AmateurRadio.com

A couple of days ago I decided to take another look at the Spectrum Communications Off-Air Frequency Standard (OAFS). It had been suggested that its failure to work might be the result of a solder bridge or similar error. I had a good look at the board using a high power magnifier and found a pair of pads that were suspiciously linked to ground. A moment’s work with the desoldering braid and sure enough there was a solder bridge exactly the width of a PCB track.

Having found a fault I was optimistic that the board would work. The setup adjustments were completed OK. But instead of hearing BBC Radio 4 in the speaker as the instruction sheet suggested I received a loud heterodyne with some speech faintly in the background, like listening to an AM signal in SSB mode with the BFO a couple of kHz off-tune.

I was looking at the Spectrum Communications advert in Practical Wireless to check how the ferrite rod was mounted and noticed that the description said “Background heterodyne whistle at 2kHz confirms lock condition.” That is exactly what I was getting. Odd that the instructions didn’t mention it though. Nevertheless I gave a cheer and went ahead with installing the board in its box.

My happiness was short-lived when I put my frequency counter on the output. It was 10MHz sure enough, but it was not phase locked to anything. I was only receiving the output of the uncontrolled 10MHz crystal oscillator which could be tuned a few tens of Hz either side of 10.000MHz. No adjustment I could make would cause lock to occur.

Comments made to my original post about this suggested that I might have problems with the OAFS as I am not in a good location to receive a strong signal from BBC Radio 4. I’m unhappy with the amount of time I’ve wasted on this. I think it would be best to write it off and forget about it. I’d rather not be bugged by it or have it taking up scarce space in the G4ILO shack. If anyone would like to have it and see if they can make it work then it’s yours for the cost of the postage.

Reminiscing about my early days in ham radio, one of the things that really stands out is a gift my parents gave me 32 years ago — a Heathkit HW-8, an 80/40/20/15 meter QRP CW transceiver! It was an utter surprise to me; I never had the slightest inkling that it was coming. I was 12 years old and had never built anything like that before. How wonderfully mysterious all those parts looked as I pulled them out and set them on the dinner table!

Looking back on it now, I realize how patient my mother was to let me take over that table in the dining room. As I recall, I worked nonstop to build the little rig and its power supply. Ten days later, on January 3, 1980, it was finally ready. My dad took a look at it and said it was ready for the “smoke test.” You can imagine how I held my breath as we plugged it in and turned it on. I was waiting for something on the circuit board to go up in a puff of smoke! Nothing exploded, so I was ready to take it into the shack and hook it up to an antenna and straight key. “Ready” is an understatement — I was so excited to get that rig on the air I was nearly bursting at the seams!

I picked up the phone and called Dr. Bernard “Bernie” Northrup, KAØDKN, a friend of mine across town, to see if he would get on the air and give me a signal report. Dr. Northrup (later NØCIE, now a silent key) was a professor at Central Baptist Theological Seminary of Minneapolis and a fellow member at Fourth Baptist Church, Minneapolis. Not long before, he had gotten his license after hearing me talk incessantly about ham radio at church (I’m afraid back then I was more interested in ham radio than spiritual things.). Anyhow, I called him (around suppertime, I see by my log!) and he graciously agreed to get on the air.

And sure enough, my HW-8 worked! After a half hour with Dr. Northrup on 15 meters I was ready for my first “real” QSO, as I thought of it. Tuning around the band, I heard ZL4KI. My heart started thumping as I prepared to call him. Could he really hear me even though I was sending with no more power than that of a small flashlight? My hand was shaking as I tapped out ZL4KI ZL4KI ZL4KI DE NØART NØART NØART KN and waited, flushed with excitement. I could hardly believe it when I heard my callsign as he came back to me! To think that the signal from this little radio, built with my own hands, was being heard 8,700 miles away in Invercargill, New Zealand! Amazing!

