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Friday, May 17, 2024
Band Imaging Rigs (Receivers and Transceivers) -- Video from WA7MLH
Wednesday, March 13, 2024
Using the RF Power Amplifier of the BITX40 Module
Thursday, December 14, 2023
Mythbuster Video for the Lamakaan Amateur Radio Club of Hyderabad, India
Friday, October 6, 2023
A Pretty Good Troubleshoot -- Fixing the Transmitter in my 75/20 meter Mythbuster Transceiver -- Mind the Gap!
All of a sudden the transmitter in my 75 & 20 meter dual band homebrew Mythbuster transceiver stopped working -- there was no output at all. I went into troubleshooting mode.
The first clue was that the receiver was working fine. This meant that many stages of the rig were taken out of suspicion: It probably wasn't the VFO, the first mixer, the BFO/Carrier Oscillator, the second mixer, or the bandpass filters. Nor was it any of the receive sections in the bidirectional amplifiers I was using.
Suspicion fell on the power amplifiers and on the transmit sections of the bilateral amplifiers.
With the output going to a dummy load, I put the rig into transmit mode and put a bit of audio into the mic jack. Then with the 'scope I started to work my way back from the antenna jack. I wasn't seeing anything. Then I got back to the transmit side of the TIA amplifier between the crystal filter and the mixer. There was a strong signal at the input, but nothing at the output. Bingo! I had found the faulty stage. But where, exactly, had this stage gone wrong?
There are three transistors on each side of a TIA amp (see schematic above) -- I just started from the input of the first one with my scope probe and moved through the circuit. Finally, at the output of the last of the three amplifiers, the signal stopped. I knew I was very close to the problem.
Looking at the components, suddenly I could see the problem: At the output there is a 47 ohm resistor (R2 in the circuit diagram above) and a .1uF cap in series. The cap went to a Manhattan pad. But when I looked at it closely, the lead was kind of floating above the pad. See it?
And when I moved it, the connection between the 47 ohm resistor and its pad seemed quite flimsy.
I quickly replaced both components and was back on the air.
I don't really know how or why the lead to that capacitor broke. Maybe I had bent it repeatedly, to the point of weakness, and, over time, it just let go.
Whatever the cause, I found this to be a satisfying troubleshoot and repair. It required me to think a bit about what could be wrong, and to use some test gear to zero on on the faulty component.
Monday, July 10, 2023
Martein's Bandpass Filters - PA3AKE
Before I built Martein's filter, my bandpass had been inadequate. Looking at the signals coming out of the diode ring mixer in my 15-10 rig, I realized that when I was on 15, there would also be an output on 10. And vice-versa. These outputs would have to be knocked down by the bandpass filters. I had been using simple dual tuned circuit filters. But when I looked at the filter shapes of these filters in NanoVNA, I could see that On 15 the 10 meter signal was only down about 20 db. And on 10 the 15 MHz output was also down only by about 20 db. That's not enough. Take a look:
Monday, September 5, 2022
Why Do Some VFOs Tune More Linearly Than Others?
This has been one of the major complaints about our beloved analog LC VFOs: The frequency tuning on these circuits is often not linear. For given amount of VFO frequency dial turn you can get vastly different changes in frequency. At one end of the tuning range the frequencies are nicely spaced and tuning is easy. But at the other end of the tuning range all of the frequencies are bunched together. This is one of the problems that leads some homebrewers to defect to the sad land of "digital VFOs."
But wait. It appears that the old designers found a solution to this problem. Just look at the tuning dial of my HT-37. The frequencies are all spaced out evenly. How did they do that?
I had been thinking that this success may have resulted from Hallicrafters' engineers using the series-tuned Clapp circuit. Here the main frequency determining element is a series-tuned LC circuit and not the parallel tuned LC circuit that we see in the more commonly used Colpitts circuit.
But hold on -- how could that be? The frequency bunching problem that we attributed to the Colpitts circuit must also exist in the Clapp, right? I went back to SSDRA where there was a good discussion of Colpitts and Clapp VFOs. The advantage of the Clapp was said to be in its use of a larger value coil which helped minimize the effects of stray inductances. But there was no mention of the Clapp offering improved linearity in tuning.
I have in front of me two transceivers: The Mythbuster uses a 9 MHz Clapp circuit (see below). The 17-12 rig uses a Colpitts Circuit. I checked the tuning linearity of both. Both appeared quite linear in tuning, with no real difference between the two.
Then I looked at the tuning capacitor in the Mythbuster 17-12 rig. It came out of an old Hallicrafters transmitter, probably the HT-44. I looked closely at the stator and the rotor plates. Both are curved. I'm guessing that this may yield a more constant change in capacitance for a given movement of the main tuning dial.
