STAR *,ECH JOURNIU"4 ________________________ J_u_Lr_H_iH_4 _ _
_ _ 2_o _ _ _ _
I_ASERD•sc REPAIR:
PART 4: ALIGNMENT AND REPAIR, PR-S21 D TYPE DISC PLAYERS
"My intent with this 6-part series on
laserdisc repair is to get a lot of useful
information into the hands of working
technicians as painlessly as possible.
Therefore I will skip over some of the more
technical matters. This approach is
necessary, I have found, because the
alternative is to require technicians to study
for months before they have the proper
background for disc player repair. Even
without all that theory, there is a lot of good
that can be done, and that's my intent with
this series; to get the information where it
will do the most good." - Bill Turner
BY BILL TURNER, CUSTOMER SERVICE MANAGER, ELECTRO-SPORT, INC., COSTA MESA, CA
Our 6-part series continues with
"Alignment and Repair, PR-8210
Type Disc Players". Part 1,
entitled "Short of Actual Repair''
covered the most frequent
problems with disc players, and
gave maintenance information
applicable to all laser games. Part
1 appeared in our April issue.
Part 2, entitled "Theory and
Safety" covered how the disc
player works. How the Pioneer
PR-821 0 type disc players work.
General safety information for
working with laser and high
voltage equipment. Part 2
appeared in our May issue.
Part 3, entitled "Voiding Your
Warranty" covered disassembly
of the Pioneer PR-821 0 type disc
players, inspection of mechanical
components, inspection of fuses,
and general troubleshooting. Part
3 appeared in our June issue.
Part 4 will cover the most
frequent alignment procedures
for the Pioneer PR-821 0 type
disc players, the most frequent
problems and their solutions.
Next month, and the last
installment of this series are
outlined as follows:
AUGUST - PART 5: SOLID
STATE LASERDISC
PLAYERS- General
information on the new
generation of disc players.
Special maintenance and
alignment considerations for
solid state laser head
systems.
SEPTEMBER - PART 6:
COMPUTER-CONTROLLED
LASERDISC PLAYERS -
Covers the Pioneer PR-7820
type disc players. Explains
test and maintenance
considerations, and RS232
conventions.
ALIBNMENT AND
REPAIR, PR-Sli!'10 TYPE
DISC PLAYERS
T
he 821 0 type disc player
was not designed with laser
game applications in mind, and
because of this there are a few
inadequacies that become notice-
able in a laser game application.
Most severe of these problems is
that, when laser games are run 24
hours a day, disc player alignment
will be required every six months,
and if the environment at the loca-
tion is severe, alignment may be
required even more frequently.
A full alignment session on an 821 0
disc player can be a frustrating,
time-consuming experience. 29
separate electrical adjustments
are required for the full set-up.
However, in laser games, the disc
player has been fully set up at least
once, so the full 29 steps are rarely
if ever required. I have found that
the most common disc player per-
' formance problems are cleared up
with a simple, 3-step quick
alignment.
Insure that the disc player has been
thoroughly inspected and no
obvious problems exist before
beginning the alignment session.
Naturally, an alignment will not
correct problems associated with
smoke build-up, defective com-
ponents, or major mechanical
problems. The alignment, rather,
compensates
for
electrical
component aging, or replacement
of disc player sub-assemblies.
The Quick Alignment
Note: Inspect disc player fuses,
laser h. v. wires, laser ballast resistor,
objective lens, etc. before begin-
ning the alignment. Test points in
the PR-821 0 are on two separate
headers, one 12-pin and one 6-pin.
The quick alignment will use the
12-pin header only. Refer to the
tables in Figure 1 for test point
signal identification.
Step 1 is to insure that the disc
player is cold. If the disc player has
been running, let it sit idle with
power off for about 20 minutes to
insure that all components are cold.
Then perform the Focus Motor
Bridge Balance.
Focus Motor Bridge Balance
Figure 2 is a schematic representa-
tion of the focus bridge circuit.
Note that the op-amp bridge is
balanced by adjusting VR3. Find
test point 1 0 (TP1 0) on the test
point header, and connect a jumper
from TP1 Oto ground (on these disc
players, the chassis is cold, so any
convenient chassis location can be
used to ground the jumper). Now
locate TP3. Hook up a DVM to read
the voltage at TP3 and set the DVM
to a suitably sensitive scale (2 volts
DC for our Fluke meters). Record
the voltage from TP3, then dis-
connect the jumper from TP1 0 and
read the voltage again. Adjust the
variable resistor VR3 so that TP3
reads exactly 30 millivolts below
the level recorded with TP1 0
grounded.
This takes much longer to describe
than to perform, but remember -
the focus motor bridge balance
must be performed with the unit
cold. This adjustment should be
completed within the first minute
that power is applied to the disc
player.
Set DC Offset Levels
The second step in the quick align-
ment is to set the offset levels of
the focus, tracking, and tangential
error amplifiers. Because the offset
adjustments are all performed in
the same way, we can consider
them as one adjustment.
I prefer to set the offset levels with
an oscilloscope, because by ob-
serving the signals, marginal or
non-functional feedback amp per-
formance can be observed. The
signal should normally be glitch
free, and line noise should be less
than 1 0 millivolts. Waveform dis-
crepancies indicate component or
circuit problems.
Set the oscilloscope to a suitably
sensitive scale, like 0.5V/div. Set
the ground level so that the trace is
exactly centered on one of the grat-
ing lines, so that the signal can be
centered accurately.
For the focus offset adjustment,
place the 'scope probe on TP1
(remember the test points are not
in order, refer to the silk-screened
labels on the circuit board to be
sure the test point you select is the
one you want). Apply power to the
disc player, but leave the player in
the idle mode (don't press PLAY).
Observe the signal at TP1 . If the
signal is not within 20 mVofground
level, adjust VR1 so thatthe level is
ground, plus or minus 20 mV.
Setting the tracking offset is much
the same. Place the 'scope probe
on TP7, observe the signal, and
adjustVR6 so the signal reads within
20mV of ground.
Sameforthe tangential offset. Place
the probe on TP1 2, observe the
signal, and adjust VR404 so the
signal falls within the 20mV range.
Jump Pulse Adjustment
The jump pulse is the signal respon-
sible for the disc player "trick"
features (still/step, scan, search,