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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Nov Mon 19, 2018 2:19 am 
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Joined: Sep Wed 10, 2014 2:01 am
Posts: 1993
Location: Costa Mesa, California
I have put a few more hours into the RF section of the receiver. First I built a separate chassis for the RF amp and use a wire to connect directly to the mixer grid. This allows me to test coils easily without any bandswitch involved. The compromise is that the wire to the mixer has some stray capacitance associated, and with the coils exposed and the wire to the mixer exposed, there is more noise. For now I can accept that. I have also revised my coil construction. I did give a run through with the primary separated from the secondary by a space on the coil form, instead of wound on top of the primary as the book specifies. This didn't work well, probably because I needed to increase the windings of the primary instead of using the number in the book. My latest method, though, seems to be a big improvement. Basically, I wind on a 1/2" phenolic form and secure the ends with zip ties. I then heat shrink a cover over the coil and wind the primary on top, again using zip ties. A 1/4" phenolic tube is then inserted into the 1/2" coil and paraffin is dripped into the center. The slug is screwed in and held in place by the paraffin. This seems to be the best way to create an adjustable coil, quickly and easily. Later, in the radio, I can screw down short pieces of 1/4" phenolic to the chassis and slip the coils onto those for mounting.

If I keep the RF section separate from the mixer, I will need to use a shielded wire to connect the signal to the mixer from the RF chassis. The circuit from the book won't allow this as the tuned output from the RF amp is connected directly to the grid of the mixer and any added capacitance from a shielded cable is significant. The variable capacitor is 50p to ground while the cable would be around 30p or more. My thought is to add a 6C4 triode as an amp/decoupler/cathode follower and feed it to the mixer cathode while the crystal oscillator feeds the mixer grid. Alternatively I can use a 6C4 there as well and also feed it to the mixer cathode. The circuit is outlined below and comes from "Rob's Web"--W5LHD.

I did have the radio up and working and listened to some 40 meter SSB and of course--WWV at 10 MHz. I really hope WWV stays around as I use it weekly to test radios. It is a constant that tells a person so much about the radio's performance.

Norm


Attachments:
1Cathode follower.jpg
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1Coil 2.jpg
1Coil 2.jpg [ 144.77 KiB | Viewed 1931 times ]
1Receiver .jpg
1Receiver .jpg [ 165.5 KiB | Viewed 1931 times ]

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Dec Tue 04, 2018 6:07 am 
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Joined: Sep Wed 10, 2014 2:01 am
Posts: 1993
Location: Costa Mesa, California
This past weekend, I implemented my plan to add the 6C4 as a way to isolate the secondary side of the last RF transformer. I fed it to the cathode of the mixer as shown in the schematic. I knew I would need to remove the cathode bypass capacitor from the 6AH6 original circuit and increase the resistor. I hooked up a 10K pot between cathode and ground and adjusted it for maximum. It peaked at 10K, but I didn't go any higher than that--just used a 10K resistor. I also added a 100K between the 6C4 grid and ground as bias. This new configuration allows a shielded cable to connect the RF stage to the mixer and crystal controlled oscillator chassis without affecting the tuned final side of the RF coil. The 6C4 adds enough gain to counter any signal loss and with a cathode follower the noise stays low. My RF coils/transformers work well with crystals in either high side or low side injection for frequencies between 9 and 11 MHz. I do not have crystals for testing frequencies bordering that range, but think I could expand a little with the same coils. The image in that set of frequencies is more than 100 times lower --in other words, it takes a signal at more than 100 microvolts to register a tone at the image frequency, whereas .5 to 1 microvolt is sufficient to get good tone at the frequency the coils were meant to cover. WWV at 10 MHz comes in loud and clear using a test lead as an antenna.

One thing I haven't figured out is why I need a .01MFd capacitor at the end of two test leads between the primary plate lead and ground for the final RF transformer. If I take that same capacitor and solder it under the chassis between the same two points--the radio goes pretty dead. I discovered this because I was noticing that hand capacitance increased the signal when I reached over to tune the capacitor for the RF stage. So I tried a few things and adding the .01 at the end of test leads connected between the plate lead of the coil and ground did the trick--but later underneath the chassis, it killed the radio.

