Sencore VC93 User manual

Learning To Use The VC93 All Format VCR Analyzer
The VC93 All Format VCR Analyzer provides
the necessary signals to troubleshoot the
luminance, chroma, and audio stages of a
VCR. It allows you to inject known good
signals to help isolate problems to the defec-
tive stage. The VC93 also provides special
Servo Analyzer Tests to help isolate servo
problems.
This Tech Tip will familiarize you with the
VC93 signals and servo tests, and where to
use them when servicing VCRs. Information
on troubleshooting specific VCR circuits is
covered in other Tech Tips.
This Tech Tip is organized according to the
major features of the VC93. You should have
the Universal VCR Block Diagrams provided
Table 1 - Key VCR Format Characteristics
with your VC93 manual for reference as you
read through this Tech Tip.
VCR FORMAT
There are many different VCR formats. The
main difference between each is the makeup
of the signals that are recorded onto the tape.
These differences include luminance frequen-
cies, the color-under signal, and the audio
frequencies used for Hi-Fi Stereo. Table 1
summarizes the key characteristics of each
VCR format supported by the VC93. The
VC93 provides signals to troubleshoot all
color-under format VCRs. Tech Tip 189,
“Comparison Of VCR Formats,” provides a
more detailed discussion of each format.
The VC93's VCR FORMAT switch
sets the signals to match the
VCR format you are servicing. It
determines the makeup of the
luminance, chroma, and audio
signals coming from the PLAY-
BACK OUTPUT jack, and selects
the frequency of the “Head
Switch” DRIVE SIGNAL. Set VCR
FORMAT switch to match the
type of VCR you are servicing.
PLAYBACK SIGNAL
The VC93's PLAYBACK SIGNAL
switch and PLAYBACK OUTPUT
jack provide the proper signals
for injecting into the stages be-
fore the FM luminance detector,
chroma frequency convertor, and
FM audio detector. The test points
that require the use of the Play-
back Signals are summarized in
Table 2.
Note: The VCR FORMAT switch
must be set to the proper format
for the playback signals to be
correct at the PLAYBACK OUTPUT jack.
The PLAYBACK SIGNAL switch selects the
type of signal available at the PLAYBACK
OUTPUT jack. The peak-to-peak level of each
signal is adjustable with the PLAYBACK
RANGE and PLAYBACK LEVEL controls. Use
the HEAD SUBSTITUTION TEST LEAD to
inject any of the Playback Signals into the
appropriate test point. Following is a sum-
mary of each Playback Signal.
Luminance And Chrominance Signals
LUM (FM Modulated): This signal duplicates
the FM modulated luminance portion of the
signal found in the luminance stages of a VCR
or camcorder. It does not include the color
information. Injecting the FM LUM signal into
working VCR stages produces only the black
and white portion of the video pattern. Use
this signal to check the video heads,
pre-
amps, and other FM luminance stages.
LUM & CHROMA: This signal includes both
the FM luminance and down-converted
chroma. Injecting the LUM CHROMA sig-
nal into a working VCR produces the COLOR
BARS pattern, or the pattern selected by the
“External” position of the MODULATION con-
trol if the VC93 is phase locked to the VCR.
This signal can also be injected into the FM
luminance stages, but its main application is
to phase lock the VCR to the VC93 while
injecting into the chroma circuits.
How To Phase Lock The VC93 To A VCR
For Color Troubleshooting
The chroma signal in color-under VCR for-
mats is phase shifted each video field to
minimize color crosstalk. The phase shifting
is controlled by the SW30 signal. In order for
the VCR to reproduce color when a signal
from the VC93 is injected into it, the color
phase generated by the VC93 must match the

Fig. 1: The Playback Signals are used to inject before the FM detector, frequency convertor, and FM audio detector.
color phase the VCR is looking for.
For example, when the SW30 pulse in the
VCR is
"+", the VCR is looking for a +180º
shifted chroma signal; when it goes "-" the
VCR chroma circuits need a -180º shifted
signal. But unless the VC93 is phase locked
to the VCR’s
SW30, the VC93 may be produc-
ing +180º shifted chroma while the VCR is
looking for the opposite phase. The result will
be no color, even though both the VCR and
VC93 are working correctly. (The luminance
will play back fine.) The CHROMA LOCK
INPUT jack on the VC93 allows you to phase-
lock the VC93 to the VCR.
