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 Post subject: NC-400 main dial bad calibration
PostPosted: Mar Thu 01, 2018 10:00 pm 
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Joined: Jun Mon 24, 2013 3:00 pm
Posts: 1129
Location: Champaign IL 61822
I've decided that there is no hope for the main dial calibration of my NC-400.
(The bandspread dial is OK).

So I'm going to make or get custom from radiodaze a new one.

But WHY would the calibration be so awful? I've tried realigning in various ways
and get good agreement in the center but not the ends, good agreement at both ends
but poor at the high end and very very bad in the range 20-50% rotation from the low end,
or almost perfect agreement from 20-100% of range and off a good 1/2 inch
on the highest frequency scale at the bottom end. I've not tried changing padder caps,
which measure spot on. I have tried using an Excel spreadsheet to see if changing the
padders would work, and it just does not. I can improve the overall agreement, but the
error at some place in the lowest 1/4 frequency range is still 3/8 inch.
The degree error is identical on all bands. In other words, the actual cap disagrees with the dial.

WHY? HOW? Did they change suppliers? Were they simply doofuses?


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 Post subject: Re: NC-400 main dial bad calibration
PostPosted: Mar Fri 02, 2018 1:59 am 
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Joined: Sep Wed 10, 2014 2:01 am
Posts: 1777
Location: Costa Mesa, California
I am working on a homebrew receiver and Bob Weaver had me try his online calculator for bandspread. In doing so, I learned a whole bunch about what goes into getting the agreement between the two (or more) tuning capacitor sections and just how little a component change can screw up the alignment. I also had to reconcile a SR 160 a few weeks ago that was hopelessly off. That one took a bit of time and the final solution required using different values for some of the caps than the VFO original design--why?--just did.

The calculator and info on bandspread is here- http://electronbunker.ca/eb/BandspreadCalc.html

Many thanks again to Bob Weaver because there is no way I would have made any progress on my radio without this tool and the information in the related pages.

Here is my homebrew thread. viewtopic.php?f=12&t=335848

Norm

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 Post subject: Re: NC-400 main dial bad calibration
PostPosted: Mar Fri 02, 2018 3:02 am 
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Location: Maryland 20709, USA
dtvmcdonald wrote:
(The bandspread dial is OK).
...
But WHY would the calibration be so awful?

Is the bandspread dial in the proper position when tuning the main dial.

Usually those are supposed to be at one end or the other, or at a calibration mark.
Having it in the wrong position will grossly screw up the main dial calibration.

- Leigh

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 Post subject: Re: NC-400 main dial bad calibration
PostPosted: Mar Fri 02, 2018 4:26 pm 
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Joined: Jun Mon 24, 2013 3:00 pm
Posts: 1129
Location: Champaign IL 61822
I had not thought about the bandspread dial position. Yes, its exactly perfectly
as supposed to be. I could try changing it ... BUT ... that's immaterial, as if wrong,
since I was checking after a full alignment, having it wrong would just move the
position of the main trimmers.

The calibration of the bandspread dial is very very good.

I should add ... I have checked the tracking of the oscillator and RF amps,
and as best as I can tell, its just fine.


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 Post subject: Re: NC-400 main dial bad calibration
PostPosted: Mar Fri 02, 2018 5:05 pm 
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dtvmcdonald wrote:
I had not thought about the bandspread dial position. Yes, its exactly perfectly
as supposed to be.
Just to confirm...the bandspread dial should be set at the red line before beginning the RF alignment procedure and also when setting the main dial before using the bandspread dial. Someone before me had not done this on my NC-400 and the main dial calibration was off as you describe. Hopefully, no one has changed the mechanical positions of your dials on their shafts.

Dave


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 Post subject: Re: NC-400 main dial bad calibration
PostPosted: Mar Fri 02, 2018 7:59 pm 
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Joined: Jun Mon 24, 2013 3:00 pm
Posts: 1129
Location: Champaign IL 61822
Yes, that's how its done, and how I do it.