Norm


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Front end.jpg
Front end.jpg [ 205.73 KiB | Viewed 1870 times ]
5a.jpg
5a.jpg [ 106.84 KiB | Viewed 1870 times ]

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Dec Mon 17, 2018 2:40 am 
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Location: Costa Mesa, California
This weekend, I started work on more RF coil sets--antenna transformer (L1 and L2) and RF plate transformer (L3 and L4). Last time I made an isolation circuit so the RF chassis can exist separate from the first mixer and crystal oscillator chassis and use a shielded cable to connect the two without affecting the final tuned circuit from the RF amp tube plate side. In fact what I did was transfer the signal coupling to the cathode of the mixer using a 6C4 to isolate the signal from the final tuned circuit--L4 and the tuning capacitor C2. Last time, I also made one set of coils that worked very well for frequencies in the range of 9 to 11 MHz. So now I have to work on up to maybe six or seven sets of coils for the bands I plan to use. My rotary switch has 12 positions, but some coils can be used for multiple bands by just changing the crystals and using high side or low side injection.

The band I chose for this weekend was the BC band. This requires a rather large coil-- between 500 microhenries to 1 millihenry. This would be somewhat large for a coil wound on a 1/2" form--and how would I make it adjustable? Why is the inductance so large? Because I am limited by a 50 p variable capacitor which needs to span something like 500 KHz or at least some sizable portion of 500 KHz at the low end of the BC band. My solution was to use a ready made 1 millihenry choke and wind my primary on top of the existing windings. Fortunately I had just the item in my stock of chokes. A 1 amp choke made of #26 wire wound on ferrite. I first covered the windings with a short length of heat shrink and then wound on top of that. I planned on using some ceramic trimmer caps to give myself adjustment between the two transformers--as accurate tracking needs a little tweaking. The stray capacitance and inductance of the two transformer circuits isn't the same so some adjustment helps track the two gang variable capacitor--C1/C2. My first try was successful in that I could easily receive stations at the low end of the BC band (determined by the crystal I chose) except that I had no adjustment with C1/C2--all the capacitance was at the low end of adjustment for a signal peak. I then unwound about 14" of wire from my chokes and tried again. This helped a little so I took another 12" of wire off the chokes and tried again. The advantage to my current system of using wire zip ties and heat shrink works well for these situations where multiple attempts are needed to get the coil right. I can clip the wire ties and even snip off heat shrink to re-do the coils in a minimum of time. The choke was easy to utilize, as well, as the windings could be pulled off from beneath the heat shrink and the new end could be soldered back to the lead.


Attachment:
BC band.jpg
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My final result was acceptable--after all it is the BC band and stations are strong, but in the future, I may return to these coils and remove even more wire to give myself a better adjustment range for C1/C2. Right now I am half engaged for the low end (540 KHz) and back at minimum capacitance by the time I am halfway back to the high end of my frequency range (860 KHz). Saturday afternoon, instead of continuing with the upper portion of the BC band--I need some more crystals, I moved on to the 80 meter band where I have a crystal very close to the correct frequency. The BC band uses high side injection because the first adjustable IF is 2.4 MHZ to 2.9 MHz-so it tunes backward. The amateur bands are planned for low side injection. By the time I had the coils wound and mounted, it was getting late on Saturday. I was then unable to get a response from the 80 meter band and struggled for several hours without finding why. That will have to wait for next week.

I get my crystals from Surplus Sales because they are cheap and I don't need exact frequencies, but can live with anything reasonably close. That is the advantage of creating your own dial--it doesn't have to start at some pre-determined frequency but only needs to cover the range you will be using somewhere in the spread.

