Sencore VC93 User manual

Troubleshooting VCR Luminance Circuits With
The VC93 All Format VCR Analyzer
A VCR contains five major sections: lumi-
nance, chroma, audio, servo, and system
control. The luminance section of a VCR
receives the FM signal from the tape and
processes it into a form that can be used by
a television or monitor. The blockdiagram in
Figure 1 shows the luminance section of a
VCR. Defects in any of these luminance stages
produce the same symptom - a snowy or
muted playback picture. This often makes
isolating the symptom to a specific circuit
difficult.ThisTechTipexplains how to trouble-
shoot the luminance section of a VCR using
the signals supplied by the VC93.
Before explaining how to use the VC93, let’s
briefly review how the luminance
stages func-
tion and what signals are found at each block
during playback.
How The Luminance Signal Is
Processed
The FM luminance signal begins at the video
heads. After the spinning heads pick up the
signal, it is coupled through the rotary trans-
former windings and fed to the head pream-
plifiers. The outputfrom the head preamps is
selected by the A/B head switcher. Only the
signal from the head which is in contact with
the tape is fed to the remaining circuits. This
keeps unwanted noise from the head not in
contact with the tape from getting into the
picture.
After the A/B headswitcher, the FM lumi-
nance signal is separated from the
down-
converted chroma signal. The color and lu-
minance signals are processed separately
until they are combined in the Y/C mixer. The
signal at the Y/C mixer output is composite
video which is fed to the VIDEO OUT jack and
RF modulator. The VC93 provides the special
luminance signals needed to troubleshoot
the circuits from the video heads to the RF
modulator.
The make up of the FM luminance signal is
different for each VCR format (VHS, Beta, U-
Matic, & 8MM). The VC93 generates the
necessary luminance signals needed for each
VCR format. For specific information on the
VCR formats refer to Tech Tip #189, “Com-
parison Of VCR Formats.”
Troubleshooting The Luminance
Signal Path
The VCR luminance signal path can be di-
vided into three major sections for trouble-
shooting:
1. The video head channel
2. The FM luminance stages
3. The video stages
Since the video heads pick up the tape signal,
the head channel is a good place to begin
signal injection.
Injecting the VC93 “Lum” PLAYBACK SIG-
NAL at the input of both preamps simulta-
neously will prove whether or not the FM
luminance circuits and the video circuits that
follow are able to process a good signal. Use
the “Playback Head Sub” position of the
PLAYBACK RANGE switch when injecting
into the head channel. If the circuits properly
process the VC93 signal (as indicated by the
“Color Bars” pattern on the playback moni-
tor), you have narrowed the problem to the
video heads or to the rotary transformer. For
additional information on testing the head
circuits refer to Tech Tip
#194, “Trouble-
shooting ‘Bad Head’ Symptoms With The
VC93 All Format VCR Analyzer.”
If your injection into the head channel fails to
return a picture on the playback monitor, a
Fig. 1: The luminance signal path in a VCR.

