Star Tech Journal

28
STAR*TECH JOURNAL/DECEMBER 1983
Troubleshooting E/ectrohome and Wells Gardner Monitors (Part I) continued from page 27.
6. NO RASTER OR HIGH VOLTAGE, HAS FILAMENT
Since both the filament and high voltage are taken off the flyback windings,
missing one of these two voltages tells us the input signal is good to the
flyback. However, that particular section of the transformer may be bad to
prevent high high voltage.
The high voltage lead which connects to the picture tube comes directly
off the fly back. The secondary windings of the transformer for filament and
high voltage are separate from each other. The focus and screen voltage
come off the same winding as the high voltage. Test whether these voltages
are present ( focus measures with a high voltage probe). If not, suspect the
transformer. With voltages present, unplug power to monitor and discharge
high voltage from the second anode of the CRT to ground.
Disconnect the second anode cap from the CRT and place the cap by
itself so it won't short to anything. Be cautious, this voltage is normally about
25,000 volts DC. Power up the monitor. If you hear it snapping, crackling,
arcing or hissing, tum off the monitor. The high voltage is present, but the
CRT is defective. If no arcing sounds can be heard when the monitor is
powered up, measure the second anode cap with a high voltage probe. High
voltage is present, suspect the CRT - if it is missing, it can possibly be a
defective flyback transformer.
High Voltage Limiter (Protection) Circuit
The HVL circuit, also known more commonly as the high voltage limiter, is
a protection circuit Federal regulations require all manufacturers to provide
a way to protect individuals from receiving X-radiation, which can be
emitted from the monitor by excessive high voltage.
CONDITION
Raster, B+ High
Normally, as the B+ rises, so does the high voltage. To prevent
excessive high voltage, a protection circuit is added to the monitor which
samples the B+ voltage. The protection circuit acts as a triggering circuit, so
that when the B+ reaches a certain increased amount, the HVL is activated,
thus allowing no high voltage to the picture tube.
This symptom was mentioned earlier as no raster, no filament, no high
voltage and finding that the B+ measured too high.
An activated HVL circuit (higher than normal B+) can be caused
basically by one of three areas: the power regulator, protection, or horizontal
circuits. To narrow down the problem, tum off the monitor and disconnect
the triggering device. On Electrohome this would be the collector of X701 .
Wells Gardner is the collector of the transistor TR353.
After the collector lead has been disconnected from the rest of the board
circuitry, power up the monitor. At this point, the screen will show one of
two conditions: a raster (lit screen) or no raster. If a no raster condition is
seen, verify the no raster l>Y turning up the brightness control.
Obtaining a raster, measure the B+ at the emitter of the power regulator
transistor (Electrohome X04, Wells GardnerTR502). Normal B+ voltage
on the Electrohome monitor is 120vdc. Wells Gardner is 127vdc.
Normal B+ with a raster - the problem will be in the protection circuit.
B+ still high with raster indicates the problem is in the power regulator
circuit. No raster condition - the problem will be definitely in the horizontal
section.
Refer to the chart below for a breakdown of conditions and suspected
components.
ELECTROHOME
WELLS GARDNER
Raster, B+ Normal
D905, X902,
X901,X04
X701, D-701
X501, TR501,
TR502
TR353, TR354
No Raster
IC501
TR351
No Raster
X501, T501
TR352, T602
No Raster
X01, T502
TR601, T701
FUNCTION
Regulator
Circuit
Protection
Circuit
Horizontal
Oscillator
Horizontal
Driver
Horizontal
Circuit
Remember to reconnect disabled triggering component when finished (collector of X701 or TR353) .
*
ATARI
COIN VIDEO CUSTOMER SERVICE NEW LOCATIONS
Atari Coin Video Customer Service Centers in California and New Jersey have moved. Please note their new locations:
CALIFORNIA
735A Sycamore Drive
Milpitas, CA 95035
408/943-1120
800/538-1530 Toll Free
NEW JERSEY
22A World's Fair Drive
Somerset, NJ 08873
201/469-5993
800/526-3849 Toll Free
*
BALLY MIDWAY
MS. PAC-MAN SPEED-UP MOD
By Jake C. Leonard, Richmond, VA
REJUVENATE MS. PAC-MAN!! You will need a good
quality SPDT micro switch attached to the front panel. Lift pin
11 at 8C, run a jumper from the lifted pin to the common lug.
Attach a wire to pin 2 at 9C (leave pin 2 in the circuit). Run
this wire to the N .C. lug. Attach a wire to pin 12 of the V. RAM board.
Run this wire to the N.O. lug. Any time you flip the switch, the game
will run about twice as fast.