We have received several reports of distorted transmitter audio when using a TNC controller such as the AEA PK-232 or Kantronic KAM, etc. The symptom will generally disappear if the transceiver and TNC units are powered from different sources.
Cause:
The audio output level form the TNC is generally too high and causes overload of the microphone amplifier circuit. In previous models such as the TS-440S the incoming TNC audio was inserted after the microphone amplifier. With the TS-850S it is inserted before the microphone amplifier. Just moving the insertion point to the output of the circuit is satisfactory since the drive level for FM packet is higher then that required for SSB.
Procedure:
- Add a 10 dB attenuator to the PKD line on the IF unit (X48-3090-XX). This will prevent overmodulation of the microphone input circuit.
- Change chip resistor R233 from 10 Kohm to 18 Kohm (RK73FB2A183J)
- Change chip resistor R234 from 1 Kohm to 8.2 Kohm (RK73FB2A822J).
- Delete chip capacitor C173 (100 pF) and add chip resistor R299, 1.5 Kohm (RK73FB2A152J).
- Add the following note to page 35 and 37 of the Instruction manual.
- When adjusting for proper ALC levels with an AFSK RTTY terminal or Packet TNC terminal you should adjust VR-13 on the IF Unit for a reading similar to the one shown in the accompanying diagram.
Note: The transceiver and RTTY or TNC terminal should use separate power supplies, in order to prevent RFI (Radio Frequency Interference).
Caution: This modification requires soldering equipment rated for CMOS type circuits. It also requires familiarity with surface mount soldering techniques. If you do not have the proper equipment or knowledge do not attempt this modification yourself. Seek qualified assistance.
Time required for this modification is ½ hour or less.
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