Monday, March 14, 2011
FT243 crystal grinding
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 -
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.
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 ...