Saturday, March 23, 2013

Vanguard I transmitter

Some time ago, a group of hams (including me) celebrated an anniversary of Sputnik I by building transmitters of a similar design and power level.  Now it's time to try the first satellite borne transmitter of the USA carried within its Vanguard I spacecraft.  This time we use a single transistor instead of a single tube.  Specifically, a germanium transistor instead of silicon.  The original was on 108 MHz, I think, but that's a bit challenging and that band isn't available to hams. We're using the 20 meter (14 MHz) band.  For authenticity, we try to hold to a single transistor, germanium, with output as a result being in the low milliwatt range.

I came a little late to the party this time because I couldn't find a suitable transistor, but eventually was pointed to some on eBay that were from the old Soviet Union.  Some kind of irony going on here, I guess.  Peaceful co-existence, anyone?

Again, I'm following the trail blazed by Mike, AA1TJ who had the original schematic and made his circuit close to it.  I was able to duplicate it pretty well, but my oscillation was slow to start, so keying would be impossible.  Impatient, I switch to a circuit from Experimental Methods in RF Design.  Here's my circuit:


I measured about 30 mW out from the transmitter, which was pretty much my goal.  I haven't yet made my first QSO, but was pleased and a bit amazed to hear my signal (from Arkansas) reported on the Reverse Beacon Network in WVA, VA, NC, OH, and PA after just a few CQs, typically 8 to 10 dB above the noise.  Update:  Just had a partial QSO with WD4HHN in Florida, so things are moving.  I'm not one of those QRP operators of great faith, so I have to admit I'm amazed.  It's fun writing in the log, Power: 0.03 W.

Not wanting to have to pound on a hand key, I also made an interface circuit to my electronic keyer using a 2N4403 PNP transistor switching the positive lead from the battery.

The transmitter went together in an hour or so on a solderless prototype board, photo below:

The silver cap thing is the transistor.  The crystal is wrapped in red tape to keep its case from shorting out any adjacent wires.

Here's a page on Vanguard activities maintained by Oleg Borodin, RV3GM.

http://www.club72.su/vanguard.html

And here's Michael Rainey's blog entry on his Vanguard project from June, 2012:

http://aa1tj.blogspot.com/2012_06_01_archive.html

72-

Nick, WA5BDU

Thursday, February 28, 2013

Antenna: Mini-loop for 20 meters

I've heard so much about small transmitting loops that I needed to eventually try one even if I have no need for a compact antenna.  I first tried it a number of years back.  I thought I'd try 40 meters since that would be appropriately challenging.  Actually, it was overly challenging and I never got it to work for transmitting, as heating of the capacitors from RF current would change the resonant frequency and drive the SWR through the roof, even at 5W.  My problem was using compromise capacitors instead of butterfly, trombone, or other high quality components.

So I'm trying again, this time on 20 meters.  My loop is made from a 10 foot piece of 1/2 inch soft copper tubing, shaped more or less into a circle of about 38 inches diameter.  It's 0.14 wavelengths long on 20 meters. This kind of antenna is fairly simple technically -- the big loop forms an inductor and a capacitor selected to resonate the inductance at the operating frequency is connected across the open gap where the ends (almost) meet.  A slight bit of technical complexity occurs in coupling the radio to the loop, but it's simple in practice.

There's lots of software to help with the design.  I used RJELOOP1.EXE by Reg Edwards, G4FGQ.  You input the loop dimensions and it gives you the capacitance required, radiation efficiency, loop current and capacitor peak voltage for a given power level, etc.  Mine shows that it will be about 52% efficient, not too bad for a small antenna.  It will have 1800 peak volts across the capacitor while operating at 5 watts(!), And the loop current will be 5.6 A.  Also, I need about 44 pF capacitance for 20 meters.

The Capacitor:

It's a known fact that you don't want your variable capacitor to have contact points such as bushings, bearings or sliding contacts that the large loop current must pass through.  But I read G4ILO's loop antenna page* as part of my research and he claimed that a good quality variable of conventional design and in good condition would work fine.  So I picked a nice one from my junk box and decided to measure the resistance across the bearings before proceeding.  I hooked up a power supply and limiting resistor to shaft and frame and passed 0.5 A DC though it. I used my DMM to measure the voltage drop across the bearings, while rotating the blades to look for bad spots.  Voltage drop was in the low millivolt range and my contact resistance was about 2.5  worst case and 1  typical.  I'm good to go!

