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==============================================================================
K E N W O O D
>>> A M A T E U R R A D I O S E R V I C E B U L L E T I N <<<
==============================================================================
Kenwood Service Centers
Kenwood Service Corporation Kenwood Service Center, East
2201 East Dominguez Street 829 Lynnhaven Parkway, Suite 130
Long Beach, California 90801 Virginia Beach, Virginia 23452
Telephone 1-310-639-5300 Telephone 1-804-340-1702
==============================================================================
BULLETIN # 990
MODEL: TS-950S/SD
SUBJECT: Final Amplifier Bias Circuit Change
It is possible to exceed the maximum Vcbo rating of the MRF429 transistors
used in the TS-950S/SD if the unit is operated at excessive transmit power
levels (greater than the normal factory output power setting). This results
in a collector to base short on the MRF-429. This causes excessive current
to be drawn thru R17 and or R18 of the final bias circuit. This can cause a
large amount of smoke from the overloaded resistors. It does not normally
cause damage to surrounding components. (You might wlsh to check Q7, VR2,
Q8 and the fuse just as a precaution!)
R17 and R18 are carbon type resistors (27 ohms 1/2 w) Carbon resistors can
emit quite a bit of smoke when overloaded in this manner and can be the
cause of considerable concern by the operator!
In order to ease operator concern we recommend replacing R17, and R18 on
the final unit (X45-3330-00) with ceramic resistors whenever replacing
the final transistors. These special resistors have been designed to open
quickly if the current rating is exceeded. This prevents smoking and the
odors associated with burning resistors!
Psrts Required:
27 ohm 1/2 Watt ceramic resistor R92-1286-05
Precautions:
1. The MRF-429 transistors generally fail one of two ways:
a) The collector/base junction opens. R17 and R18 are not damaged when
this occurs.
b) If the MRF-429's are operated at RF power levels higher than the absolute
maximum Vcbo rating the collector/base junction fails. This causes
excessive current to flow thru the base circuit.
2. If the MRF-429's short collector to base with ceramic resistors there
will be very little smoke before the resistor opens. The outward appearance
of the resistors will not change, so you must make sure to check the
resistors with an ohmmeter before installing new finals.
3. If R17 or R18 should open there will not be an appreciable change in the
base circuit current. It is essential, however, that these resistors be
in good working order as they ensure stable operation of the final
amplifiers.
4. R17 and R18 must be replaced when ever you replace the final tsransistors.
The ceramic resistors are easy to identify. They are rectangular and
white in color.
Replacement Procedure:
1. Replace the bad MRF-429 transistors.
2. Remove R17 and R18 from the component side of the Final circuit board.
3. Solder the replacement ceramic resistors directly to the base and emitter
of the final transistors. The stability of the circuit is improved by
soldering R17 and R18 directly to the base and emitter of the finals.
------ Do not install them in the old locations! ----