn on the program switch with a 400 Hz signal applied; increase its level until the
meter on the RA-1712G in the IN mode reads 100% modulation. 100% peak-to-peak
deflection of the galvanometer should be observed. If 100% deflection is not noted,
make sure the drive control has been advanced to the point necessary to achieve the
desired bias line size and that the program switch is on. If 100% deflection is still not
observed, the RA-1712G calibration should be checked. (See section on Calibration.)
Having verified performance at 400 Hz, the next step is to adjust the internal high
frequency galvanometer resonance equalization of the RA-172G. These adjustments
are made inside the unit after removing the top plate. A nominal adjustment of the
equalization circuit has been set at the factory. However, because the characteristics of
different galvanometers vary, a readjustment may be required. Apply a signal to the
input at 8.0 kHz and observe the modulation deflection of the galvanometer. Increase
the input level until the RA-1712G meter reads 100% in the IN mode. Now adjust
trimpots R-154 and R-155 until exactly 100% modulation deflection of the galvanometer
is noted. R-154 and R-155 should be adjusted together; that is, so that the small arrows
on them point parallel to each other when the adjustment is completed.
Next, the input signal frequency is increased to 9.5 kHz and its input level adjusted to
100%. Trimpot R-180 is adjusted to achieve 100% modulation of the galvanometer.
NOTE: The RA-1712G low pass filter should be set to the 12.5 kHz position in the
above alignment procedures.
RECOMMENDED USE
The RA-1712G may be used to record 35-mm, 16-mm and 8-mm positive and negative
sound tracks. For recording 35-mm sound tracks, we recommend setting the HIGH
PASS filter to 45Hz and the LOW PASS filter to either 12.5 or 10.5 kHz. The
Equalization switch should be set to either position 1 or 2. This setting will be
determined by your experience with the amount of high frequency lost in development
and printing at the lab processing the sound track. For 16-mm recording, the HIGH
PASS filter should be set to 65 Hz and the LOW PASS to 6.5 kHz or alternatively the
HIGH PASS may be set to 85 Hz and the low pass to 6.5 kHz. The equalization switch
may be set to position 3 or 4, again depending on the laboratory which will be
developing and printing the film. Finally for 8-mm recording, the 85 Hz - 6.5 kHz filter
selections should be used with maximum film loss equalization.

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For all sound track recording, we recommend a .002 to .003 inch bias line size.
Because of the use of anticipatory noise reduction in the RA-1712G, the larger bias
lines and greater margin are not necessary. The result of the smaller margin and bias
lines is a quieter sound track with no transient distortion.
CIRCUIT DESCRIPTION
INPUT AMPLIFIER AND FILM LOSS EQUALIZER
The signal enters the RA-1712G via a three terminal, Jones type, back panel connector.
The balanced differential input amplifier U-9 amplifies the input. The output of the
amplifier passes through the front panel 20-dB step-pot attenuator and to the
equalization amplifier comprising the other half of the input amplifier chip U-9. The front
panel equalization switch associated with the amplifier switches capacitors C82 - C86 to
provide from 0dB to 10-dB high frequency boost at 10 kHz.
LOW PASS AND HIGH PASS FILTERS
The signal is applied simultaneously to the 4 low pass filters comprising amplifiers U5 -
U8. These filters use a 5-pole design breaking at 6.5 kHz, 8.5 kHz, 10.5 kHz and 12.5
kHz. The desired low pass filters output is selected by the LOW PASS front panelselect
switch. The output of the selected low pass filter is applied to the three high pass filters
comprising U2 - U4. The filters use a 4-pole design breaking at 45 Hz, 64 Hz and 85
Hz. The front panel HIGH PASS switch selects the output of one of these filters. In the
MIN position the high pass filters are bypassed and the resultant system low frequency
cutoff is approximately 5 Hz.
RA-1713 INTERFACE
After passing through the high and low pass filters, the signal is routed to the other three
terminal back panel Jones connector labeled A - GND - B. This is the insertion point for
interfacing the RA-1712G with the RA-1713 Auxiliary Electronics package. For use
without the RA-1713, terminals A and B are shorted together, thus looping the signal
back into the RA-1712G.
LIMITER
The signal returned from the back panel connector strip is hard limited by the current
control clipper made up of amplifier U11 and IC U10. This circuit provides a
symmetrical clipping of the signal if it exceeds 100% modulation level by more that 2
dB. The amount over 100% modulation at which clipping occurs is set by pot R84. This
circuit prevents serious overdriving of the galvanometer, which could result in distortion
on the recorded sound track. After passing through the clipper, the signal branches.
One branch goes to signal delay; the other branch goes to the noise reduction circuit.