But that does not matter ... even if the bandspread is somewhat wrong ...
for example, all the way to the high frequency end ... it would have no effect!
This is because when I did all the tests, I aligned the bands before checking
the dial calibration. As long as the bandspread was stable (and it was) all that
would result would be that the trimmers in parallel with the oscillator main
dial tuning cap would end up in a different position. Of course, if I set the
bandspread dial wrong before alignment, the bandspread dial itself would be horribly
off and useless.

I did try changing the mechanical alignment of the main tuning dial with respect to
the capacitor shaft. It does have an effect on the problem, as you'd expect.
But the problem at the bottom of the dial (in frequency) remains no matter where
the mechanical setting is. Its as if somebody had seriously knifed the oscillator cap.
But no, they didn't.

I never changed the mechanical alignment of the bandspread dial.

Remember ... the main dial alignment is truly perfect over the top 3/4 of its
range, and pretty good over the top 5/6. Its the bottom 1/6 that's abysmal.


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 Post subject: Re: NC-400 main dial bad calibration
PostPosted: Mar Fri 02, 2018 8:53 pm 
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Posts: 1777
Location: Costa Mesa, California
By bandspread, I was not referring to the bandspread dial. I was referring to the spread of frequencies on the main dial. How they spread out over the dial. The equation for frequency is not linear. It is the inverse square root of a change in either capacitance or inductance. That is why tuning capacitors have such weird shaped fins and why the center shaft is sometimes off center. It is an attempt to get a reasonable "spread" of frequencies across the dial. This would be a difficult enough task, but then there is the oscillator that must be a frequency increment above or below the main dial frequencies. That makes the tracking very tricky, the equations much more complicated. Small value changes greatly influence the alignment. There is a reason more modern radios abandoned the need for capacitance tracking of two different frequencies with the introduction of the modern VFO.

The fact that it is the low end of the dial that is problematic could be the result of an oscillator or a RF frequency discrepancy. I would listen to the oscillator in another receiver and see if it is mis-tracking or if it is correctly tracking the dial. That would at least get you to the area you need to investigate.

Norm

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 Post subject: Re: NC-400 main dial bad calibration
PostPosted: Mar Fri 02, 2018 11:55 pm 
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Joined: Jun Mon 24, 2013 3:00 pm
Posts: 1129
Location: Champaign IL 61822
Oh I see what you mean by frequency spread.

I was testing how off the dial was by looking at the frequencies of the calibrator
harmonics, either 1 MHZ ones or 0.1 MHZ ones. I also checked known signals like
WWV, the broadcast band, and the ham band standard digital radio spots.
All of these were in agreement.

As to the frequency spread ... I started with it set correctly, i.e. with the bandspread
dial set at the red line, I followed the book as to the frequencies to align to. When
doing this, the worst errors are about 1/6 of the way from the bottom, and are badly off.

I've checked the tracking ... its just fine if I realign the RF adjustments on a given band
after doing the oscillator alignment for that band.

I suppose I could try realigning with a variable cap as padder, on one band.
This supposes the padders are off. But they have been measured and are well within specs.

And if one is off, so are all the others. If it was just one padder off, only one
band would have a problem, but in fact they are all off by the same angular error.
And .... changing the padders would ruin the current truly perfect agreement
over most of the dial.

Its an enigma.


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 Post subject: Re: NC-400 main dial bad calibration
PostPosted: Mar Fri 02, 2018 11:59 pm 
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Joined: May Sat 12, 2012 1:33 pm
Posts: 1584
Location: Rochester, NY.
Are the variable capacitor rotor end-plates slotted for 'bending' adjustment or can they be 'bent' away from the outer stator plates to give a more 'non-linear' capacitance tracking when turning the tuning shaft from stop-to-stop? This may help mid-band accuracy.
It would look something like the last rotor plate here:
https://www.techzonics.com/3-gang%20tun ... acitor.jpg


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 Post subject: Re: NC-400 main dial bad calibration
PostPosted: Mar Sat 03, 2018 6:56 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
Posts: 5615
Both my NC-400 receivers have good calibration and tracking so it isn't a standard design issue with the receiver. There is some capacitor over-travel and both of mine have all over-travel at the high frequency end of the scale.

Rodger WQ9E


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