The Other Capacitor:

My capacitor is about a 10 pF to 60 pF unit.  I mounted it in the Plexiglass plate that secures the ends of the tubing and stuck on a knob.  In initial testing, I found that I couldn't zero in on a frequency with this system.  Now the resonant frequency is 14.200 MHz, I make a small adjustment and now it's 13.900 MHz.  Playing with my RJELOOP1 program some more I see that 0.165 pF change in capacitance moves the resonant frequency by 25 kHz.  Too sensitive!  One option would be to have a 10:1 reduction drive on the capacitor.  Another would be to have a large fixed capacitor and a small value variable in parallel.  The third is just to parallel a smaller capacitor with the one I have now.  I chose the third method.

I didn't have any suitable capacitors small enough, so I decided to use a 5.5 pF to 23 pF variable and put a 2 pF fixed silver mica in series with it.  A spreadsheet showed me that this arrangement would give me 0.32 pF change for the first 30 degrees of travel from fully un-meshed, but only 0.025 pF change from 150 degrees to 180 degrees (fully meshed).  So an initial setting of 50% or so gives me a good tuning range and slow tuning.


Main & fine tuning capacitors on Plexiglass plate

OK, it's ugly but remember this is an experiment!  The big one with the knob going off to the right is the fine tuning capacitor and the smaller one with its shaft going through the Plexiglass is the main unit.  If you knew where to look, you might see the 2 pF silver mica in series with the fine tuning capacitor.

The Coupling Loop:

Literature on the subject says you can use a wire loop or a ferrite toroid threaded onto the main loop as a one-turn secondary with an appropriate number of turns on the radio side.  I'd already tried the wire loop, so I threaded a FT-114-43 toroid onto the loop. RJELOOP1 says the primary should have an impedance of two to three times that of the feeder (50 ohm coax).  So I used two turns on the radio side.

This didn't work well at all.  My null was over a 2:1 SWR and looked weird -- it came back up slowly when I tuned to the low side, instead of the sharp high-low-high null I expected.

So I went back to the wire loop.  It's just that - a loop of wire connected across the coax feeder. The loop is positioned just inside the main loop, diametrically opposite the capacitor. Nothing connects to the  main loop here, so I used a piece of insulating board secured to the main loop to support the small loop and to mount a coax receptacle on.

The sources I read said a circumference of 1/5th that of the main loop would be good, so I used 24 inches of solid #12 AWG THWN.  I had a hard time getting a good match.  Rotating the loop out of the plane of the main loop is supposed to be one tuning method, but it made things worse.  Not sure what else to try, so I tried squashing the small loop down into a football shape and got some improvement -- down to 1.5:1 at resonance.

My RJELOOP1 program had given a value of 19 inches for the small loop, so I took off five inches.  That made things even worse.  I made a new loop, this time going UP to 26 inches wire length and was able at last to get 1.1:1 at resonance.  I've read that the size of this loop isn't too critical, but I guess I have to say that's not my experience.

Now everything is ready ...


Matching loop, 26 inch circumference


Hanging from a fiberglass pole
off the upper deck of my house

Trying it out:

I made my first declaration of success before I even managed a QSO.  I was able to adjust it to the desired frequency and it stayed there over the course of a couple hours as I transmitted and was idle and it hung out in the sun and cold February wind.  Eventually I did manage a QSO with another QRPer out in New Mexico.  He was weak here and I was weak there but we managed a 20 minute QSO discussing small loops and EFHWs.

I measured my 2:1 SWR bandwidth at 22 kHz, which looks adequate to me.

What next?

If I want to actually use this antenna, I should get back to one capacitor and put on a reduction drive.  Also it would be good if the capacitor were in a weatherproof box.  As-is, I need to keep this antenna out of the weather.  A way to do remote tuning would be good -- maybe use a screwdriver type drive.  Manually adjustable would be OK, but for a big project, servo controlled nulling would be really nice.

I also need to see if it will play on 18 MHz and 21 MHz.  My capacitor should have enough range, but I'm not taking it for granted.

