Westrex RA-1712A User manual

Westrex RA-1712A Record Electronics
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INTRODUCTION
The Westrex RA-1712A is an integrated photographic sound recording electronics
package for use with Westrex light valve recorders. The system accepts balanced
or unbalanced inputs. It provides a front panel input level adjustment range of 20
dB. High and low pass filters are front panel selectable for 16mm, 35mm or 8mm
sound track recording. High frequency pre-emphasis to compensate for printer or
other loss is also front panel selectable. Percent track modulation and average light
value current are monitored on a front panel meter. The system uses a CCD solid
state delay line to delay the audio signal in an anticipatory noise reduction system.
The noise reduction and drive signals are internally combined and their relative
levels set for a 3dB margin. The margin is independent of external control setting
and is factory set. The output level to the light valve is controlled by a single front
panel ten-turn drive control. The system is capable of driving any Westrex light
valve used for recording positive or negative photographic sound track.
INSTALLATION AND OPERATION
The RA-1712A accepts either single sided or balance line level high impedance
input. It is important to follow good grounding practice in the installation of the RA-
1712A. If balanced line input is available (i.e. transformer input), the center terminal
of the input to the RA-1712A should be connected to the “water pipe” ground with
the balanced input applied between the + (high side) and - (low side) input terminals.
This terminal is the transmission ground point. Chassis ground is accessible via the
ground lead on the power plug - or via one of the 4 screws attaching the back panel
to the RA-1712A.
For operation of the RA-1712A without the RA-1713A 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-1713A to access the filtered and level set signal of the RA-1712A and as a
signal injection point for signals from the RA-1713A. The short loops the signal
though).
The modulator output on the two terminal Jones strip of the back of the RA-1712A is
connected directly to the two input terminals of the light valve using 14 GA wire.
Warning! Neither of these terminals should ever be connected to ground!
Before turning the RA-1712A 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-1712A on or off
to protect the light valve from turn-on and turn-off transients. An input signal source
capable of supplying an audio input of at least -2 dBm should be connected to the
input of the RA-1712A. The program switch should be turned off.
Turn on the RA-1712A and toggle the polarity switch to the PLUS position. Observe
the modulator opening while the DRIVE control is advanced. If the ribbons of the

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light valve begin to move apart as the drive is increased, return the POLARITY
switch to the standby position and reverse the modulator leads on the back of the
RA-1712A. 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-1712A to have a photocell or visual monitor
of the actual modulation of the light valve
Turn on the program switch with a 400 Hz signal applied; increase its level until the
meter on the RA-1712A in the IN mode reads 100% modulation. Increasing the
level beyond that point should result in perceptible distortion being noted from the
photocell monitor. The distortion is the clipping the signal at 100% modulation. If
clipping 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
some clipping is observed within 1 dB over 100% modulation, the RA-1712A
calibration should be checked. (See section on Calibration.)
Having verified performance at 400 Hz, the next step is to adjust the light valve
equalization of the RA-172G. These adjustments are made inside the unit after
removing the top plate. A nominal adjustment of the light valve equalization circuit
has been set at the factory. However, because of the age and use of various light
valves, a readjustment may be necessary. Apply a signal to the input at 8.0 kHz and
listen to the modulated output on the modulator of the photocell monitor and/or
watch it on the modulator display screen of your recorder. Increase the input level
until the RA-1712A meter reads 100% in the IN mode. Now adjust trimpots R-154
and R-155 until exactly 100% modulation deflection of the light valve is noted either
visually or audibly at the onset of clipping which in this instance will sound like a faint
screeching sound. 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 10.5 kHz and its input level adjusted
to 100%. Trimpot R-180 is adjusted to achieve 100% modulation as noted either
visually or audibly. NOTE: The RA-1712A low pass filter should be set to the 12.5
kHz position in the above alignment procedures. The above procedure should be
iterated (since R-180also affects the setting of R-154 and R-155 and vice versa) until
both criteria are met.
RECOMMENDED USE
The RA-1712A may be used to record 35-mm, 16-mm and 8-mm positive and
negative sound tracks with Westrex modulators. 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 8.5 kHz or alternatively the HIGH PASS may be set to 85 Hz and the low

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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.
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-1712A, 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-1712A 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 LOW PASS front panel select switch selects the desired low
pass filters output. 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-1713A 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-1712A with the RA-1713A Auxiliary Electronics package.
For use without the RA-1713A, terminals A and B are shorted together, thus looping
the signal back into the RA-1712A.
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.