Next I opened up the VFO on the Mythbuster. (It is the VFO from an old Yaesu FT-101.) I couldn't see the stators very well but it appears that their shape is different from the square shape we often see in variable capacitors. Could it be that this variable capacitor was also made to provide linear tuning?
Back in 2013 Norm Johnson wrote about all this in the Antique Radios.com forum:
A capacitor that has uniform increase in capacitance with rotation will have the stations at the high end of the band squeezed together. Another type known as the straight-line frequency variable capacitor has, as you might guess, a characteristic that gives even spacing of frequencies with shaft rotation. These were popular in the 1920's but weren't very good for superhets where you needed to have a dual section capacitor that would tune both the RF and local oscillator, and have them track each other properly. The midline variable capacitor is more compatible with a superhet, and easier to make both sections track properly. This is the type that you see in most receivers from the late 1930's to the end of the tube era. They don't have quite the equal spacing between stations across the band that the old straight-line frequency caps had, but they're much better than the variables that change capacitance linearly with rotation.
I wrote an online calculator that helps in the design of the tuning. It shows what frequency range you'll get with a specific type of variable capacitor, including the effects of padder and trimmer capacitors. It also displays a dial scale that shows how the frequencies are lined up accross the dial.
http://electronbunker.ca/eb/BandspreadCalc.html
Steve W6SSP also provided some really good info back in 2013:
There are three types of open, variable plate caps;
SLC= straight line capacitance where the capacitance varies linearly,
these are the most common and have half-circle plates
SLF= straight line frequency where the plates are tapered to allow
for linear tuning of the frequency
SLW= straight line wavelength, you get the idea...
SLF and SLW caps have oblong plates.
The effect on tuning a receiver can be dramatic. One example is the
Hammarlund SP series of receivers where the ham bands are very
compressed at one end of the tuning range. They used SLC caps
in the VFO. On the other hand rigs like the Kenwood TS-520
and FT-101 series have linear tuning across each band. These use
SLF variable caps. Most old 1920's battery radios used SLW
where stations were identified by their wavelength.
Steve W6SSP
The Drake 2-B also has perfectly linear tuning. I looked at the manual: "The tuning condenser is of special design..." I'm guessing that they used an SLF variable capacitor. The 2-B had no need for ganged capacitors -- the "preselector" was tuned via a separate front panel control.
I looked at the tuning dials on my Hammarlund HQ-100 receiver. It is fairly linear in its tuning, but not as linear as the HT-37 or the Drake 2-B; on all of the tuning ranges the frequencies seem to spread out a bit at the lower end. My guess is that Hammarlund used the midline variable described above by Norm Johnson. The HQ-100 did use a ganged variable cap, with one section tuning the RF amplifier and the other tuning the local oscillator.
Monday, April 25, 2022
75/20 - 17/12 Two Homebrew Rigs in Scrap-Wood Boxes
Friday, February 25, 2022
Split Stator Temperature Compensation Also Used in Yaesu FT-101
Thursday, December 30, 2021
McCoy SSB Crystal Filters (1963) -- But Apparently NOT the Real (Lew) McCoy
Friday, December 10, 2021
The Lamakaan Annual Radio Convention Starts Today!
Here's a time zone converter: https://www.timeanddate.com/worldclock/converter.html?iso=20211211T043000&p1=505&p2=250&p3=137
Here is the Lamakaan Club's YouTube Live Channel. https://www.youtube.com/watch?v=cRznKwGgvDo
I don't know how we might be able to watch or listen via the QO100 satellite. The U.S. is not in the footprint of this bird. But there is a good WEBSDR receiver run by BATC and AMSAT DL: https://eshail.batc.org.uk/
Friday, September 24, 2021
Video: N2CQR Talks About Homebrew SSB with The Vienna Wireless Society's Makers Group
Monday, August 30, 2021
Adding Automatic Gain Control to the Termination Insensitive Amplifier
Friday, August 27, 2021
SolderSmoke Podcast #232 -- Mythbuster, Pete's Tube CW Rig, Pete's DC RX and Simple SSB Rig, NanoVNA and TinySA, Very FB Mailbag
Frank Jones and the FMLA -- Possible Victory?
IBEW Stickers: NASA, Johns Hopkins APL....
Cycle 25 Lookin Better Today: SFI 93 SN 47
Toobular! A Tube Transmitter
SR-160
Simple SSB rigs around the world!
KI7NSS's Pacific 40
The Mythbuster and the Struggle Against the Urban Legend
W2EWL's Cheap and Easy SSB
W4IMP's IMP. Articles in ER by Jim Musgrove K5BZH and Jim Hanlon W8KGI
The Spirit of Homebrew SSB. From Electric Radio K5BZH December 1991
Reduced Front End Gain on the DIGITIA
Back on 17! HP3SS sells HBR receiver to Joe Walsh
Maybe another Moxon?
