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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Thu 08, 2018 9:17 pm 
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Nice work, Norm, and an interesting project.

What an education!

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Sat 10, 2018 7:08 am 
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Thanks, Chuck. I am having fun--hard won fun, but fun nevertheless. I made progress tonight. When last I worked on this, I could not get the mixer to produce a 455 KHz result and then I really wasn't sure I would know if it did. How to tell? My frequency counter seemed to pull the frequency, my scope doesn't tell you if you are right on 455 KHz, I don't have the rest of the radio built so that can't tell me, and I don't have a receiver that would receive the frequency. All my counter seemed to show was the oscillator frequency. If I pulled the grid lead to the mixer for the oscillator, then it would read the variable IF. The mixer (6BE6) is also the oscillator. The variable IF goes to grid 3 and the oscillator uses grid 1. Grids two and 4 are the screens. It really seemed like the oscillator was overpowering the variable IF. I probably should have had the tap a lot closer to the ground end. I think I was a third of the way up and I would probably go a sixth up if I were to re-do it.

I went over the circuit at least a million times. Everything was wired correctly. I measured the capacitance and inductance of both IF stages with two different instruments. They agreed and the values were what they should be. I ran the resonance calc's several times. I could not find why I wasn't getting a mixer product that I could detect. Finally, I did several things. I reduced the trimmer capacitance at the IF stages and the oscillator. Then I changed the coupling cap for the oscillator grid by a lot. Instead of a 150p cap between the oscillator and the grid, I hooked one end of the 150p cap to an unused 3-43p ceramic trimmer. (I bought a box of double ceramic trimmers cheap off EBay which meant I had one extra at the oscillator compartment.) That allowed me to adjust the coupling cap and reduced it to a maximum of 33p. Finally, I soldered a 455 KHz ceramic resonator in series with a .01 MFd cap and connected it to the plate of the mixer. This more or less simulated the filter that the radio has at that location. With my scope hooked to the other end of the ceramic resonator, I could then peak the signal by adjusting the oscillator first and then the IF 's. I was able to work my way around the dial marking the peaks for 100 KHz IF intervals. This laid out a very nice, fairly linear dial.

The ceramic resonator acts as a tight filter at or near 455 MHz. When the scope signal produces a sharp peak, I know I am pretty close. Now I feel good about my tuning stage and can proceed to give it a final run through, cleaning up any long leads, stabilizing shaky mechanical connections, and finishing the entry points for the various signal, AVC, Gain, ground and power inputs.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Sat 10, 2018 5:27 pm 
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Dumb question. Were you feeding a signal into the mixer so you could get an IF frequency signal out?

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Sat 10, 2018 7:48 pm 
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I tried everything I could think of including feeding a signal to the transformer tuned circuits individually with power not applied to see what the output frequency was, feeding the input side of the first transformer, the input side of the second transformer, the grid of the 6BJ6 IF tube, and finally settling on feeding the grid of a 6AH6 stage I threw together to match the radio schematic so that I would have something like the final conditions.

I ended up with an input signal strength of 50 to 100 millivolts at the 6AH6 grid. This may be more than the RF amp ahead of the tube will generate, at which point, I will need to wind two more variable IF transformers with better coupling (windings closer together) or add a piece of ferrite to the lower input coil to increase coupling.

I considered trying the generator at grid 3 of the 6BE6 mixer, but if that produced a 455 KHz signal, it would not have told me anything about why I wasn't getting the variable IF to do the job--other than it was just too weak compared to the oscillator. In general, from what I could tell, I was getting about 180 millivolts at the grid of the mixer from 100 millivolts input. That isn't a lot of stage gain, but I am not sure that will be a problem. To me, this stage is more about tuning and selection than gain. I will have two more IF amp tubes after this in the 455 KHz section.

When the 455 KHz output was functioning correctly, it was pretty strong, reading full scale plus on the oscilloscope 1X probe setting at the least amplified position.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Sat 10, 2018 7:59 pm 
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Norm Johnson wrote:
I don't have the rest of the radio built so that can't tell me...
Hi Norm,

Forgive the observation, but...

You build a receiver the same way you troubleshoot by signal injection, from speaker to antenna. Get one tage working, then move to the preceding stage.

That's the only way to avoid the problem you noted.

You'll NEVER diagnose a mixer with a counter or 'scope.

- Leigh

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Sat 10, 2018 9:15 pm 
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Thanks, Leigh, you are correct. The big problem with this receiver is that the variable IF was the only stage that presented (at least to me) possibly unsolvable problems. I did not want to invest a lot of time and money building a radio, where the next to last stage was an obstacle that could not be overcome. Had the necessary coils and transformers specified in the schematic been available or even had any information about their inductance, cores, winding separation etc. been available, I could have proceeded as you suggest. I have wound more than a few solenoid coils for oscillators and RF circuits, however, never a Litz wire transformer with a Pi winding. My first step was to build the coil winding machine. When that was successfully completed and I had practiced sufficiently, I then needed to come up with the component values for the tuned circuits. After that the structure of the transformers, the number of turns, spacing, taps etc. There was a lot that could go wrong.