73-

Nick, WA5BDU 
Russellville, AR
February 28, 2013

* http://www.g4ilo.com/wonder-loop.html

Sunday, December 2, 2012

Battery monitor & discharge tester


I like to use little AGM lead-acid batteries on the workbench and in the field with my QRP stuff.  The KX3 transceiver likes to have voltage up over 13V if you want full power (10W), so I bought one of those amazing little DC-DC boosters from eBay.  Using it though, I could be unaware of how low my battery's terminal voltage has gotten.  Taking it lower than about 1.8VPC (volts per cell) could be harmful to the battery, so that led to this project - a battery monitor.

The heart of the thing is an ATmel ATtiny85 8-pin MCU.  I've been moving from PICs to AVRs lately, and from assembler to C.  I originally used the ATtiny13 and did get a functioning version in its 1024 byte program memory space, but I wanted more features so I went to the '85 and the luxury of a full 8K of memory.

To keep things simple, the user interface consists of just a speaker, and LED, and a push button.  the MCU monitors battery voltage via ADC and issues a warning beep at 11.0 volts and a "tripped" beep at 10.8 volts.  A logic line output changes states when tripped and can be used to stop the battery discharge via a relay, MOSFET or transistor.  If you just want an alarm, the output line is not used.

A brief press of the switch in normal operation will cause the speaker to beep out the current voltage.  This can be in Morse, or for those who aren't Morse literate, it will give one beep for "1", two for "2" and so on, with a long beep indicating "0".  The readout is to the hundredth digit.

If you initiate a voltage report and then re-close the switch and hold until it's finished, the MCU will swap between Morse and "count beeps" reporting methods.

Holding the switch closed for > 2s while in normal operation will toggle the sense of the output line between HIGH and LOW for the non-tripped condition.  All configuration changes are saved in EEPROM and  therefore survive a shutdown and re-start.

The MCU is programmed for a trip setpoint of 10.8V and an alarm setpoint 0.2V higher.  This is also programmable:  Connect it to a variable voltage source and hold the switch while powering on the board. The voltage at the time the switch is released becomes the new trip setpoint.

The LED flashes briefly about once per second in the untripped state and changes to two brief flashes every 2.5 seconds after it trips.  I wanted to avoid the dreaded "car alarm syndrome" of continuous audible alarms driving me crazy.  The board gives two 0.5 second raspy sounds at the warning setpoint and one two second raspy sound when it trips.  Then it goes quiet.  Check the LED to see if it's tripped.

Another function is use of the board to supervise a discharge test.  After it reaches the trip setpoint and stops the discharge, it reports the discharge time in minutes. Knowing the discharge current, you can calculate the amp-hour capacity.  Additional presses of the switch while in the tripped state cause it to repeat the discharge time and the battery voltage.

So, a fair amount of function in a tiny chip & board.  One drawback is that after it trips, it's still pulling current from the battery.  But that's only about 2mA.

I tested my trusty 4 A-hr AGM and found that it had less than 1 A-hr capacity, so it went into the recycle bin.










Friday, October 7, 2011

Sputnik transmitter

For my latest, I've put aside my AVR programming and a few other projects to participate in an activity commemorating the October 4, 1957 launch of Sputnik 1, and with it, the space age. It's the brainchild of the master of archaic radio technology Mike Rainey AA1TJ, whose original post is here:

http://aa1tj.blogspot.com/2011/07/sputnik-qso-party-transmitter-prototype.html

Mike supplied me with the the two Russian "pencil tubes", a 21.060MHz crystal and a vague instruction to do something with them. I reproduced the transmitter shown in the schematic of the above link to the best of my ability.  Initially, I had trouble getting oscillation, so I reduced the size of the "lower" feedback capacitor to get it to go.  Eventually I got all the way down to 10pF, tormenting the crystal all the way up to 21.065MHz.  I had problems with low output, sometimes less than 100mW and was suspicious of my plate RF chokes so eventually went to a 1k resistor on the oscillator plate and a resonant tank on the amplifier.  That put me up to 200mW.


Here's a picture of it.  The tubes have wire leads which I soldered to a terminal strip. For keying, I did what Michael suggested and keyed the PA plate while leaving the oscillator running.  The little perf board has a TTL reed relay for keying.  By this time I'd already made my power cable for B+ and filament and realized I needed 5V for the relay.  Rather than rebuild that cable, I added the 9V battery and a 78L05.  A little foresight could have been helpful.

On the air!


