AmateurRadio.com

I’ve picked up a couple of Mirage “brick” amplifiers over the past few years.   I’ve also rewired the DC cable on each and every one.  Here’s the latest.  AWG 10 to AWG 16 transition?  Seriously?  I know it technically doesn’t matter too much for a short run, but these amplifiers suck down a lot of current and I just don’t see cutting corners on that.  Maybe I’m a purist…

I do not spend a lot of time on VHF/UHF FM and have not for many years.  My first radio in 1993 was an Alinco DJ-580T handheld and I’ve thought at various times that a new handheld might suit me well, especially for receiving.  A few weeks ago, as I tend to, I came across a Yaesu VX-3R offered “for parts or repair” on attractive terms and so acquired it—my second handheld ever.  The problem seemed straightforward enough:  full TX power, but no RX on amateur bands.  RX on FM broadcast OK.  The seller assured me, after I had agreed to purchase based on his description, that “somebody” had told him that this problem occurs when “only one component is bad.”

Based on the seller’s description of the problem and a thorough reading of the Technical Supplement, I developed a short list of candidate failure modes, components, and sources.  Fortunately, all of the components could be sourced easily from the usual sources.

When the radio arrived, I gave it a functional check and it indeed exhibited the problem that the seller had advertised.  I quickly popped it open and noticed a small red sticker in the lower right-hand (when facing the device like you would operate it) corner of the Main Unit (Side A, per the Technical Supplement’s notation).  A neatly hand-drawn arrow pointed to component Q1025, which upon closer inspection, was clearly damaged.

So, I quickly set about identifying the component and procuring a replacement.  It was a NJU7007F3 operational amplifier.  Huh…it did not contribute to any of the failure modes I had initially suspected.  However, a careful reading of the Technical Supplement indicated that this op amp drives varactor diodes in a tracking preselector—so, if it does not work, the radio will exhibit varying degrees of deafness in the amateur bands.  I found out later that the “somebody” was actually a Yaesu technician who had seen the radio for repair at the behest of a previous owner and left the sticker for me.  At any rate, this component clearly would have to be replaced if I were to fix the radio, so I set about looking for a source.

Mouser listed it in their catalog but wanted me to buy a reel of 3000.  No thanks.  At least they had it listed.  So, I did the next most logical thing—I made a list of other parts that I needed and called Yaesu.  The part was back-ordered to Japan for 4-6 weeks, but only cost 0.42 USD.  I bought three.

After however many weeks it has been, a package from Yaesu showed up on my doorstep tonight.  After repairing a damaged PCB trace (non trivial on something this small), I was able to replace it.  The little black speck in the middle of this photograph is the removed component.  For my non-US readers, the US 0.01 USD coin (“Penny”) is about 19 mm in diameter.

The radio fired right up and received NOAA/NWS right away.  The entire repair once I had the parts was about 30 minutes.  There are still two unbuilt SoftRock kits, an IC-290A with an unlocking PLL, and W1GHZ transverters for 903 and 1296 to be worked on…maybe tomorrow…

After about six years of sporadic effort, I finally finished the W3NQN filter set by building the 10- and 15-meter filters over the past two weekends.  Now I can operate SO2R with impunity on all bands.  Several band combinations are still problematic, but those are problems that filters can’t solve.

Because the geometry of our home does not permit me to have a walk-behind station desk, I had to attach the filters to the mounting board by laying on my back with a right-angle drill.  That sort of thing is always a bit exciting!

Ever since I replaced my primary station computer (a decision that may be reversed soon—details in a later post), I desired to replace the remaining 19-inch Dell Trinitron CRT monitor with something lighter and smaller.  Mom and Dad were in town a few weeks ago on a much-needed vacation and we went to the National Air and Space Museum’s Udvar-Hazy facility near Dulles Airport.  This is only a few miles from one of the best used computer shops in the DC area—CedarPC.

CedarPC is nice because they’ll sell you “damaged” stuff at a discount if you don’t care about the damage.  I inquired about a 24-inch flat panel I had seen on the web site, but they could not find it.  They did find me a nice 20-inch flat panel that was just missing the stand and the price was right.  The missing stand was no big deal because I wanted to mount the monitor on an arm so I could bring it closer to the HF end of the station desk, tuck it in at the VHF end, or even swivel it out over the couch to watch a DVD.  Sold.

Monitor arms are generally expensive…at least 2-3 times what I paid for the monitor itself, often more.  So, I went to trusty eBay and found something designed for mounting televisions for $15 including shipping.  This did require some modification of the monitor housing and liberal application of wide washers to reinforce the plastic in the housing.  But, it was done with all junkbox screws and washers.

Last night, as I have been doing lately in both the evenings and mornings, I was trawling the bands with a SoftRock (a dual-band v6.0 built for 40 and 80 meters—my first SoftRock) and Rocky.  Without fail, the waterfall enables me to see something interesting, which brings me to the point that I really need to integrate these receivers into my station.  But, I digress.  The really tantalizing, fascinating signals are the weak ones, especially when they are weak and unusual.

