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Monday, October 26, 2020
VK3YE: Solving the Direct Conversion RX -- Double Sideband TX Incompatibility Problem
Thursday, October 15, 2020
Too Simple? Deficiency of the Lafayette HA-600A Product Detector?
I've been having a lot of fun with the Lafayette HA-600A receiver that I picked up earlier this month. Adding to the mirth, I noticed that on SSB, the signals sound a bit scratchy, a bit distorted, not-quite-right. (I'm not being facetious; this is an interesting problem and it might give me a chance to actually improve a piece of gear that I -- as a teenager -- had been afraid to work on.)
Before digging into the circuitry, I engaged in some front panel troubleshooting: I switched to AM and tuned in a strong local AM broadcast signal. It sounded great -- it had no sign of the distortion I was hearing on SSB. This was an important hint -- the only difference between the circuitry used on AM and the circuitry used on SSB is the detector and the BFO. In the AM mode a simple diode detector is used. In SSB a product detector and BFO is used. The BFO sounded fine and looked good on the scope. This caused me to focus on the product detector as the culprit.
Check out the schematic above. Tr-5 is the product detector. It is really, really simple. (See Einstein quote below.) It is a single-transistor mixer with BFO energy going into the base and IF energy going into the emitter. Output is taken from the collector and sent to the audio amplifiers. (A complete schematic for the receiver can be seen here: https://nvhrbiblio.nl/schema/Lafayette_HA600A.pdf )
I had never before seen a product detector like this. One such detector is described in Experimental Methods for RF Design (page 5.3) but the authors devoted just one paragraph to the circuity, noting that, "We have not performed careful measurement on this mixer." The lack of enthusiasm is palpable, and probably justified.
A Google search shows there is not a lot of literature on single BJT product detectors. There is a good 1968 article in Ham Radio Magazine: http://marc.retronik.fr/AmateurRadio/SSB/Single-Sideband_Detectors_%5BHAM-Radio_1968_8p%5D.pdf It describes a somewhat different circuit used in the Gonset Sidewinder. The author notes that this circuit has "not been popular."
To test my suspicion that the product detector is the problem, I set up a little experiment. I loosely coupled the output of a signal generator to the IF circuitry of the HA-600A. I put the sign gen exactly on the frequency of the BFO. Then, I switched the receiver to AM, turning off the BFO and putting the AM diode detector to work. I was able to tune in the SSB signals without the kind of distortion I had heard when using the product detector.
So what do you folks think? Is the product detector the culprit? Or could the problem be in the AGC? Should I start plotting a change in the detector circuitry? Might a diode ring work better?
Tuesday, October 6, 2020
TRGHS -- My First SW Receiver Offered FREE for Pickup -- The Lafayette HA-600A (Looking for Globe VFO Deluxe)
Monday, August 24, 2020
Video on the Strange Tuning of the Radio Shack DX-390 Receiver
I'm more of a single conversion guy myself, but in working with the DX-390 I came to appreciate the benefits (especially regarding image rejection) of the double conversion technique.
While working on the DX-390, I discovered that the BFO control on the front panel DOES NOT change the BFO frequency. It was fun to try to figure out why the designers did it this way. It does make sense once you consider the limitation imposed by that PLL main tuning oscillator that only moves in 1 kHz steps. I hope the video explains things.
Here is the drawing I used in the video:
Sunday, August 16, 2020
Fixing up a Radio Shack DX-390 (AKA Sangean ATS-818) While Suffering from Fat Finger Syndrome
I've had this Radio Shack DX-390 portable receiver since the early 1990s. I bought it when I was in the Dominican Republic. It accompanied me on some interesting trips to the Haitian border, and on one very memorable 1994 trip to the Haitian capital. I have made some CW contacts with it serving at the inhaler.
Click on the diagram for a better view. It is a dual conversion superhet. First IF is at 55.845 MHz. There is a big 90's era IC-based PLL oscillator that runs from 55.995 to 118.7 MHz -- The main tuning dial moves this oscillator. Second IF is at 450 kHz. There is an oscillator at 55.395 that takes the signal down to 450 kHz. Selectivity (not a lot) is provided by ceramic filters. Finally there is a product detector and a 450 kHz oscillator that produces the audio. While there are many mystery chips in this receiver, there is also a lot of discrete-component analog circuitry in there -- it is kind of a pleasing mix.
