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AMfone's Online AM Handbook => Hints and Kinks => Topic started by: kf4qkr on April 21, 2011, 11:44:13 AM



Title: Grinding crystals
Post by: kf4qkr on April 21, 2011, 11:44:13 AM
I bought a FT243 crystal at last years Raleigh ham fest that was on 3882.5. It worked but you can not use that frequency without making people mad on 3885 and 3880 so it stayed in my drawer for a year. I read a little bit about grinding crystals and discused it a bit with my freind Phill KC4VWU so a few days ago I got brave and decided to give it a try. I have a smooth 3 inch by 7 inch knife sharpining stone that I have been sharpining mt knifes on for years. One side has a dip in it from wear but I never used the other side and it is perfectly flat. I never heard of anyone using one to grind a crystal. I read about guys using toothpaste and graphite.Well my stone looks like its made of graphite I am still not sure. Anyway here is what I did. I took the crystal apart and lucky enough it was one of the large sized old school crystals.I put on the tight thin kind of rubber gloves doctors use(keeps your oils from your fingers from getting on the quartz) and went to work.I did about 15 figure eights on the stone then changed my gloves because I wore holes in the finger tips.Then I washed the quartz in 91% rubbing alcohol and dryed it carefuly with a paper towel, put it back all back together, pluged it in my HT40 on the dummy load and checked it and it was on 3883.8. Well That was a little over half way so I figured about 10 more figure eights ought to do it so I took it back apart and did 10.I had  to change gloves again because of the holes,wash it in alcohol and put it back together,checked it and it was on 3885.1. Wow! that is close enough for AM. Now I have a useable crystal. I just thought I would share that with you all.        73s    Mike KF4QKR


Title: Re: Grinding crystals
Post by: W7TFO on April 21, 2011, 11:52:37 AM
Good tip, as I have a bunch to do down the road. ;)

73DG


Title: Re: Grinding crystals
Post by: Jim, W5JO on April 21, 2011, 12:31:50 PM
To lower the frequency, make a mark on it with a graphite or #2 pencil.


Title: Re: Grinding crystals
Post by: W2PFY on April 21, 2011, 01:48:00 PM
I wonder if you could tell us the grain of the stone you were using. I have seen stones where one side is coarse and the other fine.

I have also heard of chemical etching. Anyone else ever played around with grinding your own?

Somewhere out there is an 813 single tube oscillator. They recommend using the old FT-171- BC-610 transmitter xtals because of crystal heating from the grid current. That is something I would like to build one day. These are really big old crystals and are usually cheap to buy because very few are in the ham bands and a lot of them are in the 2-4 MC range.  


Title: Re: Grinding crystals
Post by: k4kyv on April 21, 2011, 01:49:37 PM
I have heard of people doing it using wetted ultra fine grit sandpaper (something  like #500). I recently picked up a piece of plate glass at a junk shop for the purpose, and plan to use finely powdered grit per instructions in the old books. I think the stuff they sell for polishing jewellery, available from a lapidary equipment supplier, would do the job.

I doubt the old trick of using the pencil mark to lower the frequency is as stable and permanent as grinding the crystal to the correct frequency to begin with. As you approach the exact frequency, you need to test the frequency ever more frequently - a real PITA, since you have to clean it, insert it into the holder, re-assemble the holder, and plug it in, then disassemble everything and repeat the whole process each time until you get it right.  You should test it in the same holder you plan to use permanently, and if possible, the same rig you plan to use the crystal in, when checking the frequency.  It is not unusual for the frequency to vary 300~ or so, from rig to rig, even when the same tube and oscillator circuits are used.


Title: Re: Grinding crystals
Post by: Todd, KA1KAQ on April 21, 2011, 02:09:00 PM
An OT told me he used AJAX or Comet cleaner for grinding his crystals years ago, figure eights on a pane of glass. He also told me something about listening as you grind it with a receiver somehow to hear as you're approaching the correct frequency. Not sure how this works since it seems hard for it to oscillate in that state, but he's a whole lot smarter than me.


Title: Re: Grinding crystals
Post by: kf4qkr on April 21, 2011, 02:20:49 PM
I was using the fine side of the stone.I couldnt tell you what the grain was.Its Pretty smooth.


Title: Re: Grinding crystals
Post by: Jim, W5JO on April 21, 2011, 03:22:13 PM
The powder of choice at the time was Bon-Ami.  The trick was the pane of glass and do the figure 8 with Bon-Ami.  I watched the guy who owned International Crystal do the pencil mark on the slab to lower frequency one time.  He was a ham and would help guys on weekends if they needed help.   That was back in the 50s.


Title: Re: Grinding crystals
Post by: K3YA on April 21, 2011, 05:06:26 PM
Water soluble valve grinding paste and a flat piece of glass works FB for grinding crystals.


