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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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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
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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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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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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Feb Thu 22, 2018 3:43 am 
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I wound the transformers and now have to get them assembled with a slug and adjustor and finish the shield cans. I am using 2" sections of 1.75" diameter aluminum tube with 1/16" walls. The top is a 1.625" fender washer that fits very snuggly and is then glued from the inside. My coils are wound on 7/16" diameter phenolic and have a 3/8" core. I have created a small phenolic plug that slips inside with a T-nut to hold a stainless 4-40 machine screw. The slug is a section of ferrite with a nut glue on. The top of the phenolic tube plug is slotted with a hacksaw to receive the ears of the T-nut and it is glued in place. Assembled in the cans the phenolic plug and T-nut will be up slightly through the fender washer hole. The bottom of the can will have two small aluminum angle brackets toward the inside to hold the phenolic sheet bottom. I have some terminal posts that will mount to it. The bottom of the coil will fit into a short section of phenolic tube to hold it in place.

One issue that needed resolution was how to strip the Litz wire. My final solution was to use a knife to scrape the cloth cover off and then dip the end in paint stripper for a few minutes. I then scraped with a file and after that the wire strands were easily tinned with solder.

Bob's bandspread calculator site was instrumental in determining the components and values for the oscillator and tuned IF transformers. I ended up photographing the computer screen with my camera on a base so it wouldn't move, cutting and pasting the images of the bandspread for the oscillator and IF onto a worksheet which I then printed, cut out, and placed next to each other to compare. It took a few times to find the values that would match up the two frequency spreads at all points.

Here are the photo's

Norm


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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Thu 01, 2018 5:52 pm 
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I have the transformers assembled and the oscillator coil (shown below). I am quite happy with the results as the coils are all easily adjustable to their spec inductance. My final touch will be to put a small dab of paint on the threads of the stainless adjustment screws to hold them in place and serve as a marking spot for the designed inductance value.

Norm


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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Fri 02, 2018 3:44 am 
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Keep posting your progress as it is a very interesting project.

Thanks

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Fri 02, 2018 6:36 am 
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Captivating work. Love it!


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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Mon 05, 2018 1:34 am 
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This weekend, I assembled the variable IF and associated oscillator chassis. I am building this radio one section at a time on a number of different chassis. The idea is to be able to try different versions of sections and use them in a radio that can be altered in the future. The most difficult section is the variable IF because it is the tuning mechanism and the oscillator must track the variable first IF so that the second IF at 455 KHz can function. It also involves parts that are unobtainable--Miller coils and transformers--and these same parts have no information to go off when attempting to reproduce them. Because of this, I am using Bob Weavers bandspread online calculator to establish values. I am then rolling my own coils and transformers.

The chassis is small and I need to stuff two double capacitors into it and all the other parts. I have elected to place the capacitors below the chassis and have installed them on removable partitions so that they can easily be removed for re-working the other components if needed. The capacitors in the original radio were all continuously ganged with couplers, but I could not find 3 similar capacitors of that type, so had to use two, two-ganged capacitors connected by gearing. The drive gear has half the teeth of the capacitor gears so that gives some spread of frequencies-one full turn of the knob for the half turn of the capacitors. I used a knob with a vernier base to help me see where I am on a range of frequencies.


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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Mon 05, 2018 1:49 am 
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I created a tube circuit tool to allow me to "breadboard" circuits when needed for testing. Below is one that I have hooked up to provide a signal to the variable IF from the previous stage.

So, the big question: How did the variable IF work? . . . . The results were mixed. The oscillator worked very well--at least it appeared to. Maybe it even worked too well and I could need to reduce the coupling or add a resistor at the bottom of the coil so that it doesn't overpower the IF in the mixer. The IF had problems--mostly of my making. I mis-wired it initially and then when I discovered that, it then still had problems. I had a second mistake and then a third and so on. When I eventually had it all sorted out, the input transformer and tuned circuit did peak as expected--but the second did not. I am now planning on tearing it all apart and measuring the stray capacitance where possible, check why the second transformer won't peak, and re-check my wiring and components. With a solid value for stray capacitance, I can run the bandspread calculator again and see if I need to make changes to the tuned circuits.

Norm


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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Mon 05, 2018 2:30 pm 
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How much longer before we see "Norm's Custom Coils" in the forum advertiser section? :wink:


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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Wed 07, 2018 5:05 am 
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Hopefully, I can keep my day job and it won't be necessary.

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