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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Apr Sun 01, 2018 5:05 am 
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Hooray !

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Apr Sun 01, 2018 5:53 am 
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Thanks, Leigh.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Apr Sun 01, 2018 6:42 pm 
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This morning, I added the 40 meter band--7 MHz to 7.5 MHz--and listened to some Easter SSB and amateur AM stations, as well as some international AM SW broadcast stations. When I post photos later, note that the RF tuning capacitor is inside a small utility box on top of the chassis. This does reduce stray noise. Without the antenna plugged in the radio is silent, but with the tuning cap cover off, a small amount of static comes from the speaker.

I attribute the 150 VDC B+ as a major contributor to the low noise and stability, as well as the compartmentalizing of the stages into individual modules. Originally, I worried that the low B+ would not produce sufficient gain; however that doesn't seem to be a problem. The radio draws between 110 and 125 milliamps on the B+ line and .4 amps on the 6.3 heater circuit.

I have been studying how to enclose the radio in a standard case and it will fit if I move the audio controls (volume and noise limiter switch) to the panel. Besides tightening up leads and adding bands to the RF stage, the final item on my list is the dial. I have purchased ($5) a slide rule type that I think will work and it also has a band drum. It should arrive in the next day or so and I can assess its incorporation.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Apr Sun 01, 2018 10:24 pm 
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Location: Portland, TN, USA
You may have seen this already.

I happened across it this morning and thought of it when I read your latest post. Perhaps there's an idea or two lurking in the pictures.

Looks like the builder was almost as gifted and talented as you!

https://www.ebay.com/itm/Antique-W1DGT-National-Radio-Case-Tuneable-IF-WWV-CW-SS-AM-Ham-Tube-Receiver/253449301207?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m1438.l2649

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Apr Sun 01, 2018 11:34 pm 
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Thanks for the encouragement, Chuck. Yes I did see it earlier and the first thing I tried to determine was whether it was the same schematic as mine--but it isn't similar. It is beautifully made.

I have been thinking about new bands to add, and the first one I want to try is the BC band--or bands. This will involve placing the crystal first oscillator above the RF frequency and subtracting--of course the radio would then tune backward. Which got me thinking how I could strategically add bands that were sharing the crystals used by other bands. Right now my 40 meter band uses a 4.6 MHz crystal. If I make a set of coils that are for the 160 meter band--1.8 MHz to 2.0 MHz--in other words create a set of coils centered at 1.95 MHz--I would then have the 160 meter band but tuned backwards. This is similar to what happens with the Drake R-4C. That is one advantage to making your own dial--you can have the bands tune whichever way you want and start them and calibrate them as dictated by the radio--not the other way around.

Here are the photos of my RF stage in progress and the completed radio--such that it will ever reach final completion. If it is anything like my sailboat (which I built in '86)--even after sailing around for thirty plus years--it still isn't really done--at least not in my mind.

The first photo is of the radio with the RF stage in pieces on the counter all tacked together to see if it would work. It did! That got me excited enough to spend the next 10 hours building it. From side to side, without the audio stage, the radio is 17" wide. This will fit in a standard radio case. The depth is 14" at its deepest with the audio, "hang AGC" -S meter, and calibrator sitting behind the three stages that really need their controls accessed from the panel. A separate power supply would be needed.

Norm


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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Apr Sun 01, 2018 11:51 pm 
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As far as wiring the RF stage, the two center partitions which isolate the antenna coils from the RF coils from the crystal oscillator, are not installed until all the wiring is in place. The final step is sliding in the partitions with the switch wafers mounted to them and then wiring the wafers. The space inside this chassis--3" by 5" by 7"-- is too confining to do it any other way. This brings up the question of how to add more bands. There isn't room for much more than a few more at most, so my options are either having a separate chassis with coils and crystals or using a bigger chassis. This seems the most sensible option, but I am still thinking about it. I do not have a shaft in place yet for the wafers. To change bands, I must move 5 of the wafer centers by hand. Of course there are only two bands at this point so I am not doing much band cruising. I am deciding how I will make a center shaft extension and am ordering a 1/4" phenolic round. I think I can jig up something for the table saw that will cut the flat sides.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Apr Mon 02, 2018 1:27 am 
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Amazing work, Norm. However, looking at the pix and reading your plans for that bandswitch construction, I'm envisioning some restorer generations down the road cursing you like many do Hallicrafters currently (e.g., SX-42).

