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 Post subject: A Deluxe Receiver for the DX Operator
PostPosted: Feb Tue 13, 2018 6:36 am 
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I am starting on a project to build this receiver from the pages of William Orr's 16th edition of the Radio handbook. It lends itself to modular construction and I will be using a different chassis for different sections of the radio--so that as I progress, I can experiment. A number of the transformers and coils will need to be wound--which is why I recently built my coil winder. The radio uses a tunable 1st IF and crystal oscillator for the first mixer. It can be tuned to receive any set of frequencies with the right crystal and RF coils.

My first question is that there are different air variable capacitors with some being linear and some having non-linear tuning. In other words, some have half circle plates but the rotor is off center. A half a turn is not half the capacitance. I know the formula for resonant frequency is non-linear, but are all these tuning capacitors equally non-linear or do I need to tailor my padders and trimmers for the specific non-linearity of my tuning capacitor? I have two --two-gang 50p non-linear variable capacitors. I need two 50's and a 100p for the circuit I plan to build. If I gang the second two-gang 50p to get a 100p will that work with the single 50p caps? I assume it should, but then I assume many things that turn out to be wrong.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Feb Tue 13, 2018 7:32 am 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Saskatoon
Norm Johnson wrote:
In other words, some have half circle plates but the rotor is off center.

What you are describing is what is known as a midline or centerline capacitance characteristic. It's a bit of a compromise. A capacitor that has uniform increase in capacitance with rotation will have the stations at the high end of the band squeezed together. Another type known as the straightline frequency variable capacitor has, as you might guess, a characteristic that gives even spacing of frequencies with shaft rotation. These were popular in the 1920's but weren't very good for superhets where you needed to have a dual section capacitor that would tune both the RF and local oscillator, and have them track each other properly. The midline variable capacitor is more compatible with a superhet, and easier to make both sections track properly. This is the type that you see in most receivers from the late 1930's to the end of the tube era. They don't have quite the equal spacing between stations across the band that the old straightline frequency caps had, but they're much better than the variables that change capacitance linearly with rotation.

I wrote an online calculator that helps in the design of the tuning. It shows what frequency range you'll get with a specific type of variable capacitor, including the effects of padder and trimmer capacitors. It also displays a dial scale that shows how the frequencies are lined up accross the dial.
http://electronbunker.ca/eb/BandspreadCalc.html


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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Feb Tue 13, 2018 3:36 pm 
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Joined: Jan Mon 28, 2013 9:35 pm
Posts: 920
Location: Santa Rosa, CA
There are three types of open, variable plate caps;
SLC= straight line capacitance where the capacitance varies linearly,
these are the most common and have half-cricle plates
SLF= straight line frequency where the plates are tapered to allow
for linear tuning of the frequency
SLW= straight line wavelength, you get the idea...

SLF and SLW caps have oblong plates.

The effect on tuning a receiver can be dramatic. One example is the
Hammarlund SP sereis of receivers where the ham bands are very
compressed at one end of the tuning range. The used SLC caps
in the VFO. On the other hand rigs like the Kenwood TS-520
and FT-101 series have linear tuning across each band. These use
SLF variable caps. Most old 1920's battery radios used SLW
where stations were identified by their wavelength.
Steve W6SSP


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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Feb Tue 13, 2018 6:57 pm 
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Bob,

You have some very useful information and I like the calculator. I will be studying further, because you have opened my mind up to the entire problem and possible solutions.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Feb Wed 14, 2018 4:13 am 
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Bob, your calculator is pretty amazing. I have some variable capacitors to choose from and two circuits that have to match in linearity to produce the 455 KHz second IF. The book calls out part numbers which are now unobtainable and there is no added information as to type of variable. The one photo is inconclusive as to variable type. The book does call for a ganged 0-50p variable for the IF circuits and an added gang of a 0-100p for the oscillator. I have four 0-50p ganged straight-line capacitance variables and plan on using one of the ganged variables with a jumper to make the 100p. The transformers and the oscillator coil listed in the book are also unknowable--at least to me as my Miller catalog does not contain those part numbers, and if it did, the parts aren't out there--I have searched rather exhaustively. I have looked at many hundreds of part numbers in various seller data bases.