You have pointed out the difficulty in using a scope or counter to read the mixer output. I tried a lot of configurations with no success. However, the ceramic resonator (or a 455 KHz crystal) provides a solution. When a crystal or resonator is placed in series with a signal, it shows a very strong peak at or near the crystal frequency. Factors can pull the frequency one way or another, but not far. It is close enough to know that the mixer is producing a product very near the IF and final adjustments will be successful. The ceramic resonator also eliminates other harmonics and the original frequencies. The radio has a filter in the circuit at the same place I used the ceramic resonator, so it serves as a simple version of the filter. These 455 KHz resonators are extremely cheap. They do not have the Q of a crystal, but do have a significant Q--enough to serve as a crystal replacement in a pinch or in my case here, a medium Q filter. I have inserted them in radio circuits for an amplifier effect, but they produce too much "ringing."

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Sat 10, 2018 9:23 pm 
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Hi Norm,

Yes, some version of a 455kc series-resonant circuit is needed to evaluate the mixer output.
Moderate Q probably works better than high Q for this application.

This is quite an interesting project. I'm following your progress with some envy.

- Leigh

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Sun 11, 2018 12:18 am 
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Now that I have seemingly overcome this obstacle, I will take Leigh's advice and build the audio output, then start working back through the preceding stages. For the 455 KHz IF, BFO, Hang AGC and filter, I have a set of NC 173 cans with the guts inside. The book calls for a list of unobtainable Miller transformers and coils. But since these are fairly generic, and I am a big fan of the National filter, I am going to see if I can use the whole set of National parts.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Sun 11, 2018 11:55 pm 
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Here is a picture of the set up. The scope is set to 1 microsecond per division, so you can see that it is a little less than half a cycle per division--or about 455 KHz. The wave is very clean and strong, however, if I change the scope probe to 10X then it looks pretty bad, or if I use a 455 KHz crystal in place of the 455 KHz ceramic resonator, it again looks pretty bad. I think I just got lucky with the right combination of low Q filter and matching impedance probe to the scope. I really needed to have my NC 173 filter with me to try it as well, but I didn't. As I work my way around the dial, I get this same wave to appear and peak at each frequency setting with decent linearity. I sure hope the radio performs as well as my little test set up.

Norm


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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Mon 12, 2018 1:08 am 
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Hi Jim,

I expect the probe capacitance is affecting your tuning.

Switching the probe to 10x drops the capacitance significantly.

- Leigh

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Mon 12, 2018 2:04 am 
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Great project and I am enjoying it a lot. Way beyond my skill set.

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Mon 12, 2018 4:50 am 
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The rest of the receiver schematic is below. As per Leigh's suggestion, I am building the audio and second IF stages. The audio is going on a 5 by 7 by 3 chassis like the variable IF and the eventual RF stages. The second IF is going on a 7 by 9 by 2 chassis. Because this is being built in modules, the first audio will probably be a 6C4 instead of 1/2 a 12AU7. The 12AU7 is two 6C4's combined. I believe I will need to change the cathode resistor to a 5K and the plate resistor to a 100k but any help or advice would be appreciated. The output transformer is one from a NC 173. These are very neat looking in their metal cans and they have a 6 ohm and 500 ohm output. It doesn't say in the manual, but I believe it is a 5K primary which would be appropriate for the NC 173's 6V6 single output tube. The schematic below calls for a 5K transformer for the 6AK6. I think those usually run a 10K transformer like the output circuit in a Collins 390A, but since the schematic here calls for 5K transformer I should be fine.

For a 100 KHz calibrator, I am using a 6AU6 or a 6BA6 in a circuit like the Heathkit HRA 10-1 which is pretty standard.

The Hang AGC has its own module which it shares with the S meter amp and zero control. I built that several years ago and will need to change the transformer to a 455 KHz from what I have it in for another radio.

The second IF will utilize the IF cans from a NC 173 and the BFO and filter from same. Right now I am repairing the BFO variable capacitor which has a broken ceramic plate like many of these do.

Norm


Attachments:
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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Tue 13, 2018 4:11 am 
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The BFO repair has been completed. First I used Crazy Glue to put the ceramic back together. I did this on top of a sheet of paper which I just left attached after the glue dried. I then used a 1/16" thick piece of phenolic that I glued to the the paper attached to the ceramic with contact cement. Fortunately, there was enough of the ceramic left that it would support the center shaft flange and pieces. The final result is below. I did have to get longer stainless screws for mounting.