DX-390 Main Board. Note kludged toroidal replacment for L10 (just above ferrite antenna) |
I occasionally found myself thinking of this receiver. I spotted one on e-bay not long ago, and bought it. This newer one was in very nice shape.
But that old one was kind of staring at me from the corner of the shack. "C'mon radio man," it seemed to say, "can't you fix a shortwave receiver?" So this week I took up the challenge.
First the FET. I had kludged an MPF102 in there, but that didn't seem to work well. Internet fora seemed to think that a J310 would do better, so I installed one of them -- it did seem to work better. (Note: Pete Juliano likes J310s -- TRGHS.)
Kludged in J310. And two sets of back to back diodes |
I was pleased that the old receiver was receiving OK, but there was a problem: The "BFO" control wasn't working. The BFO would come on, but turning the BFO control did not vary its frequency.
At this point I discovered that while there are many copies of the DX-390 service manual and schematic on the internet, all of them have seriously degraded copy quality right around the parts of the circuitry that I needed to study. Sometimes Murphy overpowers the Radio Gods. It took me a while to get a useful schematic of the BFO control mechanism.
BFO is a bit of a misnomer here: the control actually shifts the frequency of the 55.395 MHz oscillator that drives the second mixer. See Block diagram above). There is a varactor diode in the base circuit of a BLT oscillator circuit. Turning the BFO control varies the voltage going to the varactor thus causing the oscillator frequency to slide up and down. But mine wasn't moving. And that was a problem.
So I dove right in, trying to figure out why it was oscillating, but not shifting in frequency. At this point I discovered that I too am afflicted with the disease that Pete Juliano suffers from: Fat Finger Syndrome. That BFO control circuit has a nice big 100k pot, but all the fixed resistors and caps were surface mount and SMALL. As I poked around trying to troubleshoot, I managed to make things worse. It turned out that the lead carrying 6 volts to the BFO control circuitry had broken. But before I discovered this, I managed to do all kinds of damage to the board. I lifted two PC board pads (I should have turned down the temperature on my soldering iron). Then, when I tried to fix this, I managed to put a solder bridge across two parts of the circuit that definitely should not have been connected.
This resulted in a bizarre BFO situation. From the center position, turning the BFO to the left OR TO THE RIGHT would move the BFO in the same direction. So I could tune in an SSB station by turning to the right, or by turning to the left. That was just not right.
Lifted solder pads. And small wires that now bridge the gaps |
Some additional observations on the DX-390.
-- It really is a Sangean ATS-818 in disguise. Just look at the marking on the PLL board. If you can't find a decent DX-390 schematic, just use an ATS-818 schematic.
ATS 818 marking along the bottom (green) part of the PLL board |
-- The static discharge vulnerability is hard to understand. There is so much cool circuitry in these receivers, why not add four simple diodes? Not wanting to repeat this saga, I went in and put two sets of back-to-back small signal diodes in each receiver: one set on the telescoping antenna, and other at the input for the external antenna. Curiously, on the newer receiver, it looks like a previous owner had gone in and tried to address this vulnerability -- but he did a very incomplete job. He just put ONE diode between the external antenna input and ground. I had always thought that two diodes back to back would give you good protection from static discharge. And I don't think that single diode protects the front end in any way from discharge coming in from the telescoping antenna.
Tuesday, July 28, 2020
A 75 Meter AM Quarantine Converter for the Q-31 Receiver (video)
A while back Fred KC5RT sent me a nice collection of parts, including some 6 MHz crystals. I had been thinking of making a converter to put ahead of my Q-31 receiver. When Fred's 6 MHz rocks arrived, I knew that The Radio Gods Had Spoken (TRGHS).
I found some NE602 chips in the junk box. I used an Altoid-sized box for the case. The toroids are from W8DIZ. I use trimmer caps from KC5RT to resonate the input and output circuits.
Hooray! Now I can listen to 75 Meter AM on the Q-31. I may have to build a transmitter to go with this contraption.
Thanks again Fred.