I got it ready just in time for the October 4th start date.  I was fairly amazed to make four QSOs on a Tuesday afternoon with just 200mW out. They were:

N2JJ in NY
W1PID in NH
W5RZ 10 miles up the road
WB8YYY in MD

N2JJ recorded my simulated 54 year old transmitter on his Droid phone and sent me the audio file!
W5RZ wished me "DSW".
WB8YYY gave me a 238 RST, one of the strangest I've had.

Inspired by this project, I've done a lot of reading on Sputnik.  I remember very well when it was announced on the news in 1957.  We ran out and looked at the sky.

Some of the ideas I and maybe others had weren't really correct.  First, while the exact time of the launch wasn't known (the Soviets were pretty secretive), it was generally known in the US that the Soviets were very close to making their attempt. The US was also pretty close to launching, with an intended satellite launch planned during the IGY (International Geophysical Year) of 1957. It actually succeeded in January 1958.  But the possibly underestimated reaction of the public and media to Sputnik expedited the program a great deal.

Sputnik transmitted on 20 MHz, allowing ham operators and shortwave listeners worldwide to hear its beeps. Michael chose the 21 MHz ham band for our project to get reasonably close. The American satellites transmitted above 100MHz, which required somewhat more exotic receiving equipment to hear.

Friday, August 19, 2011

Barometer - Altimeter


For some reason I've thought for a while that it would be fun to build (or even buy) a barometer.  A friend had this cool military surplus altimeter he put on the seat of his car while driving through the mountains and you could see the elevation go up and down. That was pretty cool.  Barometric pressure is interesting too.  Can you predict a storm by seeing a rapid drop?

So I finally ran across this chip, or sensor on a chip, by Freescale, the MP3H6115A pressure sensor chip. It cost $9.15 from Mouser. The chip is SMT, 8-pin SSOP package.  It has 0.05” pin spacing, but the row-to-row width is wider than SOIC packages, so a different adapter or board pattern is needed.
It has an output of 15 to 115kPa or 2.2 to 16.7 psia. Supply voltage is 2.7 to 3.3 VDC, typically 3.0 VDC.  Output is in ratio to supply voltage, so it must be known and regulated.

In the photo you see I soldered the chip to a little adapter board and glued that to a scrap piece of PC board on which I also mounted a 3.3V regulator.

Only three pins are used: supply, ground, and output. So you just take that output to the ADC input of your micro and Bob's-your-uncle, instant barometer.  Altimeter.  Whatever.

You'll need some floating point routines in the program.  I used my Arduino board which has an ATmega168 MCU, I think.  You program it in C.  I'm trying to expand beyond assembler so this is a good project for me. The formula for pressure from output voltage is:

P = (Vout/Vs +0.095)/0.009 kPa  (Vs is the supply voltage.)

Since the output is in kPa or kilo-Pascals, I did a further conversion to inches of mercury (inHg), more familiar to me.  BTW, normal barometric pressure is about 101.3kPa.

I first just hooked the thing to my DMM and converted voltage readings to inHg with Excel or my calculator.  I pulled up the local airport's data on the web and compared readings.  They were off a bit more than I expected, but then I learned that airports convert their reading to sea level equivalent, making it somewhat higher.  After compensating for the difference between sea level and our airport's altitude of about 380 ft. I was closer.  In the end I did a small fudge factor to get closer yet. I also have access to atmospheric pressure at my friendly local power plant for calibration purposes.

Converting to altitude is more complicated. Rigorous formulas are complex and factor in a number of things like ambient temperature and relative humidity.  I opted for one that was simpler:

Z = (1-(P/29.9247)^0.19)/22.558E-6

Where Z is altitude in meters and P is pressure in inHg.  I converted the result to feet for display.

My display is a 4-digit 7-segment display surplus from a set-top box, BTW.  I like the look of red 7-segment numerals, but it has a few issues:  No decimal point, hard to see in outdoor light, can't make many alphabetic characters.

I make my program display pressure in inHg for about 1.5s, then text approximating "inHg" for 0.75s, then feet altitude for 1.5s and text "ASL" for 0.75s and repeat.

One curious result I noticed initially was that the display would show 380 feet for a while, then jump to 350 feet, alternating between the two.  I finally realized that this is the limitation of my 10 bit ADC.  One part in 1024 is equivalent to about 30 feet of elevation here in Russellville.  I tricked my way out of that a bit by putting in an averaging routine that uses the average of 5 readings taken over a 5 second period.  Now I get smoother changes and some artificial interpolation but my response time to a step change has been reduced to the 5 seconds it takes to fill the buffer.

