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 Post subject: National NC-183 dial tracking
PostPosted: Apr Mon 27, 2020 5:14 pm 
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Joined: May Tue 16, 2017 3:03 pm
Posts: 287
Location: Potomac, MD 20854
With less to do than normal I have been aligning my old sets and this NC-183 has me stumped. After alignment all frequencies track well on all bands EXCEPT the 4.3-12 MC band. I confirmed high side tracking (NC-183 set to 7.00 MHz and my shortwave radio picks up the LO at 7.455 MHz.

My dial at 11 MHz (high end) is dead on and my a dial at 5 MHz (low end) is dead on. But IN BETWEEN is driving me nuts. This is what I find (signal generator to get frequency right).

Actual RF signal.... NC-183 dial
11.0 MHz>>>>>11.00 MHz
10.5 MHz >>>>>10.48 MHz
10.0 MHz >>>>>9.950 MHz
7.3 MHz >>>>>>7.25 MHz
6.0 MHz>>>>>>5.98 MHz
5.0 MHz>>>>>>5.0 MHz

This is not the end of the world, but is curious. Nobody seemed to "knife" my tuning cap. These errors are ONLY on this band. Any thoughts?

Paul


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 Post subject: Re: National NC-183 dial tracking
PostPosted: Apr Mon 27, 2020 5:49 pm 
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Joined: Jun Mon 24, 2013 3:00 pm
Posts: 1456
Location: Champaign IL 61822
Two possibilities: the padder cap. C68 or C70 or C72 is wrong value.
Or, possibly S9A, which shorts certain oscillator coils, is not shorting them properly.


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 Post subject: Re: National NC-183 dial tracking
PostPosted: Apr Mon 27, 2020 6:08 pm 
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Joined: Sep Wed 10, 2014 2:01 am
Posts: 2102
Location: Costa Mesa, California
I think the cap in question is C-51, a 4200p padder between the coil, L-24, and ground. If you want to try something, tack on an additional cap in parallel to C-51 and see if that helps or hurts your spread. If it helps, then adjust the added cap value. If it hurts, then either replace C-51 with a lower value or add a series cap to lower the combined value. If you have a variable cap that you can tack parallel, then that might make the process easier. I would use a 365p variable which are fairly common BC band variables. If you have a dual variable, you can tack some wires across the sections to make a larger cap. Keep your leads short.

Norm


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 Post subject: Re: National NC-183 dial tracking
PostPosted: Apr Mon 27, 2020 6:15 pm 
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Joined: May Tue 16, 2017 3:03 pm
Posts: 287
Location: Potomac, MD 20854
I don't know what padder caps do. I thought they were there to make it easier for a trimmer to be "in range". Could a padder be bad/off and could I STILL be able to align the set at the top and bottom of the band?

I had a similar "one band" problem on an HQ-140X and never figured it out, maybe same problem with the mica padder cap.

p


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 Post subject: Re: National NC-183 dial tracking
PostPosted: Apr Mon 27, 2020 6:34 pm 
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Joined: Sep Wed 10, 2014 2:01 am
Posts: 2102
Location: Costa Mesa, California
Tuning is a very complex problem to solve, because it is non-linear--whereas your dial is linear. If you look at the equation for frequency, changing the value of inductance or capacitance in a linear way does not result in the frequency changing in a similar linear way. Variable caps can have off-center pivots and oddly shaped blades to try and solve the problem mechanically. You can also add capacitors in various configurations to try and help the linearity. A "padder" is added in series with both the coil and variable cap. In your case the padder is between the parallel tuning circuit and the ground. Bob Weaver has a website with online tools for solving the tuning problem and with a few hours of playing around with various values, you can design your own almost linear tuning circuits.

http://electronbunker.ca/eb/BandspreadCalc.html

Norm

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 Post subject: Re: National NC-183 dial tracking
PostPosted: Apr Mon 27, 2020 6:46 pm 
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Joined: May Tue 16, 2017 3:03 pm
Posts: 287
Location: Potomac, MD 20854
That is very helpful. I will look at the caps and play around and see if that helps at all. Curious the "problem" band is the 5-10 MHz range on both this NC-183 and my HQ-140X.

p


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 Post subject: Re: National NC-183 dial tracking
PostPosted: Apr Mon 27, 2020 6:59 pm 
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Joined: Sep Wed 10, 2014 2:01 am
Posts: 2102
Location: Costa Mesa, California
When the radio was originally built, they made a prototype. Then they marked the dial to match the actual frequency spread. At that point it matched perfectly. Then hundreds or thousands of radios were built and a few adjustments were incorporated so the top and bottom of the bands could be made to exactly match and maybe the middle was really close. After fifty years go by, things can change, coils shrink or swell, capacitors get dirty, small value changes happen. The adjustments will still allow the top and bottom to match, but the middle may be further off. I haven't thought about why the middle band may change more than others, but there may be a reason.

Norm

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 Post subject: Re: National NC-183 dial tracking
PostPosted: Apr Mon 27, 2020 8:55 pm 
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Joined: May Tue 16, 2017 3:03 pm
Posts: 287
Location: Potomac, MD 20854
Norm Johnson wrote:
I think the cap in question is C-51, a 4200p padder between the coil, L-24, and ground. If you want to try something, tack on an additional cap in parallel to C-51 and see if that helps or hurts your spread. If it helps, then adjust the added cap value. If it hurts, then either replace C-51 with a lower value or add a series cap to lower the combined value. If you have a variable cap that you can tack parallel, then that might make the process easier. I would use a 365p variable which are fairly common BC band variables. If you have a dual variable, you can tack some wires across the sections to make a larger cap. Keep your leads short.

Norm


Good thinking. I just tacked in a 2200 pF mica in parallel (bringing total to "what should be" 6900 pF. I could get 11.0 and 5.0 dead on but now the error in the middle of the range is WORSE!! Since (what I think should be the original cap) of 4700 pF is also not great, maybe I will pull it and replace with the 2200 pF cap. I'll see what happens, thanks for the tip.

Paul


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