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Beat Those VFO Drift Woes

From Swan News dtd May 1996 by K4BOV

By mid 1965, anyone comparing ham gear based on price and performance, easily saw the Swan 350 as an exceptional buy. And not surprisingly, the Swan 350 soon became the hottest selling transceiver of that period. Even to this day the model name "SWAN 350" is still the most recognizable of all Swan products.

However, within a year or two of regular use a lack of frequency stability began to appear and gradually worsened to a point of frustration and embarrassment to the user. Although increased attention was directed toward VFO integrity in later Swan equipment, the same problem tended to develop. Most attempts at the user level to thwart this nuisance instability involved changing anything and everything in the VFO frequency determining network. Unfortunately, this effort rarely achieved complete success.

So, why was it that Swan amateur equipment built a reputation of having poor frequency control while others of the same era did not? Plain and simple - the Swan VFO needed to produce four (early Swans) or five (late Swans) selectable widely spaced frequency ranges in order to cover the five different ham bands with a single conversion system. Other leading equipment of the day employed a separate crystal converter for each band which allowed for use of a single range VFO typically covering a 500KHZ spread of 5.0 to 5.5MHZ.

A single range VFO requires no switching while the multi-range Swan VFO must be switched from band to band and is therefore ganged to the main bandswitch via a flexible mechanical link. Under perfect conditions, one section of the VFO rotary wafer switch (located inside the VFO compartment) selects the correct VFO range corresponding to the desired ham band in use. The other remaining section grounds out the "not-in-use" VFO ranges so as to prevent any unwanted signals from getting into the scene. Because the switch is actually connected to components of the tuned L/C circuits, IT becomes a part of the overall frequency determining network. Remember, these types of oscillators are very very sensitive to outside forces. Just moving your hand close to an open VFO compartment will shift the frequency.

Through use and time, various actions result in weakening the positive contact areas of the VFO switch. Dirt, wear and corrosion on these surfaces introduce resistance and capacitance changes. And, it doesn't take much to cause the observable frequency (what you hear coming in and what the other members of your QSO hear coming out) to jump, warble or drift-off. Just slightly moving the VFO switch by applying pressure either left or right on the front panel main bandswitch knob will cause frequency movement.

What can be done? Well, it's not always easy. If cleaning the VFO switch does not reduce the problem, a new, unused or otherwise unworn switch of exact contact layout should be substituted. After disconnecting the old switch you can prove to yourself that the switch was at fault by hardwiring one band at the points previously made by the switch contacts. This simulates a single range VFO. Under this set-up any drift should be minimal and if evident is merely warm-up related. Should correction of warm-up drift be considered essential, positive and negative compensating capacitors are available to "tighten" things up. By selecting 80 meters for the hardwiring test on the earlier 5.1745MHZ I.F. transceivers, the 20 meter band is automatically operational as both bands utilize the same VFO range. Additionally, when connecting 40 meters in this manner, the later 5.5MHZ I.F. equipment can be easily aligned to operate on 17 meters as both bands use the same VFO range. (The green scale on late Swan VFO's is already calibrated for 17 meters)

In the event changing a worn VFO switch is not an easy task for the user, an external VFO is the next best move. Most Swan external VFO's did not develop any serious drift problem. And, any external VFO manufactured between the time of the SWAN 240 and the SWAN 700CX can be made to operate any Swan of that period, even the 600T and 600R twins. Some being directly interchangeable and others requiring one or more minor wiring changes. Some pruning of the tuned circuit coils and calibration is also required when using the late 5.5MHZ I.F. designed VFO with early 5.1745MHZ I.F. transeivers and visa versa. When using an external VFO with any drifting Swan, the control circuitry of the model 22(B) VFO adapter and similar circuitry built into later VFO's should be reverse wired for optimum results. That is, you want the external VFO to be "A" and the internal "B". This means the highly stable external VFO is transceiving in position one and is the transmit only VFO in position two. So when operating split frequency, the internal VFO is receiving and therefore any drifting on that VFO is only noticeable to the user who can re-adjust the VFO to his own liking as needed without affecting the transmit frequency.

Crystal control is another option. All Swan crystal controlled external oscillators can be made to operate any of the equipment discussed. These units hold up to 10 crystals and employ a vernier control to flex the crystal frequency over several KHZ for tuning refinements. Since production of Swan crystal oscillators was much much lower than the variable oscillators, they are somewhat hard to come by. However, all Swan VFO boards are convertible to crystal control and can be outfitted with the same type vernier as a means of tuning the crystal over a 4 or 5 KHZ range. Plus, remember about early Swans, the 80 and 20 meter bands employ the same VFO range. Thus one crystal would get you on both bands. For example, the crystal required for operating around 3920KHZ. And we say "around" because the exact freuency is factored in only when we know the precise setting of the carrier oscillator and whether normal or opposite sideband is desired. But, we can get close enough for the vernier to put us right on.

Calculate as follows: 3.920MHZ + 5.173MHZ carrier oscillator frequency = 9.093MHZ crystal frequency. Looking at that same crystal we also see the 20 meter frequency: 9.093MHZ + 5.173MHZ = 14.266MHZ operating frequency.

When properly maintained and aligned to factory specifications, Swan audio quality is very difficult to transcend by any manufacturers amateur equipment past or present. And, it would be a shame to see these handsome rigs spend any more time in those dark hideaways just because that pesky drift appeared too tough to tame.


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