Fig. 2: The VC93 provides the signals to inject into each of the luminance stages.
problem exists in either the FM luminance or
video stages. The next step is to inject a
signal into the FM luminance stages after the
head switcher to determine if the video cir-
cuits are functioning. Again use the “Lum”
PLAYBACK SIGNAL to inject into these stages.
But because the signal has been amplified by the
preamps, use either the
".5VPP" or "5VPP" range
of the PLAYBACK RANGE switch.
The FM luminance signal is converted to
video (baseband luminance) by the FM de-
tector. Use the “Lum” DRIVE SIGNAL to
inject into all the stages between the FM
detector output and the Y/C mixer. The Y/C
mixer combines the chroma signal with the
luminance signal. Use the “Composite Video”
DRIVE SIGNAL to inject after the Y/C mixer.
How To Inject The luminance
Signals From The VC93
As explained in the previous section, the
VC93 provides several signals to inject into
the VCR luminance stages (Figure 2). The
remainder of this Tech Tip shows you how to
inject the PLAYBACK and DRIVE SIGNALS,
and what you should expect when injecting
these signals.
When injecting into any of the luminance
stages, always have a test tape playing to
make the VCR circuits operational. All the
procedures explained in this Tech Tip use the
internal “Color Bars” MODULATION, so in-
jecting the VC93 signal into a working lumi-
nance stage will produce a black and white
splitfield video pattern on the playback moni-
tor connected to the VCR. If you desire, you
can use the VA62A Universal Video Analyzer
to provide anothervideo modulation pattern.
Injecting the PLAYBACK SIGNAL S
Use the PLAYBACK SIGNALSto inject be-
tween the video heads and the FM detector.
The VC93 provides the properly modulated
FM luminance signal for each VCR format.
To inject the “PLAYBACK SIGNALS” signal:
1. Insert a prerecorded work tape into the
VCR and press play.
2. Set the VC93 as follows:
a. VCR FORMAT to match format being
serviced.
b. MODULATION to “Color Bars” or
“External”.
c. PLAYBACK SIGNAL to “Lum”.
d. PLAYBACK RANGE to match the PPV
of the circuit under test.
3. Connect the HEAD SUBSTITUTION TEST
LEAD to the VC93 PLAYBACK OUTPUT
jack.
4. Connect the HEAD SUBSTITUTION TEST
LEAD to the circuit (Figure 3):
a. Red lead to injection point.
b. Black lead to circuit ground.
5. Observe the playback monitor for an
improved picture.
6. Adjust the PLAYBACK RANGE and
PLAYBACK LEVEL for the best picture.
a. Only use the “Playback Head Sub”
PLAYBACK RANGE when injecting
into the head channels.
b. Use the
".5VPP" and "5VPP" PLAY
BACK in the FM stages after
the head preamps.
This injection checks the ability of the lumi-
nance circuits to pass the signal onto the
video circuits. If the picture on the TV moni-
tor is good, the stages after the injection

point are working properly. If the picture on
the TV is bad, you are injecting before the
problem. Continue isolating the defective
stages by moving your injection closer to the
output until the injection produces a good
picture.
The “Lum
& Chroma” PLAYBACK SIGNAL is
used to troubleshoot defects in the chroma
stages. It provides both the luminance and
down-converted chroma signals for each VCR
format. The VC93 must be phase locked to
the VCR while injecting into the chroma cir-
cuits. Further details on this signal and on
troubleshootingcolorproblemsmaybefound
in Tech Tip
#195, “Troubleshooting VCR
Chroma Circuits With The VC93 All Format
VCR Analyzer.”
Injecting the DRIVE SIGNAL S
Use the DRIVE SIGNALS to inject into the
luminance stages after the FM detector. The
“Lum” Drive Signal is used between the FM
detector output the luminance input of the Y/
C mixer. Use the “Composite Video” Drive
Signal after Y/C mixer output.
To inject the DRIVE SIGNALS:
1. Inset-t a work tape into the VCR and
press play.
2. Set the VC93 as follows:
a. MODULATION to “Color Bars” or
“External”.
b.
DRIVE SIGNAL to “Lum” or “Com-
posite Video”.
C.DRIVELEVEL to "0".
d. Set OUTPUT SIGNAL MONITOR/DVM
to “Drive Signal”.
3. Connect the DIRECT TEST LEAD to the
VC93 DRIVE OUTPUT jack.
4. Connect the DIRECT TEST LEAD to the
VCR:
a. Red lead to circuit test point.
b. Black lead to circuit ground.
5. Observe the picture on the playback
monitor.
6. Adjust the DRIVE LEVEL control for the
best picture.
This injection checks the ability of the video
circuits to process the signal and pass it on to
the video output jack or the RF modulator. If
the picture on the TV monitor is good, all the
circuits between the injection point and the
video output are good. If the picture on the TV
is bad, you are injecting before the problem.
Continue to isolate the defective stages by
moving your injection closer to the output
until the injection produces a good picture.
You are now injecting before the defective
stage.
Fig. 3: Use both red clip leads on the HEAD SUBSTITUTION TEST LEAD to inject in to the
head channels
simultaneously (A). You only need to
connect one of the red clip leads
when
injecting after the A/B headswitcher (B).

Notes:
Form 4708
Printed in U.S.A.