Title: Re: Grinding crystals
Post by: K9PNP on April 22, 2011, 12:16:48 PM
IIRC the glass grinding base was the important part.  Most FINE wet grit will work, within reason.  Used to use Ajax for FT-243 blanks, but don't know if the present Ajax is the same as the 1960's and 70's Ajax.  Have used the graphite pencil, but found that long-term stability was not good.  Of course, this could be due to other things than the pencil, such as the FT-243 mounting system when it has been opened a number of times.


Title: Re: Grinding crystals
Post by: The Slab Bacon on May 17, 2011, 09:40:58 AM
I have reground quite a few FT-243 crapstals over the years. I have also found a quick and dirty way to do it that works well. I like to start with crapstals that are way low in frequency. (It gives you more room to get the feel of that particular piece of quartz.)

For large changes use #600 wet n dry sandpaper with water to "rough them in". then finish them with 2000 grit. stopping occasionally to check the frequency.
JUST BE SURE THAT THE PAPER IS ON A GOOD FLAT SURFACE.

Be sure to reassemble the quartz back into the SAME holder that you have been checking it in, as the amount of spring tension on the pressure plate in the holder does have a DRAMATIC effect on the final finished frequency. (As does sometimes even the orientation of the quartz in the holder)

If you found you've taken a little too much off, a small dot of ink from a Sharpie dead center in the middle of the quartz slab will lower the frequency.

The whole process doesn't take very long, but............. Is it really worth spending the time on? ?

I have crapstals on 3.865, 70, 75, 80, 85, 90, 95, and 3.905 as well as 7.280, 85, 90, and 7.295 that are all ground by me back when I had more time on my hands than good sense.
That's what they make VFOs for! !   ;D


Title: Re: Grinding crystals
Post by: W2PFY on May 17, 2011, 12:03:03 PM
Sure it's worth grinding your own crystals. Up here in the upstate NY and New England area, if your running a crystal, your automatically a GOD and everyone MUST zero beat you 8) 8) 8)


Title: Re: Grinding crystals
Post by: k4kyv on May 17, 2011, 02:01:11 PM

The whole process doesn't take very long, but............. Is it really worth spending the time on? ?

I have crapstals on 3.865, 70, 75, 80, 85, 90, 95, and 3.905 as well as 7.280, 85, 90, and 7.295 that are all ground by me back when I had more time on my hands than good sense.
That's what they make VFOs for! !   ;D

I have a good VFO, but it is nice to have the crapstals on hand in case of a VFO crap-out that may take some time to fix. The only rig I have with xtal capability is the HF-300 transmitter.  It uses those old Bliley/Valpey crystals with the  round holder the size of a half-dollar, and two pins that fit in pins #2 and 4 of a 5-pin tube socket. I have a drawer full of those crystals, a few of which are outside the ham bands or else the original crystal is crapped out, plus a few empty holders.  I have taken the quartz plate out of a few FT-243s and put them in the round holders, and they stayed right on frequency.  Quartz plates removed from BC-610 crystals fit into those holders very nicely. I do plan to try re-grinding a few oddball frequency crystals just to see if I can do it and then say I have done it.  Kind of a fun nostalgia thing, since I use the VFO most of the time.

I have noticed that regardless of holder (I have an adaptor that allows me to use FT-243s), the frequency is usually at least 300~ off the nominal frequency (most often in the high frequency direction, per Murphy). Not surprising, since crystals do age, just like carbon resistors, and most of these, including the once-plentiful WWII surplus crystals, are now decades old.  Plus, take a look at the ads in the old ham rags from the 30s, 40s and 50s. Crystals were usually sold with a nominal frequency, and a rated tolerance something like 0.1%, 0.01% or 0.001%.  The tighter the tolerance, the more expensive the crystal. On 75m, 0.01% calculates to 400cps and most hams purchased the cheaper coarser tolerance crystals. Even in the same holder, the frequency may be off from one transmitter to another using the same basic oscillator circuit.

I have fixed many sluggish or inoperative crystals by taking the holder apart and washing the quartz slab and metal pressure plates, using denatured alcohol or better still, a degreaser called Lektraclean that I purchased at Lowe's. The latter is about the closest thing I have found, in performance as well as in smell, to carbon tetrachloride, which is what the old timers recommended. 

If you take a crystal and holder assembly apart for any reason, be sure to carefully observe which side of the pressure plates that hold the crystal actually touch the quartz plate.  Usually, only one side is ground and polished to precisely contact the surface of the crystal.  Re-assembling with the wrong side touching the quartz plate usually results in a sluggish crystal or no oscillation at all, or a large frequency error, and could possibly damage the crystal.
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