Norm


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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Sun 18, 2018 11:45 pm 
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This weekend I built the audio and second IF modules. The audio was fairly straightforward. The book called for half of a 12AU7 for the first audio tube. The other half was used in the hang AGC circuit. This wouldn't work for me as I built the AGC a few years ago for another receiver and want to use it now with this radio. I want to keep the entire audio on one chassis. So I used a 6C4 as the first audio. It has been said that the 12AU7 is two 6C4's in the same envelope. I don't know how true this is, but I substituted a 6C4 in the circuit--except I upped the plate and cathode resistors and added a grid bias resistor. Frank's tube data seemed to call for that. The audio module worked. At first, I wasn't sure because there was zero static noise--just silence. Then I turned on the signal generator and there was a tone. It only had one problem--the noise limiter didn't work. It is a pair of 1N34A diodes with some AAA batteries in the grid circuit for the 6AK6 second audio. When I flipped the switch, it killed the sound. Being unfamiliar with this type of circuit, it took me a while to realize the schematic in the book had a mistake--the batteries were in backward. That corrected, it did as it was intended and reduced noise peaks over (or under) a certain grid voltage.

Next up was the 455 KHz IF module. This is a much more complicated section which includes the filter, BFO, and final mixer. For my version of the radio, I use a set of National NC 173 transformers, BFO, and filter. I had to make a few changes to the schematic for this, but nothing too complicated. The original radio has a pair of Collins mechanical filters. Those would have been fine--except they are expensive, I didn't have any, and I did have the National filter--which I happen to like. My real preference would have been the filter for the NC 183D, but I made do. This took a lot longer to assemble, but was a lot of fun. Finally, Saturday night about 8 it was all ready to try out. I hooked it up with the audio section and when I flipped the switch, the tone came out of the speaker. I peaked the transformers and filter and then tried the BFO. Nothing. Uh oh. This meant getting back underneath the chassis and when I did some testing, I realized I had two of the BFO wires wrong. The NC 173 schematic has very good labeling for the transformers and filter, but the BFO isn't marked and a person needs to see how the resistances read to know which wire goes where. This time when I tried it, it worked great.

My final task this morning was to add the variable IF module I had done last weekend. This also worked , but took a little adjusting to get it where there were no birdies and images. I learned something. 3-43p trimmers are not a good idea. They have too much adjustment and fine tuning is way too delicate. I now know why many radios have 0-6p trimmers. They are a lot less temperamental when trying to fine tune a circuit. I may have to do some re-working on the variable IF.

Here are the photos.

Norm


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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Mon 19, 2018 5:26 am 
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I had a chance to do some more thinking today--always dangerous and I have changed my mind on what is happening with the BFO. Earlier I thought I had just mis-wired it and when I swapped two wires it started working. However--the other odd thing was once it worked--my hand capacitance would change the tone. That led me to think I had switched wiring locations when I fixed the variable capacitor earlier and I just needed to change them back to where they belonged. When I went to do that, the wires would only reach one place--so that wasn't it.

Here are the circuits:

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BFO 2.jpg
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When the NC 173 started working in the homebrew circuit (from the book), it was actually upside down with the ground to the 6BJ6 grid and the NC 173 grid wire to ground. That was why my hand capacitance was on the wrong side of the variable cap. Now the question is:

Should I just swap the variable cap wires and run it up-side-down but working-(maybe a little weak--hard to tell) or should I take the thing apart and re-wire it some way? The caps in the NC 173 schematic C-66 and C-77 are 270p and the resistor, R30, is 47K. The variable is about 15p by the looks of it.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Mon 26, 2018 5:40 am 
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This weekend I re-built the BFO to match the schematic in the book (the resonating capacitor stayed as did the extra switch.) I also made up a set of connecting cables and installed all the banana plug jacks and RCA jacks. During the week, I had sorted out my "Hang AGC" module and replaced the coupling transformer with one I purchased off EBay that was the proper frequency. The AGC module also contains the meter S circuit, so I hooked up a 0-1 ma meter. Everything worked as it should--except the variable IF. I just couldn't get it to function correctly and eventually had to give up. The new BFO configuration was a success and the injection potentiometer worked well--allowing me to control the amount of injection to the product detector. Another item I completed this weekend was the 100 KHz calibrator. This is just a 6BA6 circuit like the Heathkit HRA-10-1. I did not have a 100KHz crystal that would fit my crystal socket so I used a 200KHz crystal. My 100KHz crystal has fat pins.

Tonight I took the variable IF apart and changed the transformers and oscillator coil. Once again, Bob's calculator proved invaluable. I changed the inductance values, employed a 100p variable capacitor for the lower frequency oscillator circuit instead of the 50p I had before (it is a two gang where I wasn't using one gang, so the gangs just get connected), and re-matched all the tracking. The new oscillator coil is a solenoid type with #30 magnet wire instead of the Litz wire and the ferrite core is a threaded slug riding on wax inside a 1/4" dia. phenolic tube. I moved the tap to 1/6th of the way up from the ground end instead of 1/3 of the way.

