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World of Ham Radio 1997
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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 # 988
MODEL: TS-940S
SUBJECT: MRF-485 Driver Transistor Notes
Some MRF-485 transistors are being supplied by Motorola with a Green or Blue
color ranking. If these transistors are installed without modifiying the
driver bias circuit there is a very strong possiblity that they will fail
within a very short time frame.
These high gain transistors cause the circuit to become unstable which
can cause the circuit to break into self oscillation, and therefore
self-destruct.
Recommendations:
Use of Red, Orange, or Yellow hfe color rankings is recommend. These
lower gain transistors work just fine and do not suffer from the circuit
instability problem. If you are only able to obtain the higer gain
transistors you will need to modify the varistor/temperature compensation
circuit on the final unit (X45-1400-00) by changing R16 from 1.2K to
2.2K ohms.
During its production the TS-940S used two different varistor values. The
original part was an STV3H (O). It was changed in mid-production to an
SV-03YS. R15 was changed from an 820 ohm resistor to a 1K ohm resistor
at the same time. Therefore the countermeasure differs depending upon the
serial number of the radio.
Serial number Varistor D2 R15 R16 MRF485 Green or higher rank
106XM or earlier STV3H(O) 820 1.2K See "Caution below"
107MX or later SV-03YS 1K 2.2K Change R16 from 1.2K to 2.2 K
Caution: If using a Green or higher hfe rank one of the above countermeasures
must be taken depending upon the serial number of the set. After replacing
the drivers check the bias current. We recommend transmitting for 1 hour
in the SSB mode with no modulation into a dummy load. After this time
frame check the bias current. It must not exceed 300 mA on the original radio.
If the current changes you must change R16 from 1.2K to 2.2K.
MRF485 COLOR CODES
RED DOT 20-32 hfe
ORANGE DOT 28-42 hfe
YELLOW DOT 38-52 hfe
GREEN DOT 48-63 hfe
BLUE DOT 58-78 hfe
VIOLET DOT 72-93 hfe