| | CB RadioHam VCO Block The green or orange coloured VCO block used in Ham International rigs is a source of many sorrows. The usual problem is that a "screwdriver expert" will destroy the special low-permeability core. Low-perm. replacement screwcores are available (but at a price). If it is replaced with a screwcore from another type of VCO coil, the chances are that the VCO will no longer cover the full range of frequencies.
If this is the case, or if you need to broadband the VCO for other reasons then read on. The VCO block comprises a small PCB potted in resin, clad in steel and housed in a moulded plastic cover. If the latter are removed you will be left with a clear block with five pins protruding. Near one corner you should be able to see a small disc ceramic capacitor of 22pF. To broaden the coverage of the VCO you must use a miniature electric drill carefully to grind away the resin which encloses this capacitor. Replace it with a 15pF ceramic capacitor and reassemble the VCO block.
The sketches, below, show the underside view of the block and its circuit, together with the connections to the TA7130 balanced mixer IC with which the block is used.
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