Star Tech Journal

27
STAR*TECH JOURNAL/DECEMBER 1983
Troubleshooting E/ectrohome and Wells Gardner Monitors (Part I) continued from page 26.
4. NORMAL B+ VOLTAGE
Here everything seems to be working properly and the B+ checks good. The
high voltage and filament are at the CRT, yet there is no raster (lit screen).
First try turning up the brightness control. On Electrohome use the screen
control. Wells Gardner has a black level control (VR201) located on the
interface board. To bring up the brightness on Wells Gardner, rotate this
control clockwise. Increase the brightness on the monitor.
With the front of the screen still dark, measure the voltage at pin 7 of the
CRT socket. The voltage is normally between four hundred and fifty and five
hundred volts DC ( depending upon where the screen control is set). If the
voltage measures zero or very low, turn off the monitor. Remove the
neckboard from the picture tube and examine it for a bad connection. Look
for either a bad trace, poor solder joint or possible crack in the board.
With the neckboard disconnected from the tube, situate the board so it' s
free from shorting to the chassis or anything else. Power up the monitor.
Check the voltage once again at pin seven; if the voltage returns to normal,
the picture tube may be defective. If the voltage is still low or zero, further
troubleshooting is needed.
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Absence of the six volts AC to pins nine and ten - work back towards it
supply. On the Wells Gardner neckboard a one-ohm resistor (R422) is in
series with the filament voltage on the neckboard. Electrohome's filament
voltage goes directly (via wires) to pins nine and ten of the CRT socket.
So, on Electrohome, trace back to pins four and five of the flyback
transformer. Here the voltage should measure about six volts AC. If not,
unplug the monitor and disconnect the neckboard from the tube. Measure for
continuity across the flyback pins four and five. If open, replace.
On the Wells Gardner, filament voltage gets to the neckboard by
connector J402, pins one and three. With no voltage at these pins, unplug
monitor and J402 from the neckboard Check for continuity at these two
pins of the connector J402. An open winding indicates a necessary flyback
replacement.
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The assembly device is a variable voltage divider. No input voltage to the
assembly would be a problem with either the wiring or the flyback
transformer itself. If the voltage is present, but there is no output, a new
control assembly might be needed.
5. NO RASTER OR FILAMENT HAS HIGH VOLTAGE
The three cathodes inside the CRT emit electrons as the cathodes are heated
up by the filaments (also called heaters). There is one filament for each
cathode. In the picture tube, there are three filaments all together. The
filaments are connected in parallel with each other. If only one or two
filaments light up, the tube is bad. If none of the filaments light and the
filament voltage reaches the heater pins on the neck board, either the socket
is not making a good connection to the CRT pins or the filament is open.
To check the filament voltage on the neckboard, measure pins nine and
ten of the CRT socket. These should be the two top pins of the tube. This is
an AC voltage and should measure around four to six volts. Voltage present
indicates no continuity between the heater pins on the socket to the filament.
Unplug the monitor, and disconnect the neckboard from the tube. With
an ohmmeter, check for an open filament by measuring pins nine and ten of
the CRT. Replace the tube if the filaments test open.
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The Wells Gardner screen control (VR406) wiper should measure about
four hundred seventy-five volts DC. The voltage supplied to the control
arrives to the neckboard by a wire from the main monitor PC board
Connector J401 pin three brings on eight hundred ninety volts DC to the
neckboard. A series resistor (R4 l 9) is connected between pin three of J401
and the screen control. Missing the 890 volts to pin three would lead one to
check the diode (X607) on the main board, the flyback, or a poor
connection.
Electrohome has the screen control mounted on the main monitor board
bracket. It is a black assembly located by the flyback transformer. The
assembly has two white adjustment controls. Top control is for the focus,
bottom adjusts the screen.
The assembly itself has four connections. The input voltage from the
flyback feeds into the assembly at one point. This voltage is well up in the
several thousands. Then another connection is held to ground. Two outputs
(which are the wipers of the focus and the screen control) are the last
connecting points.
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Continued on next page.