With the calculator, I produced a chart for the IF and a chart for the oscillator that match very closely. Now I can wind my coils and transformers and see if it works. I have a lot more confidence I am heading toward success than before reading Bob's analysis and using his calculator. I do plan on having the transformers and oscillator adjustable with a ferrite core, so minor tweaking can occur. I also have two 0-50p mid-line/center-line ganged variables and can try those at some point if the straight-line variables don't produce a reasonable dial calibration.

One question I still have is what value to use for the stray capacitance with the calculator. I have picked 4p as a number. This is based on spending all of last weekend reworking a Hallicrafters SR-160 VFO. The tuning capacitor there had a range of about 10p and everything was very sensitive to small capacitance variations. 1 KHz on the dial was .02p in the circuit. It really gave me a feel for what happens when components are shifted around, lead length, and how much to figure the VFO cover will change the circuit.

That is one good thing about a homebrew receiver--you can make the dial match the radio. A commercial radio has a printed dial and the tech has to try and make the radio match the dial.

Norm

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Last edited by Norm Johnson on Feb Wed 14, 2018 6:31 pm, edited 1 time in total.

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Feb Wed 14, 2018 6:38 am 
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Joined: May Tue 30, 2006 4:46 pm
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Location: Santa Rosa, CA
http://makearadio.com/misc-stuff/capacitors.php

Rich


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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Feb Wed 14, 2018 11:52 pm 
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Location: Saskatoon
Norm, the very earliest superhet receivers had separately tuned RF and LO stages, because no one had yet worked out how to make a dual gang variable capacitor that could tune both RF and LO simultaneously and maintain a constant IF difference frequency while tuning across the band. These tracking calculations are quite a bit more involved than simple bandspreading. I wrote a superhet tracking calculator to do this, but it's not an online calculator. You have to download it and run it.
http://electronbunker.ca/eb/Downloads.html (scroll to bottom of page)


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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Feb Thu 15, 2018 1:33 am 
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Bob,
Using your bandspreading program, I have tried a number of values for the different variables and looking at your chart and measuring the results both for the oscillator and variable IF, I have a pretty good idea what the component values should be. By making them adjustable, I should be able to dial in from a starting point to get the best match. As much as detailed calculations help--they are only a starting point. Once, I have the final tracking working, I should be able to make some of the components fixed value.

The tracking calculator locks up on my computer and I was only able to get so far with it.

The oscillator coil has about 80 turns to get 65 micro Henries. Any thoughts on how many turns in to make the cathode tap for the 6BE6 mixer/oscillator? I was going to try 8 turns.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Feb Thu 15, 2018 2:20 am 
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Location: Saskatoon
Rule of thumb is to put the tap about 1/3 up from the common end. For an 80 turn coil, that would be 27 turns.

I'll have a look at the tracking calculator and see if can find out what's wrong. What operating system are you using? I may have to recompile it to make it compatible with newer OS's.


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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Feb Thu 15, 2018 2:39 am 
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I have a Mac running OS X El Capitan.

1/3 up it is.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Feb Thu 15, 2018 3:46 am 
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Oscillator coil showing range of adjustment--not the tap. I figure I need 65 microH.


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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Feb Thu 15, 2018 5:14 am 
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My next question is. . . .when I wind my transformers, how far apart should the windings be? The way this tunable IF is drawn, the primary is untuned and the secondary has the variable capacitance. I figure about 47 microH for the coils and I want to use a slug for the secondary. Do I need a slug for the primary? being untuned, I would assume no. Should I wind it with fewer turns so that it steps up the voltage?

I do plan on changing the circuit to use a padder and trimmer as Bob's bandspreading formula suggests.

Norm


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