To Phase Lock The VC93 To A VCR:
1. Insert a blank tape in the VCR and press
“PLAY”.
2. Set the VC93 as follows:
a. VCR FORMAT to format being serviced.
b. MODULATION to ‘Color Bars”.
c. PLAYBACK RANGE to
"5VPP".
d. PLAYBACK LEVEL to midrange.
e. PLAYBACKSlGNALto“Lum
& Chroma".
3. Connect the HEAD SUBSTITUTION TEST
LEAD to the PLAYBACK OUTPUT jack.
4. Connect the HEAD SUBSTITUTION TEST
LEAD to the desired test point.
5. Connect the CHROMA LOCK TEST LEAD to
the CHROMA LOCK INPUT jack.
6. Connect the CHROMA LOCK TEST LEAD to
the SW30 test point in the VCR. (The
CHROMA LOCK INDICATOR lights when
you are connected to the correct point.)
7. Observe the monitor for a color picture.
8. If you don’t see color, press the CHROMA
LOCK PHASE button to select the other
SW30 phase.
9. Adjust the PLAYBACK LEVEL for best color.
Audio Signals
The Audio playback signals duplicate the FM
audiosignalsfound in Hi-Fi stereo VCRs. Use
these signals in the Hi-Fi audio stages be-
tween the audio heads and FM detector. The
VCR FORMAT switch automatically selects
the proper signal makeup of the playback FM
audio signal to match each format. Refer to
Tech Tip #190 "How VCR Hi-Fi Stereo Works"
for more information on the different Hi-Fi
schemes.
To simplify troubleshooting, the audio play-
back signals are continuous carriers at the
proper left and right frequencies so they can
be injected without needing to be synchro-
nized to the VCR. A 1 modulating tone is
provided for all audio playback signals when
the MODULATION switch is set to “Color
Bars”. Use the HEAD SUBSTITUTION TEST
LEAD to injectthe audio playbacksignals into
the VCR audio circuits.

STEREO AUDIO: This signal produces equal
left and right modulation. When you inject
this signal into working Hi-Fi VCR circuits
you should obtain equal left and right output.
STEREO R ONLY AND STEREO L ONLY: Use
these signals to isolate problems in the left or
rightaudiochannels. Bothchannelsare modu-
lated with the same tone, selected by the
MODULATION switch. When injecting into a
working Hi-Fi channel, you should obtain an
output only on the selected channel.
Using The Playback Signals
1. Set up the VC93 as follows:
a. VCR FORMAT to desired format.
b. MODULATION to COLOR BARS.
c. PLAYBACK RANGE to level necessary.
d. PLAYBACK SIGNAL to desired signal.
2. Connect the HEAD SUBSTITUTION LEAD.
a. Red lead or leads to injection point. (Both
red leads have the same signal.)
b. Black lead to ground.
3. Adjust the PLAYBACK LEVEL control for
the best picture or audio on the playback
monitor.
When you inject the VC93 Playback Signals
after a defective stage, the playback monitor
will return to a good output. This indicates
that all
the circuits after the injection point are
good. If you do not obtain a good picture (or
audio) your injection point is before the de-
fectivestage. Continue moving your injection
point to a later stage until a good output
returns. The stage before the injection point
that produces a good picture or audio con-
tains the defect.
Table 2 summarizes where to inject the VC93
Playback Signals on the Universal VCR Block
Diagrams. Use this table for a reference when
injecting the VC93 Playback Signals. For ad-
ditional troubleshooting information refer to
Tech Tip #194, “Troubleshooting “Bad Head”
Symptoms with the VC93 All Format VCR
Analyzer.”
DRIVE SIGNAL
The VC93 DRIVE SIGNAL switch and DRIVE
OUTPUT jack provide the proper signals for
injecting into the stages after the FM lumi-
nance detector, chromafrequency convertor
or FM audio detectors. The test points that
require the use of the Drive Signals are sum-
marized in Table 3.
Except for the HEADSWITCH Drive Signal,
the VCR FORMAT switch setting does not
affect the Drive Signals. The HEADSWITCH
Drive Signal changes from 30 Hz to 15 Hz
when the VHS-C Format is selected. The
detected video and audio signals are the
same in all other VCR formats. For simplicity,
always set the VCR FORMAT switch to match
the VCR format you are servicing.