As I stated earlier, the first IF is between 2.4 MHz and 2.9 Mhz. At the injection point for the signal to the first mixer--it is a cathode injection, I only need about 20 microvolts of signal to hear it in the speaker. So the radio can expect interference at frequencies near the first IF as the front end doesn't sufficiently reject something that close to the received frequency. I noticed this when I started hearing a familiar low background sound when the second IF was tuned at 2.5 MHz. When I connected the antenna directly to the first mixer, there was WWV--at 2.5MHz.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Dec Sun 23, 2018 3:15 am 
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Last week I left off after failing to get the 80 meter band working. This uses a set of two transformers with tuned secondaries—one for the antenna and one for the plate side of the RF amplifier. After a few more hours of trying various things—tightening up the transformer leads, adding the proper trimmer capacitors across the secondary, and using my grid-dip meter to dial in the proper slug size and adjustment to allow the resonance of the secondaries to center up at the mid-adjustment point of the tuning capacitor, I decided it was my crystal oscillator that wasn’t working. I had suspected this and already tested my crystals, but because the radio sort of worked, I wasn’t sure. I even got a hetrodyne whistle in my Yacht Boy test receiver at a frequency very near the crystal. Of course when I yanked the crystal out of the socket, the whistle remained so that sealed the deal—the crystal oscillator wasn’t working. Because it had worked at other frequencies and other crystals for different bands, I decided to try high side injection and fortunately had a crystal that could just catch the high end of my range at 4.1 MHz. That worked.

This made me suspect the tube. I concluded it wasn’t oscillating with crystals below a certain frequency. My oscillator uses a 6BJ6. I tried a different 6BJ6 with no luck. Okay, if the 6BJ6 doesn’t work, how about a 6JB6! You’re right, I am joking. My wife tells me I have no sense of humor. I don’t think this will change her mind. So I swapped in a 6BA6. As soon as the tube warmed up, the oscillator started loud and clear in my Yacht Boy. The problem then became an unacceptable number of “birdies,” and I haven’t sorted out where they are coming from. I did also try a 6AK5 and it worked also, only with even more birdies. I have some crystals coming that may hit the high side with a better range, so I will wait and see if that is the direction to go. If I use a 6AK5 like the 390a for my crystal oscillator, I will probably need to adjust a few component values to better suit the tube. I am not sure why a lot of radios in the Radio Handbook, Sixteenth Edition (1964) use the 6BJ6 but they do. So I now have another set of coils for my radio. Progress has been made. Like everything, you take a step forward and then a step back and then forward—it is like square dancing only with radios.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Dec Sun 23, 2018 4:06 am 
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I sure admire your tenacity, Norm.

Your project reminds me that I need to wind coils for 80 and 20 meters for my HBR-16. It only came with a set for 40m and once I got it restored and saw how well it performed, I began gathering the parts needed to make more coil sets. That was a couple years ago and I’ve long since accumulated the necessary parts but somehow misplaced the zeal to complete the project for now.

Fortunately that doesn’t keep me from enjoying and appreciating your exploits.

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Dec Sun 23, 2018 5:08 am 
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Thanks, Chuck. It turns out plugging away on this radio is easier on the pocket book than buying more radios—that I don’t have any more room for. Plus, I am learning things that restoring radios hadn’t taught me.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Jan Sun 06, 2019 1:44 am 
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Today, I have been measuring adjustable chokes. The crystal oscillator has a choke in the plate supply that keeps the generated signal at the crystal frequency from returning on the B+ line. The adjustment of this choke is specific for each frequency crystal and has a definite peak. The peak is a result of the choke value and the parallel capacitance across the choke. In many cases the tube circuit will not oscillate without the choke adjusted close to the resonant frequency. I am able to use a 6JB6 at lower frequencies if the choke is correctly adjusted. My problem before was in using the choke specified in the book. It just had 1mH chokes for 40 meters and 80 meters, but those do need to be adjustable—especially at 40 meters for the 4.6 MHz crystal. That choke at about 58 microhenries is much too far off the 1mH value. At 80, my choke measures about 858 microhenries, so that may be okay at 1mH, although my tube won’t oscillate with the 1mH choke.

I have wound a series of chokes based mostly on the specifications in the book, but also some intermediate chokes for frequencies outside the ham bands. I insert a crystal, adjust the frequency of the generator, peak the RF plate and antenna tuning capacitor, and then peak the choke with a 1/4” ferrite core that screws into the coil form center. Finally, I turn everything off and measure the inductance of the choke in the circuit—where it is fairly isolated. In the circuit and out of the circuit doesn’t change the value by much. With this data, I have made a graph and plotted the crystal frequency against the choke inductance. I get a nice curve that is fairly flat for crystals under 4MHz and fairly steep for crystals over 6 MHz. Going forward, I can decide the number of windings based on my plots. This will alleviate the need for trial and error for any crystal I select. I installed a set of banana jack posts for one position of the crystal switch wafer. These allow me to insert a choke on top of the chassis and then measure the inductance without having to get underneath. Further testing of many crystals was done using a scope on the grid of the mixer. This sped up the process and has given me a much expanded set of data points. The scope numbers vary from the non-scope numbers by being lower above 7MHz and higher below, but are much more accurate as to exact peak.