Today I gave it a trial run in the Miata to the top of Mt. Nebo.  My local elevation is 380 feet.  When I got to the sign at the top of the mountain saying "Elevation 1800 feet", my altimeter was displaying 1749 ASL.  So, pretty good.  

BTW, I looked at a week's worth of barometic data and saw that the equivalent elevation change between the high and low readings was 186 feet.  So I might reasonably expect extremes to make my altitude reading off by up to +/- 93 feet if I haven't entered any compensation.




Will this project ever make it into a box with a dedicated MCU chip?  Maybe.

Saturday, March 26, 2011

PIC powered filaments - my first PWR / switching project.

This project satisfies a couple things I'd been wanting to try - 1) using PWM on the PIC, and 2) trying my hand at a switching power supply circuit.

I should say that I first tried the Arduino board, but it's C-language front end only allowed a PWM frequency of about 500 Hz.  I felt the need for a higher frequency / shorter period.  So back to the PIC, and I chose a favorite, the 16F88.  It has 18 pins and incorporates ADC, PWM and lots of other good stuff.  To make sure I could get the PWM speed I wanted, I used a 20MHz crystal instead of one of the very handy built-in oscillators.

First I worked on learning to get the PIC working in the PWM mode.  It didn't take long to be able to produce a "demand" output voltage of good accuracy that I could measure with a DMM and/or see on the scope.  I used a simple RC filter on the output pin to convert the pulse train to DC.

PWM is pulse-width modulation.  For example, if I set it for 25% output, the output pin will be HIGH for 25% of the selected period and LOW for the remaining 75%, and repeat.  Good for power supply voltage regulation.  (Similar methods are even used in your digital (CD, MP3) music players to generate audio.)

Now, what about the switching circuit components?  I scanned several references (complicated!) and came up with a simple appearing "buck" circuit.  The PIC controls a pass transistor that "charges" a series inductor during the ON time, through the load, and when the PIC turns off, the inductor continues to source current through the load with a diode providing the conduction path that opened when the pass transistor turned OFF.

The references indicated that design of PWM circuits is pretty complex and not likely to work with homebrew hacker techniques.  I decided to play with my proposed circuit in LTSpice for a while before risking blowing stuff up, since I was pretty clueless.  That helped me a lot.  I just simulated the PWM pulse drive from the PIC with a square wave source, changing the duty cycle manually until the output voltage was right.

I was using a 20us (microsecond) period, which is the total ON plus OFF time for one cycle.  Things were looking good and I was thinking it ought to work, but then I started using LTSpice to look at the dissipation in components such as my transistor.  Wow, it was dissipating twice as much as my load. My overall efficiency must have been about 25%.  Looking closer, it appeared that the losses occur during switching periods, the rise and fall times.  When fully ON or OFF there's no device dissipation, which of course is what makes switching power supplies attractive. Do I somehow have to figure out how to make those rise and fall times smaller?  Fortunately, no.

It occurred that my efficiency is related to the number of switching transitions (OFF to ON, ON to OFF) per unit time.  So maybe my period is too short.  I had just assumed that faster was better.  I cranked down the period in my simulation and efficiency improved considerably.  I eventually went all the way from 50kHz PWM frequency down to 4.88kHz. So maybe I could have skipped the 20MHz crystal and just used the internal 4MHz oscillator.

One negative is that the peak transistor current gets larger as the ON time stretches out.  More inductance can help.  My transistor has a rating of 1A continuous and 3A peak and I was trying to hold to that.

Now I'll show the schematic and discuss the component selections-
I originally wanted to use full wave rectified voltage right off the AC line, like PC supplies do.  But that would give me about 170VDC and I didn't have a PNP transistor to handle that with margin.  My closest shot was the TIP30C, rated 100VDC.  So for my experiment I also had to build a power supply of about 50VDC output.  I have a good junk box, so no problem.

Next comes the diode, I guess it's a commutating diode.  I think it needs to be one of those "fast" diodes used in switching supplies and capable of 2A or more.  Junkbox to the rescue again, I have a RHRP860 rated "hyperfast" at 8A and 600V.  It's in a TO220 package like my transistor.  The cathode of the diode and the collector of the transistor are both on the metal mounting tabs of their packages, so I could bolt both to the same heat sink and board area to make that connection.

