SB 300 LMO

[Bob Burns]

Hi Mark
 
If you mechanically align the 0 or 500KHz point at one end of the  tuning 
range, is the rest of the dial calibration always out in the same  direction? 
If so you may be able to make some minor corrections using the preset  
trimmer or fixed capacitors and slightly moving the position of the dial  
mechanism on the LMO shaft. If the LMO moves too far in frequency then you will  
have to increase the trimmer capacitance and if too little then decrease the  
trimmer capacitance. I believe that this was also the purpose of the small 
fixed  capacitors mounted directly on the LMO coil that were adjusted during 
test, the  trimmer being for a final tweak when the unit was sealed up. One 
or more of  those caps may have aged a bit.
 
If you can get the calibration at each end to be correct you may have to  
accept a small error at the mid point. All depends how much effort you are  
prepared to expend.
 
The original three point tracking concept (long before Heath made their  
LMOs) had a slightly S shaped curve (instead of a straight line) that 
correctly  met the three alignment points at start, mid and end of scale and 
deviated in  one direction between start and mid and the other direction between 
mid and end.  As you say, the purpose of the split vanes on the tuning 
capacitor was to  minimise the calibration errors.
 
Not sure what the calibration spec was but you may ultimately decide that  
2-3KHz is too small to worry about as long as the calibrator is correct.
 
73

Bob 

G3OOU
G-QRP  6907
Joint site with Crystal Palace Radio & Electronics Club: _www.g3oou.co.uk_ 
(http://www.g3oou.co.uk/) 
My technical site: _http://www.qsl.net/g3oou/_ (http://www.qsl.net/g3oou/) 

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