The DRIVE SIGNAL switch selects the signal
available at the DRIVE OUTPUT jack. The
peak-to-peak level is adjustable with the
DRIVE LEVEL control, and is displayed in the
right-hand LCD display when the OUTPUT
SIGNAL
MONITOR/DVM switch is set to
“Drive Signal”. The voltage is variable from -
10 VPP to +1 0 VPP with the "-" signals being
inverted from the "+" signals. Use the DI-
RECT TEST LEAD to inject any of the Drive
Signals into the appropriate test point. Fol-
lowing is a summary of each Drive Signal.
COMPOSITE VIDEO: This is the standard
NTSC composite video waveform including
luminance, color, and sync. Use it to inject
into the RF modulator, Y/C mixer output, and
these test points. Universal VCR Block Diagram.
Table 2: Inject the VC93 Playback Signals at
other stages that normally have both lumi-
nance and color signals. The MODULATION
switch determinesthevideo pattern selected.
LUMINANCE: This signal is the luminance-
only portion of the composite video signal
selected by the MODULATION switch. Use it
in the stages between the output of the FM
luminance detector and the input of the Y/C
mixer.
Tech Tip #193, “Troubleshooting VCR
Luminance Circuits with the VC93 All Format
Analyzer,” provides additional information
on troubleshooting these stages.
CHROMA: This signal is the chrominance-
only portion of the composite video selected
by the MODULATION switch. Use it in the
stages between the output of the frequency
converter (color detector) and the Y/C mixer
inputs. Tech Tip #198, "Troubleshooting VCR
Chroma Circuits with the VC93,” provides
additional information on troubleshooting
these stages.
AUDIO: The Audio Drive Signal is a baseband
(20 Hz-15 kHz) audio signal at the frequency
determined by the setting of the MODULA-
TION switch. Inject this signal into audio
stages after the FM audio detector.
3.58
MHz: This Drive Signal is a continuous
sine wave that can be used to substitute for
the 3.58 (3.579545) MHz color oscillator, or
the 3.58 MHz reference for the servo circuits.
HEADSWITCH: This signal duplicates the
square wave signal found at the playback
A/B headswitcher. Most VCR formats use a
30 Hz headswitching signal. VHS-C format
VCRs, however, use both 15 Hz and 30 Hz
switching signals. When the VCR FORMAT
switch is set to VHS-C or SUPER VHS-C the
HEADSWITCH Drive Signal frequency is
15 Hz. The frequency is 30 Hz for all other
formats.
SW30: This signal substitutes for the 30 Hz
(29.97 Hz) input to the chroma conversion
circuits. The actual frequency is the compos-
ite video frame rate, 29.97 Hz. Use the SW30
Drive Signal and the LUM
& CHROMA Play-
back Signal when troubleshooting chroma
stages.
VCR to the VC93 using the CHROMA LOCK
CHROMA KEY PULSE: The CHROMA KEY
PULSE Drive Signal provides a substitute for
the horizontal sync input to the chroma con-
version circuits. Always inject the LUM &
CHROMA Playback Signal and phase

Fig. 2: Drive Signals are injected after the FM detector, frequency converter, and FM audio detector.
INPUT jack when using this signal to
troubleshoot the
chroma conversion cir-
cuits
Using the Drive Signals
1. Select the desired DRIVE SIGNAL.
2. Set the DRIVE LEVEL to
"0".
3. Connect the DIRECT TEST LEAD to the
DRIVE OUTPUT jack.
4. Connect the test lead to the circuit.
a. Red lead to the injection point.
b. Black lead to circuit ground.
5. Set the OUTPUT SIGNAL MONITOR/
DVM switch to DRIVE SIGNAL.
6. Adjust the DRIVE LEVEL control to match
the level of the circuit into which you are
injecting.
When you inject the VC93 Drive Signals
after the defective stage, the playback moni-
torwill show a good picture. This indicates
that all the circuits after the injection point
are good. If you do not obtain a good
picture, your injection point is before the
defective stage. Continue moving your in-
jection point to a later stage until a good
picture returns. The stage before the injec-
tion point that produces a good picture
contains the defect.
Table 3 shows where to inject the VC93
Drive Signals on the Universal VCR Block
Diagrams. Use it as a reference when in-
jecting the Drive Signals.