Monday, July 13, 2020
"The All Japanese 6" Receiver
Sunday, July 5, 2020
The Ceramic Spurs (not a rock group)
+/-6kc filter upper left, 455B wide filter to the lover right. |
Q-31 with can for first IF amps and filters open |
Sunday, June 14, 2020
Dilbert, Shep, Dex, Pete, Farhan, and Wes! N2CQR Presentation on Homebrewing to Local Radio Club
Dean KK4DAS asked me to speak to our local radio club, the Vienna Wireless Society. It was a lot of fun. I talked about my evolution as a homebrewer, some of the rigs I made, the moments of joy, and the tales of woe. You can watch the presentation in the video above.
I was really glad to be able to explain in the presentation the importance of people like Pete, Dex, Farhan, Wes, Shep and even Dilbert.
I was also pleased to get into the presentation the N2CQR sign that Peter VK2EMU made for me. Thanks Peter!
Here is the URL to the YouTube video (also above):
https://www.youtube.com/watch?time_continue=3414&v=VHSr-v4QO7Q&feature=emb_logo
And here are the PowerPoint slides I used:
https://viennawireless.net/wp/wp-content/uploads/2020/06/VWS-presentation-Rig-here-is-homebrew.pdf
Saturday, May 2, 2020
Idaho Homebrew -- Brian KE7LOY
Monday, April 20, 2020
DX WaveScan
I heard this program this morning on my Q-31 receiver. It was on WRMI 9955 kHz at around 1330 UTC.
It reminded me a lot of the DX listening program of HCJB. Good stuff.
You can listen to the programs on-line at: https://awr.org/program/engmi_wav/
Sunday, April 19, 2020
Q-31 Quarantine Receiver -- All Boxed Up, Almost Done
Almost done. A few odds and ends remain, but now I have all the circuitry in their boxes.
As I was taking my walk the other day I was thinking of how I didn't have to build a BFO for this superhet. That's because the signals coming in on this rig bring with them their own BFO signal (the carrier).
Saturday, April 18, 2020
Q-31 Quarantine Receiver -- +30 db and a Germanium Diode Help a Lot (video)
Today I added two additional stages of IF amplification. This added 30 db to the receiver's total gain. That helped a lot. I also discovered that Germanium (1N34A) diodes work a LOT better as AM detectors than do silicon diodes. This receiver is starting to sound decent. Currently listening to the VORW program on WRMI Miami.
Thursday, April 16, 2020
Q-31 Quarantine Receiver -- First Signals (Video)
It still needs a lot of work, but today it pulled in its first shortwave signals. See video.
Tuesday, April 14, 2020
Quarantine Rig Q-31 -- Putting the Stages Together (Most of Them)
I've been recording short videos on my progress with the Q-31 Shortwave AM Quarantine Receiver.
Yesterday was a bit of a milestone -- I put five of the six sub-assemblies together and did some testing. You can see the video above.
All the other videos are on my YouTube site:
https://www.youtube.com/user/M0HBR
Please subscribe and give me a "thumbs up."
SITS! Stay in the shack! Flatten the curve. Hang in there. 73 Bill
Saturday, April 11, 2020
Videos on the Q-31 Quarantine AM SW Receiver Project (and some pictures)
I've been making some short, stage-by-stage videos of my Q-31 receiver project. So far I have seven videos. They are here:
https://www.youtube.com/user/M0HBR/videos
Please subscribe to my YouTube Channel. And give me some "thumbs up" if you like the videos.
Thanks. SITS! FlattenTheCurve! 73
Pads from Pete, toroids from Farhan |
The diode ring |
Altoids-sized tins will hold the circuit boards |
Stay In The Shack -- Or in the front yard. |
Thursday, March 26, 2020
Radio History Question: Why 455 kHz as the IF frequency?
My work on the S-38Es, on the HRO-dial receiver, on the Mate for the Mighty Midget, and on various mechanical filters has caused me to think (once again) about why we ended up with 455 kHz as the IF frequency for so many radios. I've heard many explanations for this, but unfortunately I've forgotten the explanations and lost the sources. I started digging into this again today. I found the below e-mail from Al N3FRQ on the Boatanchors mailing list (2008).
I contacted Al to find out if he had learned anything else on this topic. He has not. So if anyone out there has answers to Al's questions, or anyother info that would shed light on why they went with 455, please let us know.
