Sencore VA62A Tech Tips

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~CJ.~
Means Success In Electronic Servicing
tech ...
This Tech Tip explains how to use the
Sencore VA62A Universal Video Ana-
lyzer, the SC61 Waveform Analyzer,
and a test tape recorded with VA62A
signals to identify the source of servo
troubles. The step-by-step procedures
use a combination of signal tracing and
signal substitution to qu ickly identify the
bad circuits. If you need background on
how servos operate , ask for a copy of
Tech Tip#176, which takes you through
circuit operation.
Organized Troubleshooting
Methods
The main difficulty in isolating servo
problems is the way sevetal feedback
loops overlap each other. It's common
Fig. 1: Servo problems usually cause the
picture to tear, or to change between clear
and snowy. If the tearing remains stationary,
or
it
the picture stays snowy alt the time, the
servos are not a likely cause.
Troubleshooting VCR Servos
to spend time troubleshooting one loop,
and to learn later that the trouble was
really in a different loop. To avoid this ,
follow these steps (each is covered in
detail later) :
1. Confirm a servo problem by analyz-
ing the symptoms with a test tape.
2. Confirm the 3.58 MHz reference sig-
nal is correct with the SC61 's digital
measurements .
3. Confirm if the cylinder and capstan
loops are locked to each other by com-
paring the head-PG and the control-
track signals .
4. Separate problems in the capstan
servo from those in the cylinder servo by
testing the 30 Hz feedback signals to
.. 1ps
see which one is wrong .
5. Separate speed from phase loop
problems with a combination of signal
tracing and signal injection.
6. Narrow the problem to a particular
component with signal tracing or com-
ponent tests.
Confirming Servo Symptoms
Most servo defects cause picture tear-
ing , and may also affect the linear (non-
hi-fi) audio . Some servo troubles also
blank the picture , hiding the symptom.
Picture tearing may take several forms .
Sometimes , the picture is clear for a
while , gradually turns to noise , and then
gradually returns to normal. In other
VCR Symptoms
Probably are servo related:
1. Linear (not Hi-Fi) audio is at wrong speed.
2.
Linear audio is changing in pitch (warbling).
3.
Picture alternates between clear and snowy
at a regular rate.
4.
A horizo ntal noise bar passes through the
picture from time to time, either moving from
top to bottom or bottom to top.
5. Picture noisy, and adjusting the tracking
control has no effect
May or may not be servo related:
1. Jitter, or noise near picture top or bottom .
2. Screen is blank, or audio is muted.
3.
Tape loads, but does not play.
4.
Machine begins to move tape, but then
shuts down.
5. VCR plays tapes it recorded okay, but will
not play tapes recorded on another machine.
Probably not servo related:
(Probable cause shown for each)
1. Picture is always snowy or constantly noisy,
but tracking control changes results. (Bad
head signal)
2. Picture has a noise bar which remains in the
same position on the screen. (Tape path)
3.
Picture has a line running horizontally across
the screen from left to right. (Scratched tape)
4.
Color is intermittent (or missing) but black
and white (luminance) signal is okay. (Bad
color circuit or weak head signal)
5.
Picture changes in brightness as tape plays;
especially on copies of tapes. (Copy protec-
tion on copied tape)
Fig. 2: Relating each symptom to its likely cause prevents troubleshooting servo circuits when
some other circuit is at fault.
ij@fj
~CJ.~