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                        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

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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!  ----