-------------------------------
Every so often the question comes up: Why are all the IF’s 455 KHz? I’d like to get an article together that solves this riddle while the people who know are still with us. I know parts of the story, but I need help with a couple of issues. There are two major consideration is the choice of the intermediate frequency used in a superheterodyne receiver. The lower the frequency, the easier it is to attain high selectivity. Also, in the early days, before tetrode and pentode tubes, it was easier to achieve a high degree of amplification at lower frequencies. Conversely, a higher IF frequency results in better image rejection. Early superhets had the IF at 100KHz or lower in order to get adequate gain from the available triode tubes. They suffer severely from “two-spot tuning” (images). By the early 1930’s, broadcast set had settled in at 175KHz, and automobile receivers would later adopt 262KHz as a standard. The advent of the short-wave craze, and multi-band broadcast receivers dictated a higher IF frequency to achieve adequate image suppression on the short-wave bands. The broadcast band occupied 550-1500KHz at this time, and the designer encounters sever problems if his radio tunes across it’s own IF. Some shortwave sets used 1600-1700KHz for better image rejection, but one couldn’t go higher if the 160-meter ham band (1800-2000KHZ) was to be covered. Most multi-band receiver settled in near 450KHz, a comfortable distance from the first broadcast channel at 550KHz. Questions: Odd multiples of 5KHz, 455, 465, etc., were usually chosen so that the image of the carrier of a broadcast-band station could be zero-beat with the carrier of the station being tuned to achieve minimal interference. (This assumes 10KHz channel spacing. Did the Europeans (9KHz) do something else?) The Radiotron Designers Handbook, Third Edition, p. 159, states “A frequency of 455 Kc/s is receiving universal acceptance as a standard frequency, and efforts are being made to maintain this frequency free from radio interference.” (1) Do FCC and international frequency allocations reflect this? (2) I’ve heard the term “Clear-Channel IF.” Can anyone cite references? (3) At lease one news group posting claims that broadcast frequencies in a particular market are assigned to prevent strong inter-modulation products from falling near 455KHz. Is this factual? Need reference.” (4) Was this (3) at least part of the reason for “Radio Moving Day” in 1941? See: http://www.dcmemories.com/RadioMovingDay/ 032341WINXFreqChange.jpg (5) Many National Radio sets used a 456KHz IF’s and I think I remember a 437 somewhere. Why? Are there different considerations for short-wave CW operation? Further input, corrections, and elaborations are greatly appreciated. Scolarly reference will be looked upon with great favor. Regards, Al -- Al Klase - N3FRQ Flemington, NJ http://www.skywaves.ar88.net/
Wednesday, March 25, 2020
Mr. Carlson Restores an All-American Five -- Tribal Knowledge! SITS! Flattening the Curve! (video)
It is always a pleasure to see a new video on Mr. Carlson's awesome YouTube channel, especially in these days of Staying-In-The-Shack (SITS). Obviously Mr. Carlson is doing his bit in this area. FLATTEN THE CURVE! Thanks OM!
My recent bout of S-38E madness has peaked my interest in the All American Five design, so this March 10, 2020 video was especially interesting to me. Mr. Carlson puts out so much great tribal knowledge. I didn't know about "rounder" resistors. I didn't know that you have to be careful not to short out (to the IF can case!) the 455 kc transformers. I really like his approach to dial cord restoration.
Mr. Carlson's discussion of the adjustment of the front end tuner circuit on this broadcast band radio was very interesting. Unlike the S-38 radios, there are no front end coils being switched in as you change bands. In fact, it appears that that big coil/antenna inside the back cardboard piece IS the front end coil. This discussion has caused me to question my front end alignment technique for the S-38E. Did I have an appropriate antenna or antenna substitute across the antenna terminal when I set the peak on the input LC circuit? I will check on this. Hooray! One more thing to do during the COVID-19 SITS period.
UPDATE: I checked on this using the test set up described in an earlier post, but this time with my antennas connected. First with a 40 meter dipole, then with my 130 foot doublet, then with a 50 ohm dummy load I was still able to see the resonance dips at exactly where I wanted them to be.
My favorite bit of Carlsonian wisdom from this video? Mr. C's confirmation that some hum in All American Five receivers IS NORMAL! (This may be too much for the folks who find normal band noise to be offensive.)
For Inspiration and Education: Dean's Radio Blog (with video)
Be sure to check out the blog of Dean KK4DAS. He is a new homebrewer who is having great success with one of Pete Juliano's ingenious SSB designs. Dean has a video of his receiver working -- AL FRESCO -- as construction on the full transceiver proceeds.