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ANTI-ALIASING FILTERS
Amplifiers U23 and U22 act as anti-aliasing filters for CCD delay. The delay uses a
2000 element CCD delay device, RS101. Its clock input comes from the free running
multivibrator U19 and the 2:1 divider flip-flop U20. The clock operates at 150 kHz at
test point 10, the clock input point to the CCD chip. Amplifiers U23 and U24 are the
post amplifiers and filters. U25 is a distortion correction circuit whose output provides a
delayed signal to the noise reduction circuit to assure that under no circumstances will
the noise reduction close below the amount necessary to provide clearance for the
current signal. The main delay signal output appears at test point 7. Trimpot R151
adjusts the amount of audio signal summed into the final composite signal and thus sets
the margin.
NOISE REDUCTION CIRCUIT
The noise reduction circuit comprises a dual input resettable peak follow-and-hold
circuit made up of amplifiers U12, U13 and U14. This circuit stores the highest peak
positive signal received in the preceding 25 ms and buffers that value out to test point 8
(TP8). U15 and U17 are timing circuits that reset the peak follow circuit after 25 ms if its
stored value is not exceeded by the incoming signal during that period. U17 adds a DC
offset to the held peak value set by pot R99 and simultaneously hard limits at ground.
U18 is a two-pole low pass filter that removes all audio frequency components from the
noise reduction signal.
GALVANOMETER HIGH FREQUENCY RESPONSE CIRCUIT
The Audio signal, which has been delayed by 25 ms by the delay circuit, is passed to
the two pole low pass filter circuit a U26 and the four pole high pass filter at U31.
Trimpots R155 and R154 adjust the low pass filters cutoff frequency; pot R180 adjusts
the amount of high frequency added by the high pass filter. The outputs of these filters
are summed together along with the noise reduction signal at amplifier U30. The high
and low pass filters are adjusted to compensate for high frequency response variations
of the galvanometer. These circuits comprise the galvanometer high frequency
resonance equalization circuit. Trimpot R174 is a DC offset pot which allows
electronically changing the zero deflection setting of the galvanometer. It is factory set
for no offset.
DRIVE AMPLIFIERS
U29, U32 and transistors Q1 through Q4 form the output amplifier (Drive Amplifier) of
the RA-1712G. The two complementary drivers for a bridge output amplified capable of
85 volts peak-to-peak swing using a moderate supply voltage of ± 24 volts. The output
level of the system is set by the front panel 5K ten-turn potentiometer called DRIVE
control. The output amplifier is a 600Ωoutput impedance voltage source. The output is
connected to the galvanometer through the front panel 3 pole toggle switch labeled

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POLARITY. One pole of the switch is used together with one pole of the program
switch to light the RECORD light when the galvanometer is connected in the forward
bias record direction and the program switch is on. Capacitor C76 provides high
frequency compensation for the increased inductive reactance of the galvanometer at
high frequencies. For some galvanometer applications its value may be changes or it
may be eliminated altogether
METER CIRCUIT
Dual Amplifier U1 is a buffered slow decay peak follow-and-hold meter circuit. A peak
follow-and-hold circuit is used so fast transient overloads of significant duration (a
duration log enough to be heard on the recorded track) will be indicated on the meter/
POWER
Power for the system is provided by a bipolar regulated 24 volt supply. This voltage is
delivered directly to the output amplifier section of the RA-1712G. Power for the rest of
the systems circuitry is regulated down to bipolar 15 volts by on-board IC regulation.
CALIBRATION
Necessary Equipment: The following equipment is required to calibrate the RA-1712G:
Low distortion Audio signal source whose distortion is less than .05% across the
audio spectrum
Digital voltmeter
Distortion analyzer
Oscilloscope
Frequency counter
INPUT SIGNAL LEVEL CALIBRATION
Connect the signal source to the input of the RA-1712G. Adjust the input level to
achieve -2 dBm at Test point 1 (TP1). Turn the program switch on and the equalization
switch off. Set the step pot attenuator to the 0 position (the 12 o'clock position) and
adjust trimpot R31 so that the signal level at TP2 is 0 dBm.
NOTE: All measurements made during the calibration of the RA-1712G are made
between the stated test point (TP) and the RA-1712G ground. A
convenient ground is provided as a loop of bare wire near the center of
the printed circuit board.