NanoVNA -- Alan W2AEW helped solve mystery of why NanoVNA not providing accurate readout of circuit impedance. Over driving. Need attenuator.
TinySA -- Limited Resolution Bandwidth. But you can listen with it! See video on blog.
-- Google Feedburner to end e-mails from the blog :-(
-- Paul VK3HN -- TIA AGC? Farhan and Paul looking into options
-- Ciprian's Romanian Mighty Mite
-- Dino KL0S SolderSmoke GIF and graphical presentation on sideband inversion
-- Allison KB1GMX helped me on 24 volts to IRF 510 issue.
-- Dave K8WPE Wabi Sabi and Martha Stewart. And thanks for parts! 40673s!
-- Steve N8NM building a 17 meter rig with 22.1184 crystals in a SuperVXO and a 4 MHz filter.
-- Dean KK4DAS restoring an old Zenith. One hand behind your back OM.
-- Pete Eaton debating SSB or DSB for 17. Go DSB Pete!
-- Richard KN7FSZ a FB HBer. Asked about my solid-stating of Galaxy V VFO.
-- Walter KA4KXX on benefits of no-tune BP filters like Farhan's FB.
-- Jack 5B4APL on Time Crystals and Homebrewing in the 4th dimension. FB OM!
-- Moses K8TIY listens to the podcast with his young son Robert. Crank it in Robert!
-- Farhan and the SBitx on Hack-A-Day
-- Also Tom's receiver from junked satellite rig on Hack-A-Day
-- Todd K7TFC sent in beautiful message about the spirit of homebrewing. On the blog.
-- Grayson KJ7UM was on Ham Radio Workbench with George Zaf
-- AAron K5ATG running a uBitx with a homebrew tuner and antenna. Hope I can work him
-- Heard Mike WA3O last night on 40 DIGITIA. Water cooled amplifier
Tuesday, August 24, 2021
Mythbuster Video #17 Boxing it Up, Tuning Filters, Tapping a Heat Sink, QRO Dreams....
Saturday, August 21, 2021
W2EWL's "Cheap and Easy SSB" Rig -- And The LSB/USB Convention Myth
In March 1956 Tony Vitale published in QST an article about a "Cheap and Easy" SSB transmitter that he had built around the VFO in an ARC-5 Command Set transmitter. Vitale added a 9 MHz crystal-controlled oscillator, and around this built a simple phasing generator that produced SSB at 9 MHz. He then made excellent use of the ARC-5's stable 5 - 5.5 MHz VFO. His rig covered both 75 meters and 20 meters. Here is the article:
Because it used the 9 and 5 frequency scheme, over the years many, many hams have come to think that Vitale's rig is the source of the current "LSB below 10 MHz, USB above 10 MHz." This is wrong. An example of this error popped up on YouTube just this week (the video is otherwise excellent):
Tuesday, August 10, 2021
Mythbuster Video #16 -- GLOWING NUMERALS! In Juliano Blue!
I added two San Jian frequency counters to the front panel. In addition to making the rig a lot easier to operate, they add a classy touch of Juliano Blue to the project.
I got my counters here: https://www.ebay.com/itm/224223678132
There is a limitation of some of the the San Jian counters: https://soldersmoke.blogspot.com/2021/02/a-problem-with-san-jian-plj6-led-counter.html
But this limitation didn't cause any problem with this rig: In this case I just plugged in the IF frequency of 5.2397 MHz. I connected the input to my VFO running around 9 MHz. For 20 meter signals, I select the "up" option; the San JIan counter just adds the IF frequency to the VFO Frequency. For example 9 + 5.2397 = 14.2397 MHz . For 75, I select the "down" option. Here the San Jian just subtracts the IF frequency from the VFO frequency -- for example 9 - 5.2397 = 3.7603 MHz.
The band select switch operates relays that select the proper band-pass and low-pass filters. This switch also alternatively turns on either the 20 meter San Jian or the 75 meter San Jian.
Monday, August 9, 2021
Mythbuster Video #15 The Mythbuster Signal As Seen in the NA5B WebSDR
Sunday, August 8, 2021
Mythbuster Video #14 -- ON THE AIR!
On August 5 I put the Mythbuster on the air, making QSOs on both 75 and 20 meters. This video is from the following day -- I was on 75 meters. My first QSO with this rig was on 20 with S57DX. That was a good omen. TRGHS
I had no feedback or spur problems with the BITX40 module amplifier circuit. And I didn't release any IRF510 smoke. There is, of course, a lot more output on 75 than on 20. That's just the nature of the IRF510. I get about 4 watts out on 20 and about 9 watts out on 75 (with a 12 volt supply). At Pete's suggestion I might replace the IRF510 with a Mitsubishi RD06HHF1.
I had a couple of minor problems getting the transmitter going. I will describe these in a future video.