If I may be permitted a slight O.T., it looks like we have more than a love for boatanchors in common. When we lived in Florida a number of decades ago, I owned and sailed a couple of fun boats. The first, a Danish King's Cruiser, was a beautiful lap strake folkboat requiring way too much maintenance effort to keep the marine borers at bay and the bright work shipshape. The second was a Columbia 26 MKII, a great family boat. Many great memories of sailing on Tampa Bay!

We now return you to your regular programming as I think Mark (Pix) is fond of saying.

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Apr Mon 02, 2018 8:11 pm 
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Fortunately, Chuck, my boat has a fiberglass hull. Over the years, I have learned to be a pretty fair hand with a varnish brush. I also keep the boat under a full cover much of the time. My bandswitch? Well, like the SX 42--it is the gift that keeps on giving.

Norm

The air vent on my hatch boards


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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Apr Mon 02, 2018 9:19 pm 
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Location: Jackson, TN
Norm,

That's a remarkable project! The set is just beautiful.

Thanks for taking the time to provide the photos and the tortuous path of finding the bloopers. Its those sideroads of getting things to work correctly that are the most fun.

I'm looking forward to seeing the final product.

Tim


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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Apr Mon 02, 2018 10:51 pm 
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Looking great. Really nice work.

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Apr Tue 03, 2018 2:18 am 
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Very excellent, Norm. I hope you have as much fun operating it as building it.


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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Apr Tue 03, 2018 4:55 am 
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Thanks, all.

As I work out the RF stage, it has occurred to me that the ganged 50p capacitor that tunes the antenna stage and RF mixer coils, offers some opportunity for adding bands without winding more coils. For the lowest bands--like part of the BC band, the 50p just barely covers the 500 KHz segment allowed by the variable IF. For higher bands, the coil and 50p capacitor more than tune the RF segments and there is left over tuning to also cover an additional band by just adding a crystal. Because I am creating my own dial, I do not have to use only specified crystals that are becoming scarce or unobtainable. I can use crystals that are off a few KHz or tens of KHz one side or the other of the segment and create my dial to match. I can also use one crystal and two different coil sets or one coil set and two different crystals having the crystal above the frequency and the dial tuning backward. The crystals are in the grid circuit of the oscillator and have no voltage potential across them, which gives me pretty wide latitude as to which types I can choose from.

My latest acquisition for the radio is a tuning mechanism. It has a rotating drum and pointer that slides across. The drum would index off the bandswitch and could easily hold 12 bands. It is now indexed as though it would be for 10 bands, of which 6 were used. The current drum was lit from inside, but I am not sure that would be how my radio would work as it would be much easier to cover the existing drum with printed card stock glued to the surface. The window is 11" across--more than enough to show a 500 KHz segment with good detail. One issue is that the dial slider travels fully across with only about 300 degrees of rotation of the shaft that would connect to my variable IF tuning. The gears I am using now operate with one full turn to rotate the capacitor 1/2 a turn. There is about 5% of that where the capacitor is at zero capacitance so I really only need about 320 degrees for the full range. I am not sure there is enough adjustment in the oscillator and IF transformers to get to 300 degrees and still have 500 KHz of band. Using a little less band would not be the end of the world as most amateur bands are less than that--or I could split the band if needed--or I could see about different gearing.

The slide rule dial has some distinct advantages over a circular dial. All the bands get equal space--especially when they are all 500 KHz segments--and the drum can hold 12 bands--something a circular dial can't really do. But most importantly--circular dials are really, really hard to do graphics for; whereas a linear dial is a lot easier to organize and print from my computer.

Norm


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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Apr Fri 20, 2018 6:11 am 
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I was back with the radio tonight after spending last weekend camping. During the past two weeks, I rebuilt the RF section, tightening all the leads and securing the coils to the chassis. I also re-wired the trimmer caps for the variable IF so that the disc on top where you adjust the capacitance was going to ground. This stabilized them considerably and I can now use a metal screwdriver. It is a huge improvement.

