A Pretty Good Troubleshoot -- Fixing the Transmitter in my 75/20 meter Mythbuster Transceiver -- Mind the Gap!

Bidirectional Termination Insensitive Amplifier by W7ZOI and K3NHI

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? 

Mind the Gap 

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.  

Out with the old...

Modular Magic from AK2B

http://www.youtube.com/watch?v=mrDYEbRGEds

This video really made me yearn for my shack and soldering iron (all my gear is still on the high seas). OM AK2B is doing amazing modular things in an apartment in NYC. Richard Fenynman would be proud! I was pleased to see the circuits and kits of so many FB radio amateurs (including KD1JV, W7ZOI, and KA7EXM) in this rig. Thanks to Jonathan-san, KC7FYS, for sending this video to me.

Some Interesting QSLs from W7ZOI

Wes, W7ZOI, just put up on the web some QSLs from his collection. They are all very interesting. Take a look: http://w7zoi.net/qslcards/qsls.html I especially liked the "lemon QSO" card shown above.

While visiting the QSL page, I also came across the story of Wes's first encounter with homebrew ham radio. As a kid, he he would listen to a local 75 meter AM operator who was a hardcore homebrewer. On this guy's rig, "even the knobs were homebrew." Wes was listening with a crystal radio! What a great way to get started. More on this here: http://w7zoi.net/shack.html
Thanks Wes!

Links from #111: W7ZOI, VU3ICQ, AA1TJ

Here is the excellent discussion of bi-directional amps I mentioned. From the web site of W7ZOI:
http://w7zoi.net/Bidirectional%20Matched%20Amplifier.pdf

Here is Farhan's new JBOT 5 watt final amplifier:
http://www.phonestack.com/farhan/jbot.html

Here is the mysterious (and perhaps diabolical) Xtaflex from AA1TJ (the first rig EVER to promise "buns of steel.")
http://mjrainey.googlepages.com/xtaflex

K1JT Spots My DSB QRPp WSPR Signal

2009-05-03 16:42	N2CQR		-29					K1JT		7035	4371
2009-05-03 16:38	N2CQR		-24					K1JT		7035	4371

I got into WSPR because I wanted the gratification that comes from seeing a map readout indicating that my QRP signal has crossed various oceans. Well, yesterday I crossed the pond for a second time, but this time the receiving station heard the signal more than once, so the rx station callsign appeared on the display (if the signal is heard only once, you get the line showing the path, but NOT the rx station call). And what a fine callsign it was: K1JT, Joe Taylor, Nobel Prize winner and the inventor of WSPR.

In the last SolderSmoke I got Joe's name wrong (I called him John). It must have been too early in the morning here. Anyway, I was gently corrected by Wes, W7ZOI, and I promissed to make ammends. Sorry Joe! (Thanks Wes).

Gil Cartoon of W7ZOI on the Mountaintop

Steve, WA0PWK, sent this one in. He found it in an old issue of "Hints and Kinks." Note the callsign on that FB HB portable rig. Wes tells me that this cartoon was actually a Doug DeMaw initiative. Doug took an older cartoon by Gil, W1CJD, and put "W7ZOI" on the rig. The drawing appeared next to a short article by Wes about an electronic keyer using a pair of 741 Op Amps. Wes also reports that his kids got a real kick out of it -- his son Ron was taking an elective printing course in Jr. High School at the time, and printed up a bunch of W7ZOI QSL cards featuring the modified Gil cartoon.

I thought that Steve, N0TU (now WG0AT !), would find this especially interesting.

Calibrating my W7ZOI/W7PUA Power Meter

In the course of testing my 20 meter DSB rig, I discovered that the chart I prepared for my

W7ZOI / W7PUA power meter had some problems. In my first effort to get some calibration points, I used a simple diode and cap peak reading signal probe to measure voltage across 50 ohms, but I may have neglected to convert from peak to RMS before calculating the power. That would certainly mess up the chart!

This morning I used my old Heathkit SG-8 signal generator and my HAMEG (20 Mhz) 'scope to get a couple of points for my graph of the response curve (line) for my meter.

I drew the new line, then used my trusty HW-8 at 1 watt to see if my chart was accurate. It was right on the money!

I look forward to happily measuring milliwatts with this very FB piece of test gear. See the June 2001 issue of QST for more details. Thanks to Wes and Bob for the great project. And thanks to Thomas in Norway for sending me the AD8307 chip.