The other thing I have been working on is the situation with heterodynes—“birdies.” I ordered and received a better dual air variable capacitor that has much better isolation between sections. The original radio in the book used individual air variables with enough of a shaft at the rear that a coupler could be added and the individual air variables would then gang together. This allowed a lot of isolation and the sections could be in their own enclosures. The new capacitor helped. What has become obvious is that high side injection generates almost no birdies while low side injection seems to create many, especially for the lower bands. I am not sure why. My final design for the front end will certainly need to have as much isolation between the tuned sides of the RF amp tube as possible.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Jan Mon 07, 2019 12:09 am 
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To clarify the choke issue: The capacitance across the choke is measured capacitance. There is no discrete capacitor. This threw me off for quite a while and it wasn't until last night, when I measured the capacitance with the choke removed, that I saw what was happening. I have been using the 390a circuits to some extent to understand what is happening with my receiver--because the 390a has some of the same basic structure--with a crystal oscillator to mix down the RF to where a variable IF tunes the frequency. The 390a uses a very similar crystal oscillator to my radio, except it has a discrete capacitor, 100p, in the plate circuit of the oscillator to resonate with a coil at the crystal frequency. It couples the signal forward with a step-down transformer into the cathode of the mixer while my radio uses a 3p capacitor to couple to the grid of the mixer. Obviously, using stray capacitance to resonate with the coil, leaves a lot of variables to chance. My circuit now measures 18.8p with a few more pF added with the choke coil windings when it is connected. I think I may use a discrete capacitor going forward--probably connected across each choke--at least for the lower bands.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Mon 04, 2019 3:12 am 
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With the crystal oscillator worked out and understood a little better, I was ready a few weeks ago to tackle that module in its final form. Rather than waste a chassis that I had already drilled and configured for the entire front end, I decided to re-purpose it. This is something I have done before and it works well. I cut out a section and add an aluminum plate there with the new layout and components. One huge advantage is that sockets and terminal strips can be wired with the plate on the bench and a person does not have to work down in tight spaces.

To attach my coils to the chassis plate, I use some more of the same phenolic tube. Since it is a tight fit, the tubes can be pressed into each other trapping the coils to the chassis plate. In the future, they can be pried off and replaced when needed. To hold the slugs in place inside the coil tubes, I use wax. Paraffin wasn't quite right as it is too brittle. I tried bees wax and found it to be a little too sticky. My compromise was a mixture of the two. I melted a small amount and applied it to the inside of the coils with a Q-tip. I then ran a steel core through to thread the wax and finally used my ferrite cores. Don't apply an excess of wax. What seemed to work best was a thin coating. The steel cores are durable and can be threaded through the form with an allen wrench. Ferrite is quite brittle and will break if forced through the wax.

I checked the circuit for a capacitance value and then based my coil choices on that measurement. In the end, the stray capacitance was too low and I had to add a 3-12p ceramic trimmer below the chassis to bring the number up to about 22P. Once the coils were in place, I could no longer measure the capacitance directly and instead measured the inductance and calculated the capacitance using the formula.

My initial setup is for 4 bands--80 meter, 40 meter, 20 meter, and 15 meter, but I have room for 11 bands. The band choice depends on the crystal selection so to aid my decisions, I have created a chart based on which crystals I have collected for the radio. Here are the photos.

Norm


Attachments:
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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Mon 04, 2019 3:20 am 
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The last few photos. After this, I will be working on the RF amp section. I have one coil set for 40 meters that works very well. Unfortunately, my 80 meter and 20 meter sets break into oscillation and don't yield good results. I will need to wind some new coils and experiment a little. If all the coils broke into oscillation--then I could make some guesses about why. The fact that one set of coils in the middle of the range works so well leaves me a little stumped. The RF section will be built on another separate chassis and exist at the rear of the crystal oscillator and first mixer chassis. Either that, or I will need to have a way to transfer the rotation of the bandswitch shaft so that the RF chassis can exist to one side of the oscillator/mixer chassis.

