Modifications for the ICOM - RogerK

Fig. 3: The circuit extract from ICOM shows where the potentiometer is to be found that allows the phase shift to be adjusted.Fig. 4: This photo shows where the parts should be locatedThe VCO of many transceivers will have been modified already by the manufacturer. This is in the form of a RC-network of 470ohms / 1 nanofarad, which will have been inserted into the sourcecircuit of the oscillator transistor.These two components are accommodated on the conductor side of the PC-board, after breaking the required conductor lane.Figure 5 shows this ICOM-modification, as well as the modification recommended by the author, in the form of a circuit diagram.This can be seen from the measured sidebands of transceivers A, B, and E in FigUre 1. ICOM probably wanted to redUce theoscillator power with this modification in order to reduce the RF-voltage across the varactor diode.This in turn will reduce the leakage current through the diode, and subsequently the voltage drop across the 47 kQ resistor.After carrying out the recommended modifications listed in this article, the ICOM-modification will no longer be required, sincethe voltage source of the varactor diode will exhibit such a low-impedance for lower frequencies that the 1 /f component of theleakage current will not be able to cause any voltage drop.