To match the tracking, I did as before--setting my camera on something stable, photographing the computer screen of the transformer calculation and the oscillator calculation, and then printing the photos so I could cut them out and set the two bandspreads next to each other to see if they match. This took about four attempts until I had it dialed in sufficiently close. Now I will wait until next weekend to add the final components and re-connect all the stages for testing.

The final stage is the RF front end, with crystal oscillator and a mixer to lower the frequency down to match the variable first IF. I have made up one set of coils for the 20 meter band as that is the one I have the correct crystal for. I have some other crystals for the 80 meter band and 40 meter band, but they are "off" frequency a little and those coils will need to be slightly adjusted from the ones described in the book. I figure I will get the radio working on 20 before proceeding with other frequency ranges. My Centralab ceramic six-wafer switch has 12 bands I can use. I have a Collins crystal deck on the way so I can use all the bands. The last item on the list once the radio is operating is the case and dial.

Norm


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BS1.jpg
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BS2.jpg
BS2.jpg [ 124.52 KiB | Viewed 564 times ]

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Sat 31, 2018 12:42 am 
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Today I gave my rebuilt variable IF a try. It didn't work. I hooked the scope to a rough coil of wire which I set over the 6BE6 mixer and the variable IF part of the stage worked exactly as it was designed. The inductances peaked at the right spot and the scope showed good strong signals at the correct locations of the tuning capacitor. The oscillator was the problem. It appeared to be weakly oscillating at a much, much higher frequency than intended.

While connecting the IF trimmer silver mica capacitors, I found yet another wiring error. That really had me hoping I was through solving problems. I have never made this many mistakes wiring anything in my life. This particular stage has been one mistake after another. All the other stages went perfect and work 100% as they should. I might have made half a dozen or at most a dozen wiring errors on all the radios I ever worked on. I think I made a dozen wiring this one stage alone.

Now to fix the oscillator. I decided to pull the coil cover and use a grid dip meter to test the coil in place--see what it came out as. I also disconnected a wire to the tuning cap and checked the coil inductance. Then I checked the capacitance. Everything was pretty close. The coil was about 10% low but my ferrite core could easily correct the amount. With the coil cover off, I did a quick check of resistance. That was when I spotted yet another wiring error. The tap and ground were reversed. OMG. Big oops. No wonder it wouldn't oscillate--there was no feedback--even if I could adjust the inductance, no return signal was being injected back into the resonant circuit to keep it oscillating.

When everything was back together the stage started working. I adjusted my oscillator to the top and bottom of the range and then peaked the IF circuits. I then went back and readjusted the second IF and filter. Right now, I get a very nice tone out of my speaker with 5 microvolts input to the variable IF. 1 microvolt will still produce a tone. At this point there is no RF stage and the radio is quite sensitive. There is absolutely no noise or static coming from my speaker. It is hard to tell the radio is even working unless I do something to produce a noise. I am not sure whether to be thrilled or worried.

On to the RF and first mixer stage. I have all the components and a set of coils so I should get it completed very soon---as long as there are no wiring mistakes. If there are, I am going to blame it on Leprechauns.

Norm

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Sat 31, 2018 12:49 am 
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Great progress, Norm.

The finish line is in sight. We're all rooting for you.

- Leigh

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Mar Sat 31, 2018 1:08 am 
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What Leigh said.

Thanks for taking us along for the ride.

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 Post subject: Re: A Deluxe Receiver for the DX Operator
PostPosted: Apr Sun 01, 2018 4:43 am 
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I will post photos tomorrow, but tonight I am listening to Jim, E51JD, from Rarotonga in the South Cook Islands on twenty meters in SSB. The RF stage is complete and has been adjusted a bit, although at some point I need to go through the whole radio again. Everything works except the RF gain pot needs to be replaced with something a little beefier--at least a 2 watt wirewound. I have just shorted across it for now. The S meter is quite sensitive, the radio responds to AM at less than 1 microvolt, the noise limiter works, the BFO injection pot is a nice feature-- especially when the RF gain is on the fritz. I am still trying to figure out the tuning for the front end. It sort of acts like a preselector as well as mixing the signal down to the variable IF range.

I have been comparing the reception to my Hammarlund HQ 40XA and the sensitivity is about the same, however the bandspread on the HQ makes tuning a little easier. The biggest difference is that the HQ has better audio--especially better highs and crisper reception. I wonder if this is something I can adjust--and I am not sure it is the audio itself or other stages that are at fault. Unfortunately, the only band I have coils set up for is twenty meters. Since it is now past 8 PM, there isn't a lot to hear--a very few SSB stations, a few CW stations, and a very weak AM broadcast near 14.5 MHz.

Norm

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