Norm


Attachments:
90.jpg
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91.jpg
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92.jpg
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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Apr Sun 14, 2019 11:16 pm 
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Very nice work. I am envious. I am following this thread as I have coils I wish to build and an following your advice.


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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Apr Thu 18, 2019 3:33 am 
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I am still working on the radio. Right now, I am wiring the RF chassis. This has 22 coils, of which I am starting with 4 sets of 2 to match the 4 crystals I have in my crystal oscillator module--which is now complete. The crystal oscillator has room for 11 crystals and plate coils. In the RF chassis, I have decided to use as much physical separation with shielding between the grid and cathode side of the RF (6DC6) tube and the plate side. I have also inserted a second switch wafer to insert a silver mica capacitor ahead of the variable tuning cap section for the grid tuning. This will allow a better "spread" for tuning the grid secondaries--especially as the bands progress to higher frequencies.

The extra efforts at separation and shielding are to try and reduce feedback oscillation of the RF stages--something that has plagued progress to this point. By separating the two sides of the RF tube--grid/cathode and plate--into their own chassis sections with a metal plate down the center of the tube, and by also dividing the compartment that houses the ganged tuning capacitor, I will see if that is sufficient to stop feedback oscillation of the RF.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Apr Thu 25, 2019 12:46 pm 
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Norm...keep up the great work. I think that you may have developed a cult following, those that truly appreciate the thought process in design and experimentation, that went into the creation of high performance receivers.

One important point is that, unlike so many of the past design engineers at Heath, Collins, Hallicrafters, National etc., you are not driven or restricted by targeted release dates, parts count/cost/price points/profit margin dictated by senior management and/or the competition/marketplace- which would result in compromised designs (corner cutting), less experimentation (what if?), cheaper components and finally as a result, a lower performing product.

Todd
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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Apr Fri 26, 2019 8:35 pm 
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Thanks, Todd. I am saving tons of money by building this radio. I get to work on a tube radio without having to buy a junker, which still would cost a hundred bucks or so. Yes, I have to buy all the parts for the radio, but in dollars per month, I am orders of magnitude ahead of where I was when I restored radios for my small collection. The main thing is what I learn by the process. Simple things like IF and RF transformers and coils are totally demystified when a person starts building them, and then testing the transformer in their own circuits. I also spend a lot of time comparing the circuits in my radio to other known designs—like the 390a—a tunable IF radio. The key decision was building the radio as a set of stages and separate modules. This has allowed me to complete some and keep modifying others, without having to rebuild or work on the whole radio each time I want to make a change. It also means that when I complete the basic radio, I can still go back and improve some areas—like the audio output stage or the noise limiter section.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Apr Sun 28, 2019 11:18 pm 
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I wired up the RF chassis this weekend including installing the first set of coils--the 80 meter band. This put me into a position where I could see how my coils worked in the final version of the layout--one where the two sides of the 6DC6 are totally isolated and the tuning capacitor is shielded from the coil transformers. Unfortunately, the bandswitch index I had planned for this section, was frozen solid--the shaft could not be rotated. I soaked it in Liquid Wrench overnight, but even trying to drive the shaft out of the collar was impossible--so I gave up. It wasn't a huge setback, as I was still able to rotate the wafers manually and try the radio. I ordered a new index and then came to the conclusion that I may not need an index, as the crystal oscillator index could be coupled to the RF amp shaft without a second index. Originally, I had just figured I would have a split bandswitch, where the band setting had to be made with two unconnected switches. This would not work for a commercial design, but for a personal design, I would be able to live with it.

