Zenith CM-150 Series Service Manual page 4

PRODUCT SAFETY SERVICING GUIDELINES FOR AUDIO-VIDEO PRODUCTS
SERVICE SAFETY NOTES
When servicing chassis, under no circumstance should
the original design be modified or altered without
permission from Zenith Electronics Corporation. All
components should be replaced only with types identical
to those in the original circuit, and their physical
location, wiring, and lead dress must conform to original
layout upon completion of repairs.
Special components are also used to prevent
X-Radiation, shock, and fire hazard. These components
are indicated by the letter "X" included in the item
number. This designates critical components in circuits
which are required to maintain safe performance. No
deviations are allowed without prior approval by the
product compliance and evaluation department.
The circuit diagrams may occasionally differ from the
actual circuit used. This way, implementations of the
latest safety and performance improvement changes into
the set is not delayed until the new service literature is
printed.
B+ AND HIGH VOLTAGE SHUTDOW N CIRCUIT
These sets utilize safety circuits for protection. In case of
a failure involving the seal B+ adjustment, high voltage
adjustment, or high voltage shutdown, follow these
instructions closely. Failure to do so many violate
Federal X-RAY limits. Some sets may use a high voltage
shutdown adjustment potentio in place of fixed
resistors. If it should become necessary to replace one of
these factory controls use only the exact Zenith factory
replacement part number. You must also adjust the
replaced controls to factory specified values (B+ to VDC)
and high voltage adjust to 31.5kV). For the high voltage
shutdown adjustment after replacing the high voltage
shutdown adjustment potentiometer or fixed resistors,
follow the following procedure. A functional test of the
high voltage shutdown circuit must be performed to
check for proper operation if you service these sets in the
shutdown circuitry.
HV SHUTDOW N FOR PROJECTION TV
1. Measure the DC voltage across capacitor CX3006. It
Caution: It will normally not be necessary to perform
2. Connect a 40kV high voltage meter in place on one of
NOTE: Any board which cannot be repaired to pass
iii
should be approximately 7.6VDC. Temporarily
connect a 1K 5% 1/2W resistor across RX3015. High
voltage should shutdown. This checks out the
shutdown circuit operation. Remove the temporary
resistor and recycle the ON/OFF button. A more
elaborate test to determine the actual high voltage
value at which high voltage shutdown will occur can
be made as follows:
this test unless the set keeps shutting down
under normal operation, the high voltage
shutdown resistors or adjustment pot had
to be replaced, or the operational test des-
cribed in the previous paragraph was
unsuccessful. It should be recognized that
significantly elevated high voltage will be
produced during this test and adequate care
should be taken to avoid high voltage
arcing.
the CRTs at the high voltage block. Connect the
negative lead of a 500mA 12 volt power supply, with
a 1K OHM resistor in series, to the junction of RX3438
and RX3439 and the positive lead to ground side of
RX3439.
Turn the set ON and ensure that the brightness and
contrast are set at minimum. Increase the 12 volt
bench supply until shutdown occurs, Shutdown must
occur in the range from 34.3 to 36.5kV, If not,
shutdown occurs, the circuit must be repaired and
the test repeated.
When you have completed all the adjustments and
tests, use a "hot melt glue" gun to permanently seal
the B+, HV and/or shutdown, if any of these controls
had been used, to render them not adjustable. This is
to ensure continued compliance with the Federal TV
X-RAY Standard.
the functional test above should be
exchanged.