MODULATION
The MODULATION switch determines the
video and audio modulation for the Play-
back and Drive Signals. It also determines
the modulation of the composite video and
audio signals available at the STD VIDEO
OUTPUT and STD AUDIO OUTPUT jacks.
COLOR BARS: In this position, the VC93
provides an internally generated “Split
Field” color bars pattern and 1
kHz audio
modulation.
EXTERNAL: In this position, external video
and audio may be used to modulate the
VC93. The modulation is supplied through
the SYNCHRONIZING SIGNAL INPUT jack
located on the rear of the unit. Connect the
optional SYNCHRONIZING INTERCON-
NECT CABLE to the ACCESSORIES OUT-
PUT SIGNALS jack on the VA62 or VA62A.
STD VIDEO And STD AUDIO
OUTPUT
The standard output jacks provide signals
to feed into the video and audio inputs on
a VCR or camcorder. Use these jacks for
making “work tapes” and to check the
video and audio input circuits.
Servo Analyzer Tests
The VC93 provides five servo tests which
identify problems in VCR servo circuits.
Each test checks out a specific servo func-
tion. Detailed information on using the
servo tests is provided in Tech Tip

cient amplitude, excessive
noise, or that is missing will
result in a dashed line dis-
play. If this occurs, use the
SERVO TROUBLESHOOTING
TEST LEAD.
The SERVO TROUBLE-
SHOOTING TEST LEAD con-
nects to the CTL and SW30
signals inside the VCR. Use
this lead to confirm any BAD
test result obtained when us-
ing the Performance Test
Lead, and when the audio or
video signal is too poor to
obtain a reading with the Per-
formance Test Lead. The
Servo Test Leads can be used
to isolate servo problems to a
specific servo loop.
The SERVO ANALYZER read-
out displays the results of the
Servo Analyzer Test in a per-
cent-of-error and a “Good/
Bad” indication. The “Good/
Bad” indication result is a
quick way to determine servo
operation, while the percent-
of-error reading provides
more specific troubleshoot-
ing information. The capstan
and drum “Speed Error” tests
also display a
"+/-" indication
to show if the speed is too
fast or too slow.
Perform the Servo Analyzer
tests starting with the "Ser-
Table 3: Inject the VC93 Drive Signals at these Universal VCR
vos Locked" test and con-
Block Diagram test points.
tinue clockwise through all
switch positions. If you ob-
tain a BAD result on a servo
“Testing VCR Servos with the VC93 All
Format VCR Analyzer.” The following in-
formation is a summary of the servo tests.
The servo tests may be performed with
either the SERVO PERFORMANCE TEST
LEADS or the SERVO TROUBLESHOOT-
ING TEST LEADS. The Performance Test
leads connect to the VCR’s audio and
video output jacks. They provide a fast,
overall checkout of the servos when the
VCR is producing an audio signal and at
least a partial video signal. You must use
the SERVO PERFORMANCE TEST TAPE
when using the Performance Test Leads.
An audio or video signal that has insuffi-
test, continue with the remaining tests to
further isolate the problem. Following is a
brief summary of each of the servo tests.
SERVOS LOCKED: This test determines if
the servo capstan phase loop and drum
phase loop are locked to the reference
signal.
CAPSTAN SPEED ERROR: This test deter-
mines if the capstan servo is operating at
the correct speed. It identifies speed select
circuit problems and speed related
problems.
CAPSTAN JITTER: This test measures how
Fig 3: Perform all the Servo tests, starting with
the SERVOS L OCKED test.
constant the capstan movement is. It helps
identify capstan phase problems and me-
chanical problems including motor bear-
ings, bad idlers, etc.
Note: 8MM VCRs do not use a CTL pulse
or linear audio. Therefore, the capstan
servo tests DO NOT APPLY to 8MM VCRs.
DRUM SPEED ERROR: This test deter-
mines if the drum is operating at the cor-
rect speed.
DRUM JITTER: This test measures how
constant the drum speed is. It helps iden-
tify drum related problems including bad
motor bearings and a bad drum phase
loop.
To use the Performance Test Lead:
1. Connect the Performance Test Lead to
2. Connect the yellow VIDEO phono plug
to the VCR’s “Video Out” jack.