Other radios have come and gone, but that trusty ol’ HW-8 is still with me. As a boy I brought it with me to church camp and set it up in the lodge, tapping out CW while the other boys played games. Once on a trip to Louisville, KY I set it up on the second floor of my grandparents’ house — with a TV-twin-lead dipole my father had built — and worked a station in Poland. When I moved into my first apartment as a newlywed, I set it up with that same dipole in my (below-grade!) apartment. On a couple of memorable, crisp, autumn days, I brought it to a local park with a thermos of hot cocoa, sat down on a carpet of pine needles, and thrilled to the sound of soft static and CW.

And last summer, when I just couldn’t wait until I got my shack set up at my new QTH, I set it up on the picnic table in my backyard with an OCF dipole tossed into the trees. Even though that antenna was so low its feedpoint rested on the picnic table, I still worked both coasts on 20m with my trusty ol’ Heathkit HW-8! What a great little rig. Thanks, Mom and Dad, for giving me such a great gift!

(Click here to view the HW-8 Manual)

After building the “Accurate LC Meter Kit” from Electronics-DIY.com, I turned to their “1Hz – 2MHz XR2206 Function Generator Kit”. All parts necessary to complete the kit were included, though not exactly as pictured on their webpage — two of the WIMA capacitors had been replaced with substitutes and there was no IC socket. All components were through-hole; soldering the kit together went quickly and easily.

If you build one of these kits you’ll need to provide your own power source as well as your own pin-connectors (if you choose to use the pins provided). As with the LC Meter, I used a size M coaxial DC power jack to accept a plug from one of the wall-wart power supplies I have around here. I didn’t bother to install a power switch in either unit since I won’t be using them very often; I won’t leave them plugged in between uses.

The fellow at the local Radio Shack gave me some pin-connectors for free, clipping them off of some battery packs that were in a box for recycling, though he only had two-pin connectors. Since one of the pin-sets has three pins, I just soldered a piece of hookup-wire to the third pin. If I had to do it all over again, I wouldn’t bother with these pins — I’d just solder hookup wire right to the PCB. By the way, if you ever try soldering to a pin make sure you clip a heat-sink to the pin before heating it up. The plastic base of those pins melts pretty quickly!

I chose a plastic project box from Radio Shack to house this function generator. Using a Dremel tool with an engraving cutter (at the lowest speed — 5,000 RPM), I put three notches in one side of the box for the potentiometers, a notch on one end for the two switches, and ground down all four stanchions on the floor of the box since otherwise the potentiometers would have extended too high to allow the lid to fit. That Dremel tool sure is handy! A few knobs from Radio Shack finished off the project.

The two outboard switches allow you to select between three waveforms — sine, triangle, and square. I don’t have an oscilloscope so I can’t tell you how the waveforms look, but I can at least tell you that the sine wave sounded pure when I hooked my headphones up to the output with a matching pad. I am pleased to report that the signal generated by this function generator is very stable. Four DIP switches on the PCB allow you to select between four frequency-ranges, and two potentiometers allow you to tune within the selected range. One of these two potentiometers provides coarse tuning, and the other provides fine tuning. The third potentiometer controls the amplitude of the signal generated (note: amplitude decreases as you turn this potentiometer clockwise).

If you build this kit you’ll want to hook it up to a frequency counter. Two pads on the PCB are provided for this purpose. I have a piece of coax hanging out of the back of the box for connection to my own frequency counter — not that you have to use coax, but it was handy for terminating with a BNC connector. (If I were really classy I would have put this coax through its own hole in the project box, but hey, this is a piece of test equipment — I just ran it through the big hole I made for the RCA connector.) When I hooked up my frequency counter I noticed that the published ranges for each DIP switch were just rough approximations, but I was pleased to see that this frequency generator covered the entire published range and more — up to about 2.4 MHz, if I recall correctly.

Here is a slideshow of photographs I took of the completed function generator:

Click to view slideshow.

The source impedance of the generator is 600 ohms and the output is intended to be terminated in a 600-ohm load. In my next post, I hope to discuss the construction of a minimum-loss matching pad to hook it up to a piece of equipment that has a different input impedance.