Next comes Q2, which switches the main transistor ON and OFF by grounding and opening its base circuit.  When this transistor is OFF, it sees the full supply voltage, so I wanted a small transistor that could take fairly high voltage as I look forward to my 170 volt version.  The MPSA43 is a small transistor in a TO92 package that can take 300 volts.

Finally the inductor.  Books and articles indicate that this can be critical.  I picked one from my junk box which seemed to have heavy enough wire and measured 933uH.  I have no idea of the intended use although I suspected it was part of a switcher of some kind.  I should mention that the inductor should not saturate at maximum current.  A recent project of mine was a saturation tester similar to that made by Alan Yates, VK2ZAY.

http://www.vk2zay.net/article/200

I may document mine here one of these days.  Anyway, I had tested this inductor and it could go several amps without saturating.

That network of three resistors and two capacitors between the output and the PIC's ADC input does two things.  First, it's a /2 voltage divider since 6.3 volts would over-range my PIC's 0 to 5 volt ADC input.  And second, it's an RC filter to smooth out any ripple on the output so the PIC gets a consistent average reading.

I lashed everything together, including the power supply, PIC board, switching circuit and a 10 ohm load with a 6.3 volt #44 bulb in parallel.  (My setpoint in software is 6.3 volts.)  Somewhat to my amazement, the lamp came up to normal brilliance and my DMM measured 6.27 volts. And nothing blew up, smoked, or made scary noises.  My homemade sheet metal heat sink got to about 125F near the transistor and diode -- not too bad.  I added a second 10 ohm load in parallel to get closer to my desired output of a bit over 1.5A and all was good.  Regulation with load changes looks good, and as I ran the input voltage from 35V to 65V the output voltage did not waver.

My PIC software changes the output duty cycle in 5% steps until output voltage gets within 10% of setpoint, then slows to the minimum step of about 0.1%.  Updates occur about every 5 milliseconds.

It occurs to me that if I power on the PIC before the big supply, the PIC will run up to maximum duty cycle trying to get to setpoint.  Then when the main P/S suddenly comes on, output may go high before the PIC is able to sense the overshoot and run the duty cycle back down.  Or maybe not.  But I may add another input to give a "power supply voltage normal" signal which would be required before the PIC comes off zero.

Let me show you the drive signal from the PIC, as read at the collector of Q2.  When the voltage goes low, it allows base current to flow out of PNP transistor Q1, turning it ON.  You can click these pictures to make them larger, BTW.
What's it all for?  This one is just an experiment, but one application is in making power supplies for tube type equipment.  Both the B+ (high voltage or plate supply) and filament supplies are getting harder to come by for someone without a huge junk box.  I'm seeing several approaches.  One guy took a 12V to 240V auto inverter and rectified the output to give about 250VDC.  Most of us have 12VDC available.  But inverters with 240VAC output are not common in the USA.  I think using switchers for both the HV and filament voltages could be interesting although I can see that a purist might insist that everything be authentically from the period, including power supplies.

Using such a power supply for filament voltage could have a couple advantages.  I recently checked an old tube type power supply and found that my filament voltage which was supposed to be nominally 6.3 volts was actually 6.8 volts.  The  could be due to having less load on the transformer than it is rated for, but it's partly due to the rise in house line voltage over the years.  People still say "one-ten" and "two-twenty", but probably for the last 30 to 40 years, actual house voltages have been 120 and 240 volts.  Hard on antique tube gear.

Another value a PIC controlled supply could give would be a "soft start" feature, bringing the filaments up slowly instead of hitting them cold with full voltage.

But if the PIC controlled supply fails, it might destroy some rare tubes it was supposed to protect.  In such cases some kind of protection in software or hardware might be advised.

73-

Nick, WA5BDU
3/26/2011

Monday, March 14, 2011

FT243 crystal grinding

Back in 2006, I was highly impressed by a crystal grinding workshop given at the OzarkCon QRP convention. Unfortunately, I didn't participate directly and missed some of the details.  It was impressive though that crystal resonant frequencies were raised much farther than I'd thought possible.  Possibly 50 to 100 kHz or more, IIRC.

So now I have a "new" DX-20 and I see that my FT-243 crystal cache includes a couple of worthless (to me) 40 meter crystals on 7073.33 kHz and 7080 kHz, in the data mode area between lower CW segments and a slow speed (usually) segment just below the phone band starting at 7125 kHz.  Could I raise these crystal by 40 to 50 kHz for use in that higher CW segment.