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This circuit prevents serious overdriving of the light valve modulator, which results in
spurious ringing distortion on the recorded optical 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.
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.
LIGHT VALVE 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 light valve. These circuits comprise the light valve high
frequency resonance equalization circuit. Trimpot R174 is a DC offset pot, which
allows electronically changing the zero deflection setting of the light valve. 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-1712A. The two complementary drivers for a bridge output amplified
capable of 85 volts peak-to-peak swing using a moderate supply voltage of ± 24

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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 current regulated
design. The amount of current passing through the light valve being driven is
measured as a voltage across the precision resistor R156 and used as negative
feedback to the driver amplifier U29. The output is connected to the light valve
through the front panel 3-pole toggle switch labeled POLARITY. One pole of the
switch is used together with one pole of the program switch to light the RECORD
light when the light valve 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 light valve at high frequencies. For some light
valve 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 long 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 15-volt supply. This voltage
is delivered directly to the output amplifier section of the RA-1712A. Power for the
rest of the systems circuitry is regulated down to bipolar 12 volts by on-board IC
regulation.
CALIBRATION
The following equipment is required to calibrate the RA-1712A:
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-1712A. Connect a short piece of
wire from the terminal marked "A" on the back panel to the terminal marked "B".
Connect another short piece of wire from the modulator "+" terminal to the modulator

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"-" terminal. Set the HIGH PASS switch to the MIN position and the LOW PASS
switch to the 12.5 position. Set the POLARITY SWITCH TO THE STBY position.
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-1712A are
made between the stated test point (TP) and the RA-1712A ground. A
convenient ground is provided as a loop of bare wire near the center
of the printed circuit board.
CLIPPER LEVEL CALIBRATION
In this calibration step the level at which the RA-1712A 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-1712A 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, 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.
Adjust trimpot R72 so that the voltage observed at TP9 just switches to
approximately -12 volts with no input signal in to the RA-1712A (i.e., with the
PROGRAM switch in the OFF position. Do not rotate the trimpot to its limits; it
should be near midrange. 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 D6. Adjust trimpot R95 for 0
millivolts offset at TP8, again with the program switch off. After completing this
procedure ensure the lifted ends of both diodes have been returned to their proper
location and are soldered in position. Readjust trimpot R72 so that the voltage
observed at TP9 just switches to -12V and remains there with the PROGRAM switch
OFF.

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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 25-msec delay
in the CCD delay. With a 100% signal at 1 kHz applied as noted on the RA-1712A
meter in the IN meter mode, trimpot R119 is adjusted to give -2 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. Adjust R168 so that the voltage at TP11 is 0 mV. With PROGRAM switch
OFF, set trimpot R99 so that the voltage at TP11 is -.790 volts.
NOISE REDUCTION 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.
Connect the scope probe to TP13. Set the scope on DC and have the trace in the
center. Turn the PROGRAM switch to OFF and set the POLARITY switch to PLUS.
Increase the DRIVE control to achieve a -3.4 VDC deflection. (Be sure zero is the
center line.) Now turn the PROGRAM switch to ON and apply a 100% signal at 1
kHz to the RA-1712A. Adjust R168 so that the positive and negative peaks are
equal. (DC average voltage is zero.) Now turn the PROGRAM switch OFF and
readjust the DRIVE control for a -3.4 VDC deflection. Turn the PROGRAM switch
ON again and adjust R151 so that the peak signal levels at TP11 are ±3.8 volts
OUTPUT METER ADJUSTMENT
Turn the PROGRAM switch OFF and turn the DRIVE control to maximum. Set the
POLARITY switch to PLUS and set the METER switch to OUT. 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

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SCHEMATICS

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