3. Connect the red AUDIO phono plug to
the VCR’s “Audio Out” jack.
4. Set the SERVO ANALYZER TEST switch
to SERVOS LOCKED.
5. Place the SERVO PERFORMANCE TEST
TAPE into the VCR and press play.
6. Perform all five Servo Tests and note the
results.
If the Servo Tests give a BAD indication on
any test, or if the display shows dashes,
connect the Servo Troubleshooting Test
Lead to confirm the problem.

voltagespeciallydesigned
fortroubleshooting servo
circuits. The voltage is var-
ied with the SERVO SUB
BIAS LEVEL control. The
level is displayed in the
OUTPUT SIGNAL LEVEL/
DVM readout when the
OUTPUT SIGNAL MONI-
TOR/DVM switch is set to
“Servo Sub Bias.”
This DC supply provides
up to
1 amp of current at
10 volts and will sink (pull
down) current to 500
milliamps. These current
levels are necessary to
“take control” of some
servo circuits, but if used
incorrectly they may dam-
age components such as
the motor drive.
Table 4: Use this table to isolate servo troubles to either a capstan
or drum servo problem.
When using the SERVO
SUB BIAS LEVEL supply,
observe these
precau-
tions:
To Use the Troubleshooting Test Lead:
1. Connect the Troubleshooting Test Lead
to the SERVO ANALYZER INPUT jack.
2. Connect the black GND test clip to a
servo circuit ground point in the VCR.
3. Connect the red CTL test clip to the VCR
CTL test point.
4. Connect the yellow SW30 test clip to the
VCR SW30 test point.
5. Set the SERVO ANALYZER TEST switch to
SERVOS LOCKED.
6. Place a test tape recorded at any speed into
the VCR and press play.
7. Perform all five servo tests and note the
results.
Compare the results with Table 4 to isolate
the servo problem. If all five Servo Analyzer
Tests give a GOOD indication, the servos are
functioning properly.
SERVO SUB BIAS
The SERVO SUB BIAS provides a variable DC
1. Set the bias voltage to the measured circuit
voltage before connecting to the circuit.
2. Never adjust the voltage more than
+/- 0.5 volt from the measured circuit
voltage.
To use the SERVO SUB BIAS.
Measure the voltage at the test point
into which you will inject.
Set the OUTPUT SIGNAL MONITOR/DVM
switch to SERVO SUB BIAS.
3. Adjust the SERVO SUB BIAS LEVEL
control for the voltage measured in step 1.
4. Connect the BIAS SUPPLY TEST LEADS
to the SERVO SUB BIAS OUTPUT jack.
5. Connect the test leads to the circuit test
point:
a. Connect the black lead to circuit
ground.
b. Connect the red lead to the injection
point.
c. If a negative voltage is required reverse
the lead connections.
Measuring Drive Signals And
External DCV And PPV
The autoranged DVM monitors the internal
Drive Signals, Servo Sub Bias, and external
voltages. The OUTPUT SIGNAL MONITOR/
DVM switch selects the desired measure-
mentfunction. The measurement is displayed
in the right-hand, OUTPUT SIGNAL MONI-
TOR/DVM readout.
Use the DVM TEST LEADS to connect the
PPV & DCV INPUT jack to measure external
PPV and DC voltages. The PPV meter has a
wide, 5 MHz frequency response that allows
the
PPV function to accurately measurevideo
signals and offers low-level sensitivity to test
rotary transformers.
To measure an internal Drive Signal:
Set the OUTPUT SIGNAL MONITOR/DVM
switch to DRIVE SIGNAL or SERVO SUB
BIAS.
Read the level in the OUTPUT SIGNAL
LEVEL/DVM readout:
a. DRIVE SIGNALS are measured in PPV.
b. SERVO SUB BIAS are measured in DCV.
Adjust the corresponding LEVEL control
for the desired amount.
To measure external PPV and DCV:
1. Connect the DVM TEST LEADS to the PPV
& DCV INPUT jack.
2. Connect the test lead to the circuit test
point.
3. Set the OUTPUT SIGNAL
switch to PPV or DCV.
4. Read the level in the OUTPUT SIGNAL
LEVEL/DVM readout.
5. A flashing “888” display indicates the
applied signal is greater than 199 volts.
Form4806
Printed in U.S.A.