The subject weak and unusual signal is shown in the figure headlining this post.  Rocky’s cursor shows the approximate bandwidth of the CW filter (250 Hz, if I recall correctly) and the center frequency (7026.25 MHz).  I tuned it in and caught a CQ from “AA1T…”  Recalling that I head read about Mike, AA1TJ‘s, Das DereLicht transmitter built mostly from parts scavenged from a dead CFL, I suspected it was him.  After a moment the QSB came up and I caught the ‘J.’  Thanks to the fact that the SDR is not fully integrated into the station, I scrambled to plug my 15-year-old Small Wonder SW-40 in and get it online.  Mid-scramble, Sarah handed Evan off to me for a diaper change, but I did manage to snap the screenshot above.  By the time I returned to the shack awhile later to close things down, AA1TJ had disappeared.

Through the (ubiquitous) magic of the Internet, I sent Mike an apologetic e-mail QSL, which yielded a nice response.  It turns out that he was trying a 40-meter version of the first transistor transmitter described in the amateur literature.  The design for 146 MHz by K2AH appears in March 1953 QST.  If you are an ARRL member, pull up those old QSTs in the archives and read about it.  K2AH is on the cover of February 1953 QST with the same transmitter.  Mike figures he was making 20 mW, or 20650 miles/Watt on the path from his station to mine.  The more remarkable thing is that he was using a 1956-era point-contact transistor (the same type as in the K2AH article).  Fascinating stuff!  Hopefully, we’ll connect for a real over-the-air QSO sometime soon.

Thanks, Mike, for making my day by doing something interesting and sharing it on the air!

Over the past week in the evenings, I’ve managed to cobble a little PA together for the 50-MHz transverter.  It’s a near-clone of the PA in the Elecraft XV50 using the Mitsubishi RA30H0608M.  Last night, I got it all hooked up and installed in the cabinet.  It broke into oscillation when keyed on CW.

The oscillation was about 50.040 MHz.  I reasoned that it might be the PA output coupling into the nearby TX RF bandpass filters which are followed by 12 dB of gain before returning to the PA.  So, tonight, I added a litle shield between the BPF and the PA board.  That seemed to clear things up and I got about 10 watts out.  There is a 6-dB pad between the last driver stage and the PA, so I should be able to get it up to about 18-20 watts.  But, the first goal will be to check the linearity on SSB.

In other transmit capability news, some boxes and heatsinks arrived for the W6PQL PAs for 903 and 1296 this afternoon.

As I mentioned previously, the receive capability in my 50-MHz transverter seemed to be a little bit anemic.  So, I decided to add a little bit of gain.  Looking at the block diagram for the transverter (shown below in updated form—jumper across the SGA-4586Z to understand what the circuit looked like initially—by the way, full-sized PDF versions of all of my notes and schematics will eventually be available here), I reasoned that I had the following losses and gains:  +10 dB for the LNA (this might be as much as +15 dB, but 10 seems more realistic, if not generous), -3 dB for the bandpass filter, -7 dB for the mixer conversion loss, and -3 dB in the diplexer, leaving me with -3 dB overall conversion gain.

So, I endeavored to find some more gain in the form of a MMIC (which is really what all gain blocks in this transverter would be if I were to do it again).  I have a nice (but dwindling) supply of SGA-4586Z’s that produce about 20 dB gain with a 1-dB compression point around 16 dBm, which is probably a good part for this location.  The idea is to overcome the losses in following stages with robust gain in earlier stages (in order to keep system NF low).  So, I ripped out the existing two-pole BPF and replaced it with this:

I was relatively satisfied that I had everything working with the cover of the transverter off to peak the BPF on the W3APL beacon.  So, I replaced the cover and surprise!  To borrow a poetic description of RF circuit doom from N3UM, the MMIC “amplifier burst into song.”  Bursting into song is a bit of a charitable description for something that sounded more  like S9 power line noise in the IF receiver.  I did not actually know that it was the MMIC oscillating at first, but I started wiggling cables and finally found that if I touched the 12RX (+13.8 volts on RX) line, the problem disappeared.  So, I added the 0.01-uF bypass capacitor to ground on the supply side of 180-ohm bias resistor and the problem was solved.

With the completion of the HF SoftRock with Si570 LO, I now have a tunable IF to play with the transverter.  We had a nice opening to the Southeast and Gulf this morning and I even heard my first DX with the transverter and SoftRock combination—CO2WF.  With the appropriate software, I can configure a the SoftRock as a panadapter with the TS-930S as the transmitter.  More on this in the future.  Best of friends:

The real next step in making the transverter useful is building a 20-watt PA stage.  This should be good for driving a Mirage or TE Systems brick or even something bigger like a 3CX800 or 50-volt solid-state amp…