============================================================================== 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