This is amazing. Just a short time ago Dean was taking his first steps as a homebrewer with his version of the Michigan Mighty Mite. He has followed the advice of the Tribal Wizards and has proceeded slowly, step by step, stage by stage, gaining the experience that has allowed him to actually build a superhet receiver and be on the verge of completing a full SSB transceiver.
Lots of inspiration to be found on Dean's blog. Check it out:
https://kk4das.blogspot.com/2020/03/dean-kk4dass-furlough-40-ssb-rig.html
Monday, March 23, 2020
AM Diode Detector + 41 and 49 meter Shortwave Bands for HRO-dial Receiver (videos)
The COVID-19 emergency is a good time to look around the shack for projects you have been meaning to take on but didn't have the time for. We have the time for them now!
When I first built my HRO-dial receiver (using an HRO dial given to me by Armand WA1UQO and an enclosure from Tim KI6BGE) my hope was to have the 40 meter ham band and some shortwave broadcast bands. But it didn't work out that way. I had trouble getting an AM detector to work properly, and I had a hard time getting a sufficiently broad filter to work right. I ended up adjusting the VFO so that the receiver would cover only the 40 meter ham band.
My recent S-38E adventures and a video from VK3HN have alerted me to the nice programming that is now on the shortwave broadcast bands (I really like WRMI's afternoon rock music program). So I decided to take another shot at getting this receiver to cover SW BC frequencies.
When I built this receiver, I made the front-end bandpass filter tune-able. There is a two section variable cap behind that "Pre-selector" control you see on the front panel. That lets me tune two loosely coupled LC circuits from about 5.5 to about 8 MHz. So without any mods to the front end, I could cover the 49 meter band (5.9 -- 6.2 MHz) our 40 meter band, and the 41 meter band (7.2-7.5 MHz)
Here is how I do it:
For 49 meters: I now have the VFO set to run from 6.34 MHz to 7.120 MHz. The IF is .455 Mhz. So to get down to the lowest frequency in the 49 meter band, I tune that front end preselector down to that frequency (variable cap in filter almost fully meshed). Then I take the VFO down to 6.355 Mhz. I take the difference frequency out of the mixer -- .455 MHz.
For 40 and 41 meters: I just tune the pre-selector to this range (variable cap about mid-range) and tune the VFO accordingly. For a signal at 7.5 MHz, for example, I put the VFO at 7.045 MHz. 7.5 - 7.045 = .455 Mhz. Note: There is no sideband inversion in this case -- this is important because 40 meter SSB is lower sideband. The Kokusai mechanical filter that Pete N6QW gave me is a lower sideband filter. I have my BFO set at the right spot relative to the filter passband for LSB.
As you can see, I just tune to the "image frequencies" with the preselector. This gives me double the frequency coverage.
As for the filters, well Pete's Kokusai filter works great on 40 SSB. My problem was, ironically, getting a filter that was broad enough to let AM sound good. I concocted a filter using old 455 kc IF cans, but I wasn't happy with it. Paul VK3HN used a ceramic .455 MHz filter that was 6 kHz wide at 6 db down. I ordered some from Australia. That should have been wide enough for AM, but I had gotten spoiled by the very WIDE bandwidth of my S-38Es (no real filters at all, just the two 455 kc IF cans). At this point The Radio Gods interceded. Bruce KK0S heard me talking about this on the podcast and kindly sent me some 10 kHz .455 kHz filters. Now we're talking! I put one of them in this receiver and AM started sounding as good as it does in my S-38E. BTW -- a look at NA5B's WebSDR receiver shows that most of the SW broadcast stations are running at 10 kHz wide. See video below:
Finally, I had to get a decent AM detector going. The SBL-1 product detector I have in there works great, but I had tried several AM detectors and none of them worked well for me. This was puzzling -- it should be so simple, right? Just a diode. But I would get weak and/or distorted audio. I realized that I really needed was something that looked to the rest of the circuitry like an SBL-1, but with just a diode and an RC filter section instead of the SBL-1's diode ring. I ended up using a small 455 kc IF transformer that Michael Rainey (AA1TJ) had sent me a long time ago. My detector looks like this:
But you know, I find myself thinking that there are many stations I like on the 39 meter band. I think it might be best to build a separate receiver for those frequencies. Maybe throw in 30 meters. Hmm, let me see what's in the junk box...