The test of the radio went just okay. I was able to adjust the tuning capacitor (really a pre-tuning capacitor as the main tuning is at the first IF) to a peak and then fine tune the coils. The radio receives an AM signal at 1 microvolt just fine. However, the coil tuning tended to hit a cut-off point and kill the radio as did some positions of the tuning capacitor. Bypassing the RF stage, I can feed the crystal oscillator-first mixer stage with about 20 microvolts for a good strong signal response. This means, I am looking for a gain at the RF stage of about 20 times the input at the antenna. Why is the radio cutting out? What happens is I adjust the coil slug for a peak and then hit a cut-off where the radio dies. I back off the adjustment and let it sit twenty seconds or so and it comes back to life. At first I thought the radio was going into oscillation at the RF amp tube. Now I think the grids (6C4 cathode follower, and 6DC6 RF amp) are getting too much signal voltage and cutting off the tube. This means I need to do one or more of the following--reduce coupling at the coil transformers by separating the secondary and primary more (this is probably the best option but may be least possible), change the 47 ohm resistor between the coil secondary and the grid to a higher value, or add a coupling capacitor (about 33p) between the grid and the tuned circuit and add a resister (100k) to ground at the grid. OF course, I may be completely wrong about why the stage cuts off. That will have to wait until next time.

It is hard to see in the underneath photo, but the RF tube is split down the middle by the mid-chassis divider with the cathode and grid on the antenna side and the plate and screen on the 6C4 cathode follower side. The two-section tuning capacitor is on top inside the small utility box. There is an extra switch wafer on the tuning capacitor side for a possible in-series silver mica capacitor to allow greater bandspread with the tuning cap depending on the frequency of the band.

Norm


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RF chassis 1.jpg
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RF chassis 2.jpg
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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Apr Sun 28, 2019 11:37 pm 
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Lookin’ very good, Norm.

Going from (weak) memory, but doesn’t the HQ-110 use the method you’re employing of putting an under-chassis shield to split the 6C4 sections? (Forgive me if you already mentioned this.)

Still following your educational adventure with great interest. It’s adding to my motivation to get back to my HBR-16 and make up the coil sets needed for the other bands besides the 40m set it now has. It’s a great receiver, like yours, for tinkering and learning.

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Apr Sun 28, 2019 11:59 pm 
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Thanks, Chuck. Splitting the RF tube is a common strategy--usually with just a small shield--but you are correct the HQ 110 has a major divider between the two sections with the tube divided.

Norm


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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: May Mon 13, 2019 1:19 am 
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Posts: 1993
Location: Costa Mesa, California
This weekend I made progress. I installed antenna and RF plate coils (transformers) for the 40 meter, 20 meter, and low broadcast band. The 80 meter band was already in place. The low BC band is between about 400 kHz and 875 kHz. It required winding a choke to go with a 3.274 MHz crystal (high side injection). As I stated before, the choke resonates with 22p stray capacitance in the plate circuit of the crystal oscillator—so I guess it shouldn’t be called a choke, but a plate coil for the oscillator. I call it a choke because I originally believed the designer placed it in the plate circuit to keep RF from feeding back into the B+, except the oscillator won’t oscillate (or oscillates weakly) if the coil and stray capacitance aren’t at the resonant frequency of the crystal.
The BC band antenna and RF transformers were made using 1 mH chokes that had been wound on ferrite cores. The windings were removed from the chokes until the inductance was about 425 microH. Then heat shrink was placed over the winding and the primary was wound on top of that and secured with wire ties. A 50p silver mica capacitor was soldered in parallel to the choke secondary and one of the choke secondaries had a 5 to 50p trimmer added in parallel as well. The tuning capacitor (also in parallel) is 0 to 100p. The problem the BC band poses is trying to use the same 100p tuning capacitor across the 500 kHz frequency spread that one would also use on much higher frequency bands. I have added a second switch wafer at each gang of the tuning capacitor so I can insert a silver mica capacitor padder in series to adjust the spread of the 100p tuning capacitor depending on the band. The extra trimmer was added to allow adjustment for any difference between the two transformers and stray LC in their respective circuits. These transformers do not have adjustable ferrite cores as do the ones I wind. Because of the number of windings, it is impractical for me to wind transformers for bands with frequencies in the BC range.

All of the bands have sensitivity at about 1 microvolt for modulated AM with good noise levels. The receiver is turning out to be an excellent performer with zero drift and good sensitivity. One bug I still need to deal with is that the AVC doesn’t seem to function properly. It is a “hang AGC” circuit and requires adjustment for attack and release. I haven’t focused on it as I have worked through the more fundamental stages of the receiver. I also want to use a more powerful audio amplifier. Mine is pretty basic.