So, tonight 20 metes worked well, with 5 microvolts AM producing a tone in the speaker and almost no noise. One thing about a modular structure for a radio is that the separation between stages and shielding are very good. I heard a few amateur stations on SSB. 40 meters did not work well. I get a ton of birdies up and down the dial--even when the BFO is off they sound like it is on. I assume these are heterodynes. I would conclude that the coils or crystal are not right--except that Radio Havana came in loud and clear on AM. I also received a religious station. The problem with this is that Radio Havana is at about 6.165 MHz. My 40 meter band is supposed to be 7.0 to 7.5 MHz. I think my crystal is bad. It isn't an HC6/U but is one of the smaller HC18/U's.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Apr Fri 20, 2018 6:06 pm 
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This morning I substituted crystals on the 20 meter band. With crystals of 12.2 and 12.5 MHz, I was able to get the WWV at 10 MHz and a number of AM SW broadcast stations below 10 MHz. This is "backwards" tuning where the received frequency is subtracted from the crystal frequency to arrive at the first variable IF of 2.4 to 2.9 MHz. I compared the reception to my Hammarlund HQ 140XA which I use for SW listening. The homebrew out performed the Hammarlund in sensitivity and signal to noise level. This surprised me as the HQ 140 had been my standard for performance. The homebrew is also very, very stable. I attribute this to the rigid tuning capacitors. They have very solid plates--not thin sheet metal--and are well constructed. The low noise has to be related to the modular construction and enclosed tuning capacitors. I even have good separation between sections within the individual chassis.

My next problem is to try and better understand the RF tuning. Why the 20 meter coils in conjunction with the RF tuning capacitor work so well at 9 to 10 MHz, is beyond me. I had expected the 12.2 MHz crystal to produce reception around 15 MHz as that would be closer to the 11.6 MHz crystal producing reception on the 20 meter band--but it didn't. Right now, 15 MHz is pretty dead, so maybe I have reception at both spectra, and the higher one just doesn't have signals I can receive so I only hear the lower one.

I may need to create the RF stage in a way that I can plug in coils and crystals. This will give me a better way to analyze what is taking place.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Apr Sat 21, 2018 2:42 am 
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I have sorted out what is happening with the RF section tuning. I am receiving both high side and low side injection at the same time. With a 12.521 crystal, I can get WWV at two locations a few degrees of dial rotation apart. These would be the 10 MHz and 15 MHz stations--12.521 - 2.521 and 12.521 plus 2.479. My variable iF goes from 2.4 to 2.9 MHz. Fortunately, the band above 15 MHz is not very active or strong. However, using my 4.6 MHz crystal for 7 to 7.5 MHz, I get a whole set of stations--because it is a HC18/U and I suspect it is an overtone marked on the case. I get many, many heterodynes (birdies) up and down the dial and carefully adjusting the tuning capacitor for the RF section, I can select out a number of different stations at each birdie.

This is not linear tuning but some kind of multi-dimensional radio where all frequencies are next to each other or on top of each other on the dial.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Apr Sat 28, 2018 5:03 pm 
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Another week has passed. If you look at the last photo, you will see screws sticking up from the variable IF cans. Those are gone. Last week, I needed to tap the variable IF chassis several times so as to get it to operate at maximum gain--otherwise the gain fell off. I suspected this was due to something related to the IF cans as they, and actually one more than the other, were sensitive to vibration and sporadic performance. I spent more than a few hours designing and building my original slug adjustment system, but I am over it now. Fortunately the alternative was a very easy change. I pulled the old tops off the coil forms--these were short sections of 3/8" phenolic tube with the slug screws mounted in T-nuts. I replaced these with longer sections of phenolic tube and lined the tube with some melted paraffin. Then I screwed in threaded slugs. This is much more stable and has less ferrite--further from the wire, more phenolic in between, and less of an inductance change when adjusting. The result is a very stable variable IF, no tapping the chassis, and accurate fine adjustment. I also wound a new transformer for one of the cans and installed it at the same time.

The RF section continued to be problematic. It worked well at 20 meters, but still covered two ranges--high side and low side injection. Fortunately the high side injection placed the received frequencies in a section of the spectrum without a lot of stations. On 40 meters, there were a thousand birdies and oscillations. While trying to sort out the cause, I bumped the mixer tube, a 6AH6 connected as a triode. The 6AH6 is a typical pentode. In this circuit, the crystal oscillator signal is fed to the grid though a small value capacitor along with the RF signal--like a typical BFO injection at the plate of a detector. Because the tube worked normally at 20 meters and not at 40 meters, I had not suspected the tube as a problem, but it was. If I pulled the tube shield and waved my hand over the tube, the birdies went wild.