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CLIPPER LEVEL CALIBRATION
In this calibration step the level at which the RA-1712G clips the audio signal after 100%
modulation is achieved is set. We recommend that the clipper be set to 2 dB over
100% modulation to provide maximum safety and long term stability of the light valve
modulator. The input signal is increased 2 dB over 100% modulation by advancing the
front panel step pot two steps. Observing the signal at TP4 with a scope, adjust trimpot
R84 until the onset of clipping is just noted. This sets the clipping level of the unit.
INPUT METER CALIBRATION
In this step the RA-1712G meter is set to read 100% in the IN meter mode when the
modulation level is 100%. Return the step pot to the 0 position and turn the meter
switch to the IN position and adjust trimpot R5 so that the meter reads 100%. Verify
that the signal level at TP3 is 0 dBm. If it is not, set it for 0 dBm by adjusting the input
level and repeat the above procedure.
NOISE REDUCTION OFFSET CALIBRATION
This step adjusts the off set of amplifiers U12 and U14. It is unlikely that the adjustment
of the offsets discussed in this paragraph will ever need to be done in the field. Their
effect on the operation of the system is very subtle and is not critical. Connect a jumper
across diode D5 and lift one end of diode D6 from the circuit board. Adjust trimpot R90
for 0 millivolts of offset voltage at TP8 with the program switch off. Next lift one end of
the diode D5 and reinsert diode D6and connect a jumper across it. Adjust trimpot R95
for 0 millivolts offset at TP8, again with the program switch off. After completing this
procedure ensure all jumpers are removed and the lifted ends of both diodes have been
returned to their proper location and are soldered in position. Adjust trimpot R72 so that
the voltage observed at TP9 just switches to -12 volts and remains there with no input
signal to the RA-1712G.
DELAY CALIBRATION
Connect a frequency counter or sweep time calibrated oscilloscope to TP10 and adjust
trimpot R106 to yield a 150 kHz square wave. This results in a 20 ms delay in the CCD
delay. With a 100% signal at 1 kHz applied as noted on the RA-1712G meter in the IN
mode, trimpot R119 is adjusted to give 0 dBm signal at TP5. With a scope probe
attached to TP6, adjust trimpot R144 so that the two sampled 1 kHz waveforms are
positioned directly on top of one another and appear as a single waveform. With a
100% signal applied at 1 kHz adjust trimpot R135 to yield minimum distortion as
measured at TP7. This completes the delay calibration.
NOISE REDUCTION CALIBRATION
With a 100% signal applied adjust trimpot R68 so that the voltage at TP8 just begins to
increase. For best results, trimpot R68 should be set just before this increase occurs.

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With no signal applied set trimpot R99 so that the voltage at TP11 is .864 volts. Apply a
100%, 1 kHz signal and adjust R174 so that the DC average voltage at TP11 is 0. Turn
the signal off and reset R99 to the specified .864 volts.
MARGIN ADJUSTMENT
This step adjusts the relative level of the audio and noise reduction signals by adjusting
the amount of audio signal summed into the pre-driver amplifier, U30. With a 100%
signal applied R151 is adjusted so that the peak signal level at TP11 is .965 volts. This
corresponds to a level of approximately -1 dBm.
OUTPUT METER ADJUSTMENT
Connect a 600Ωresistor between the modulator output terminals on the back panel of
the RA-1712G. Remove the input signal and turn the DRIVE control to maximum. Set
the meter switch to out and adjust trimpot R166 so that the meter reads 100%.
Note: The adjustment of the high frequency resonance circuits comprising
amplifiers U26 and U31 are discussed in the manual section on
INSTALLATION AND OPERATION.

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