Grinding agent?  I searched the grocery store and came up with some Comet cleanser and some Colgate toothpaste which include silica in its ingredients.  It's a problem that these days, scouring powders want to be "scratch free" and don't include good abrasives in them.  I also bought an 8x10 inch piece of glass (Hobby Lobby, for a picture frame) as a working surface.  The idea is to wet the glass, add the abrasive agent, and grind your crystal using a figure-8 motion with your finger tip moving the crystal.

Monitoring the frequency?  Another cool thing about the OzarkCon workshop was that it included oscillators and frequency counters ... you just cleaned up a crystal in progress, sat it on a ground electrode, and sat a second, weighted electrode on top of it to check the frequency.  Much better than re-assembling the whole thing and plugging it into an oscillator having a FT-243 connector.  I built an oscillator from Experimental Methods In Radio Frequency Design (EMRFD), figure 4.23.  I later changed the top (base to emitter) capacitor from 470p to 220p in hopes of increasing gain and getting a reading if the crystal became reluctant to oscillate.


Above you see the corner of my oscillator with the crystal blank, two electrodes from the FT-243 holder, my own top electrode (made from circuit board material) with the wire attached.  To check, I sandwich all four items and clamp with the orange and black clamp (or a clothes pin).  Initially I did not use the internal silver colored electrodes, but after I learned that they are machined to contact the crystal on its corners and let the middle section free to vibrate, I started using them in the stack.

Early results - The Comet was OK to use.  The toothpaste tended to stick the crystal to the glass so I'd have to pry it loose.  I also tried Turtle Wax auto buffing compound.  It was very slick and my finger kept slipping off the crystal.  None of them ground the crystal fast enough. Doing lots of figure 8s only moved the frequency a few hundred hertz.  I also tried some crocus cloth.  It discolored the crystal and made it quit oscillating until I ground it some more with abrasive compound.

Better / faster results -


I did some web searches and found Dave's Crystal Grinding Page, where he documented doing a lot of the same stuff I was doing, plus coming up with a better abrasive compound -

http://home.netcom.com/~wa4qal/crystal.htm

The better compound was Dremel Polishing Compound.  I went out and got some to try.  It's kind of a putty and didn't want to mix with water, instead forming globs and sticking to the crystal.  Then I added a few drops of dish washing liquid to the mix and it spread out nicely.  This time I was able to move a crystal 15 kHz fairly quickly.  I ground on it some more and ... it quit oscillating!  (This was before I changed the feedback capacitors, so who knows?).  I ground some more to try to bring it back, but no luck.

Next I went to my remaining crystal, ground a while, read the frequency, looking good.  Figured out about how many more figure-8s I'd need to reach my target and did about half of that.  Checked and ... I've over shot and am in the phone band at 7138 kHz.  One crystal dead, one useless to a CW operator.  But wait?

Tried the dead crystal in the DX-20 and the good news is that it came to life.  The bad news is that it is also at about 7138 kHz.

Someone on the 4SQRP list put me on to Hans Summers' page on what he called crystal "penning", meaning you lower the frequency by painting the crystal with a Sharpie marker.  I'd heard of the pencil lead trick before but didn't think it would do 15 kHz.

http://www.hanssummers.com/penning.html

I tried the formerly dead crystal first, but discovered yet another mistake to make.  Trying to get the stack of electrodes and crystal aligned and clamped, I broke a corner off the crystal.  No more rabbits to pull out of the hat for this one.

Now the remaining crystal.  I blacked about 80% of one face and the frequency dropped about 8 kHz.  Promising.  Did the same on the other face and got right down to the edge of the phone band.  I just need a few more kHz.  BTW, you have to allow some time, maybe 30 minutes to an hour, for the ink to try or the crystal might not oscillate.

So I fully inked both faces.  It didn't want to oscillate even after an hour's worth of drying.  The ink looked kind of lumpy, so I polished both faces on a sheet of typing paper.  Now it's oscillating on 7122.7 kHz in the DX-20, close to my original target.

So there you have it.  Everyone talks about over-shooting, you decide to be careful, but do it anyway.  So be more careful -- that's probably better than resorting to "penning".

Another post-script ... another 4SQRP poster said Bon Ami cleanser works well.  I'm not sure you can get it in Arkansas, but I'm keeping an eye out.

That's it, happy grinding ...

Nick, WA5BDU