SB 300 LMO

[Kenneth G. Gordon]

On 18 May 2009 at 8:25, Mark Johnson wrote:

> The LMO needed work as the dial drive was loose. I had to remove the
> actual variable cap to get the rotor set screw tightened up.

That is somewhat unusual, but I have had to do that at least once.

> After re-assembling the LMO and testing it, my actual linearity is off
> a bit. From the bottom end of any given band, say 14,000 to 14,100
> khz, the dial accuracy is off by about 2.5 khz.

That seems a bit excessive...

> Over the rest of the
> band, 14,100 to 14,500 the accuracy between calibration points is off
> by about 1 khz. 

That is about right.

> Is this reasonable?

The first is not: the second is about right.

> I hate to think about taking that LMO out again
> and trying to adjust the linearity or 'tracking' by bending the rotor
> plates on the variable cap. 

There may be another way to take care of it, and this usually works 
for me:

Connect a frequency counter to the LMO output.

Operate the LMO from one stop to the other.

You should see a frequency range of 4995 Khz to 5505 Khz, or 500 
Khz with about 5 Khz extra at either end.

I think you will find that the range is MORE than 5 Khz at one end, 
and LESS than 5 Khz at the other end.

What you need to do is "center" this, since the non-linearity normally 
is most obvious at the ends.

If you look closely at the LMO capacitor while you tune it, you will 
see that there is only a very narrow range of the movement of the 
capacitor rotor needed to cover the full 500 Khz range.

Remember that the LMO tunes "backwards" with respect to the 
transmitter output frequency.

What you need to do is to adjust the tuning range to take advantage 
of the most linear portion of the tuning cap.

If you have gotten the "stops" at either end too far one way or the 
other, you will be out of the linear portion of that curve.

Therefore, the first thing I would do is to attempt to "center" that 
tuning range by loosening the set-screw on the dial stop "tagged 
washer" setup, and adjusting its position in relation to the shaft so 
that the tuning range is centered, then re-tighten the dial stop set-
screw. You SHOULD be able to loosen this setscrew without 
removing the capacitor or even opening the LMO.

If that doesn't cure the problem, you can may be able to "tweak" the 
range onto center by adjusting the small coil that is in series with the 
larger one inside the LMO, but this is NOT a job for the faint of 
heart. As I remember it, there are no trimmer caps in the LMO 
either.

So far, I have not had to tweak the coil or adjust capacitance.

If I have not clearly explained the process, e-mail me again.

BTW, I have always wanted to spend a few days with a set of files, 
a frequency counter, and some graph paper to "tweak" the capacitor 
into as close to being linear as I can make it.

Also, be aware that the true frequencies of your heterodyne 
oscillator crystals can effect centering of the tuning range, so get it 
right at the LMO first, then adjust other things if necessary.

Please let us know how it goes.

Ken Gordon W7EKB

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