I have a temporary solution and that is to run pin 2, the suppressor grid 3, to ground, and run a 4.7k resistor to a .01 MFd capacitor between the connected screen and plate to ground. This has "tamed" the birdies and 40 meters works normally--but still with both high and low side injection. 20 meters works as before.

The receiver is very sensitive--able to receive .25 microvolts of AM at the antenna. The S meter registers almost full scale for strong station like WWV and ticks up for the weakest stations. The AVC does what it is supposed to do. The noise blanker works for impulse noise. The BFO functions perfectly with USB and LSB received when the knob is about 15-20 degrees either side of TDC. The BFO injection control is a nice feature. The stability is rock steady, no drift on SSB stations. Overall, this is quite a nice receiver. I just need to resolve the RF section before I can move forward with a case and panel. On the one hand, I like that a crystal can be used for two frequency ranges because this reduces the number of coils I need. The RF tuning cap has two different settings for high side and low side. On the other, there isn't really enough separation between high side and low side. When I tune for receiving a signal from my generator and then switch injection sides (go to the other side frequency on the generator), I am only an order of magnitude or so down. Strong stations still bleed through.

I need to think and experiment some more to resolve this--especially if I want to use the receiver for more than just ham bands. I may need to switch in traps when I switch bands.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Apr Sat 28, 2018 6:58 pm 
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Very good, Norm.

When you get it finished, you might run some photos and final schematics, then submit it
to the ARRL for possible inclusion in the Handbook. It would be an inspiration.

- Leigh

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Oct Mon 08, 2018 2:13 am 
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I put my radio aside for a while because I couldn't make progress. It works great--super sensitive, but suffers from poor selection. It can't differentiate between high side and low side injection at the RF stage. In other words, with a 12.5MHz crystal, I can hear WWV at 10Mz and 15Mhz on the same band. I tried a RF band block filter to see if that would help, but it didn't. Which is fine, because that would be too much to have to rely on a different filter for each band. And that would be a bandaid for the real problem. The first mixer results in a frequency between 2.4Mz and 2.9MHz--which is then tuned at the first IF. I have done a lot of head scratching over the last few months, trying to decide how to proceed. For now, the radio functions because there are not stations that conflict for the few bands I am using. However, I get twice the noise. Since the radio is very quiet to start with, this isn't a huge problem, but I want to correct the situation.

This weekend, I started on the radio again and my plan of action is two fold. First, I am going to separate the RF stage chassis into 3 sections so that I can address the RF amplifier, crystal oscillator, and first mixer as three entities. The oscillator and mixer work, but I am not sure I like the design of either. The mixer I would like to try as a cathode follower 6C4 (like the R390a) and the oscillator is probably fine, only needing adjustment for coupling to the final mixer design. But those will come later, for now, I need to solve the RF amplifier, because that is where the selectivity fails--and I think I know why. The tuning is in the grid circuits (secondaries) at both transformers. The primary side is wound on top of the secondary and is a fraction of the turns for the secondary. It is a voltage step up transformer with a tuned secondary. This is fine for the grid side of the 6DC6, basically an antenna trimmer, but for the plate side, I think I need a tuned plate circuit and a tuned mixer grid circuit. Or just a tuned plate and a capacitor coupling to the grid of the mixer. I think it is the tuned plate that will get me selection.

If anybody has thoughts about this, please advise me. It is new territory, but I have a plan now and can move forward again.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Oct Mon 08, 2018 9:26 pm 
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Before doing anything else, I'd take a serious look at the antenna RF transformer. It appears that the Q of this coil is very low, leading to poor selectivity. Do you have access to a Q-meter? Two possibilities that come to mind are:
- The metal of the coil shield is too close to the coil;
- The primary is overcoupled to the secondary.

To check the first case, try removing the shield and see if it improves things.

To check the overcoupling case, try winding another primary beside the secondary instead of on top. Maybe about 1/8" to 3/16" gap in between. You could also try removing a few turns from the existing primary, but I don't like the idea of having the primary wound on top of the secondary. That sounds like a real Q killer to me.


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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Oct Tue 09, 2018 1:32 am 
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Thanks, Bob. I had that as my plan for winding the new coils--separate the windings by 1/8". The book had called for the primary on top of the secondary at the "cold" end of the coil. This may be because they were using stock Miller forms (with adjustable cores) and there wasn't enough room. Your pointing out the low Q configuration with over-coupling the secondary and primary confirms my suspicion. There is no metal shield on these transformers, so that isn't an issue.

Norm

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