Some MRF-485 transistors are being supplied by Motorola with a Green or Blue color ranking. If these transistors are installed without modifying the driver bias circuit there is a strong possibility 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.
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 high gain transistors you will need to modify the varistor/temperature compensation circuit on the final unit (X45-1400-00) by changing R16 from 1.2 Kohm to 2.2 Kohm.
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 1 Kohm resistor at the same time. Therefore the countermeasure differs depending upon the serial number of the radio.
|Serial number lot||Varistor||R15||R16||MRF485 Green or Higher rank|
|106xxxx or earlier||STV3H(O)||820||1.2 K||See "Caution below"|
|107xxxx or later||SV03YS||1 K||2.2 K||Change R16 from 1.2 to 2.2 K|
Caution: If using a Green or higher hfe rank one of the above countermeasure 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 SBB 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 1.2 Kohm to 2.2 Kohm.