Update (3/7/12): Yesterday I prompted [email protected] for a reply, mentioning the number of pageviews this post has received. I received a prompt and polite response. I learned that I was mistaken in expecting the meter to read capacitors 1 uF or higher, since the published range of the meter is only 0.1pF-900nF. There was no explanation of why I am having problems with inductors that are within the published range of the meter. However, I was quite favorably impressed by an offer to test and fix the kit at no extra charge! I shall take them up on this offer and keep you updated.

Update (2/8/12): I am having trouble with this LC Meter. It gives me the same reading for all capacitors 1 uF or higher, and the same reading for all inductors higher than about 70 mH (this last value is just a guess): 838.8 nF and 83.88 mH, respectively. As you can see the digits are the same. It seems to work for really small capacitors and inductors, but anything bigger and these are the only readings I get. I emailed [email protected] on 1/8/12 about this, but as of 2/8/12 I have received no reply. Unless and until I learn the problem is due to some error of my own in constructing this kit, I recommend against purchasing it.

Yesterday evening I finished building the “Special Edition Accurate LC Meter Kit with Blue Backlight LCD”, available from Electronics-DIY.com for $69.95. I have no experience with such devices; a more experienced fellow told me he was impressed by its specifications, so I ordered the kit. Soldering it up was a snap. The main printed circuit-board is all through-hole construction, and the LCD-board that mounts over the top of it requires nothing but a connector.

If you want to build one of these you may want to order this version of the kit instead of the one I purchased: Accurate LC Meter Kit with Green Backlight LCD, for $59.95. My kit’s “Blue Backlight LCD” turned out to be green anyway, and I think the two kits have the same circuit, save an adjustable potentiometer on mine that controls the contrast of the LCD (which I just set to maximum anyway). Certainly the cheap case that comes with the kit I ordered is not worth the extra $10 — to use it you have to carve out a bunch of stuff (to make room for the circuit-boards), including two of the four stanchions that attach to the lid. After going to all that trouble (I used a Dremel tool) you are left with a case that requires adhesive tape to hold down one side of the lid!

The instructions that came with the kit were pretty sketchy, mostly limited to how you need to carve up the case (by the way, the measurements were wrong, so ignore them). The only thing that got me into trouble was the voltage regulator, which gets in the way of the LCD-board (and protrudes too high to seat the lid of the supplied case) if you solder it in the way you normally would (which I did!). By bending the voltage regulator out at angle I managed to get the LCD-board mounted, but the lid still won’t seat properly. Learn from my mistake, and bend the leads of the voltage regulator into a Z so that they lay flat on the board and allow the voltage regulator to sit just off the edge of the board. (Of course, this only matters if you try to use the case provided.)

You’ll need to supply your own power to this unit. There isn’t enough room in the case for a 9V battery, so I purchased a DC socket. You’ll also need to supply your own connectors for testing inductors and capacitors; the photograph on the Electronics-DIY.com website shows them in the case, but they aren’t supplied. I used banana-plug sockets. You’ll also need to supply your own pin-connectors if you use the supplied pins on the circuit-board, and you’ll need your own stand-offs if you want to support the LCD-board (only two of the four screw-holes match up with the lower PCB, but that’s probably good enough).

There is no way to select the units displayed on the screen, e.g. pF vs. nF. But the dearth of selector switches is actually one of the nice things about this unit. There is no need to select a range of capacitances or inductances. The only thing you have to do is plug it in, hit the reset button whenever you want to calibrate it, and stick in a capacitor to get a reading. If you want to test an inductor, you simply press one button to select inductance-mode, then attach your inductor. It just works — and it works with precision.

Here is a slideshow of some snapshots that I took with my cell-phone. They didn’t turn out very well, but they’re good enough to give you an idea of what it looks like. Notice that I used black electrical tape to mask the edges around the LCD. That’s because the opening I made was downright ugly. Next time I’ll try using a cutting wheel on my Dremel tool instead of a grinding tip!

Click to view slideshow.