Next up—more bands. I intend to add a high BC band, the 15 meter band, and some shortwave segments around 6 MHz and 9 MHz. That will bring me to 8 or 9 bands and I have room for 11. The 3.274 low BC band crystal can also function for a 5.675 to 6.175 MHz band, as well as the high BC band crystal working for a band above 6.175 MHz.

Here are the photos.

Norm


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RF bottom.jpg
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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: May Sun 19, 2019 2:19 am 
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Joined: Sep Wed 10, 2014 2:01 am
Posts: 1993
Location: Costa Mesa, California
I added the high BC band yesterday. The RF coils were easy enough as I had some wound which I was able to use. They weren’t perfect—the inductance was too high, but at the BC frequency it doesn’t matter much as long as the RF stuff is close. The oscillator was problematic. My crystal is a CR-1A type— the old clunky style. I mounted it in a holder attached to an adaptor that fit the crystal board I am using from a Collins. I then pulled some turns off a choke I had wound that was too high an inductance. It was quite a struggle trying to get the oscillator working. I ended up using my James Millen to get the oscillator tube plate coil close, and then did a final touch-up after the radio was working.

Today I added the 15 meter band. I repurposed some 20 meter transformers I had wound and pulled turns off to get in the range for the 15 meter band. 50p paddlers were used to bring the 100p tuning capacitor down to work with the transformers. The cheap Chinese LC meter that measures real time values at very low inductances and capacitances is indispensable for this stuff. It is quite accurate and a person can adjust proper values while watching the screen. Doing these LC measurements in circuit as much as possible is important, because there is significant added stray capacitance. The inductance is less crucial. 1/3 or more of the capacitance (8 to 18p) can be stray capacitance for the LC circuit. I measure the stray capacitance before the final coil connection. Unforunately, I have yet to get the 15 meter crystal to oscillate. It is a HC18 that I have attached to an old HC6 base. I spent several hours and have yet to get it to work. I use a scope probe attached to a coil of wire draped over the oscillator tube to see the frequency if it is there. I have substituted my signal generator for the crystal to adjust the plate coil and when I replace the crystal in the circuit it still won’t work. I have also tested the crystal with my signal generator and the scope and it does seem to function. I have even used a different HC18 crystal of the same frequency with no better luck. So now I am stumped, but that is what I like and why the radio is so much fun—I have a new problem to solve. If it all worked perfect, what fun would that be?

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: May Sun 19, 2019 6:02 pm 
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Joined: Sep Wed 10, 2014 2:01 am
Posts: 1993
Location: Costa Mesa, California
This morning I again tried to figure out the oscillator at the 15 meter position where it uses a HC18 crystal at 18.6MHz. First I added 7p to the cathode to grid feedback which moved it to 20p—same as the R-390a oscillator. This failed to get things oscillating. Then I pulled the plate coil for the oscillator and changed it to one that was closer to the needed inductance, requiring less from the slug to bring it into range. I rough tuned the oscillator plate coil with the grid dip meter (in circuit) and then finer tuned by injecting the frequency of the crystal at the grid of the oscillator. I did not see any improvement on the scope, but moved to injecting a signal at the antenna. I had a small measure of success working my way down to about 2 millivolts, adjusting the RF resonant circuits and finally the oscillator plate. Everything peaked nicely, where it should—except I am about a multiple of 1,000 away from where I should be. My current conclusion is that the oscillator is working, just very faintly.

The mixed result of the oscillator and the RF depends on both incoming signals being of sufficient strength to produce a strong enough result. The product is dependent on the weakest signal coming in. Apparently my HC18 18.6 MHz crystals don’t produce strong signals. They do appear, though, to be producing a very weak signal. I have tried bypassing the RF stage and feeding directly to the mixer with my signal generator and the required amplitude is 20 or more times that required at the antennna. This seems to imply that the RF stage is doing its job and the fault is the signal strength produced by the oscillator. All the lower bands function normally. I am using one other HC18 crystal, but it is at 4.608 MHz as low side injection for the 40 meter band. It works fine.

Norm

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