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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 12th Oct 2020, 4:13 pm   #1
G6Tanuki
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Default Grinding crystals

The crystals I offered here a week or so back will be on their ways to two new owners shortly, but while packing them up I started wondering if anyone on the forum has experience in crystal-grinding?

All the 'classic' ham-radio handbooks talk about lapping crystals using a very fine abrasive (Jeweller's Rouge, Ajax) or etching them using Hydrofluoric Acid (oooer!) but that was back in the days of the olf FT241/FT243 crystals whose holders could be dismantled, the rock lapped, then refitted into its holder (which made contact with the rock at only a few points)

I've got a 5.something-MHZ crystal I would like to shift HF by a few hundred KHz but it's one of the HC6/U type in a solder-sealed can: last one of these I remember opening-up had the faces of the crystal silvered and suspended on very fine wires that were somehow affixed to the silvering.

Any ideas how I'd go about grinding it? I don't know what 'cut' it is, so not sure where I would want to remove material either!
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Old 12th Oct 2020, 6:56 pm   #2
Bazz4CQJ
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Default Re: Grinding crystals

I've done my fair share of FT243's with Ajax (or Vim), but I've never tried to do anything similar with modern crystals which, as you point out, usually have a metal film deposited on the active surfaces. With that film present, quite how any abrasive or etching process could be carried is hard to say. Perhaps that film needs to be removed at least in one side, then the quartz processed, and film replaced by some procedure. That sounds like quite a learning curve.

Do you have any pictures of what's inside the can?

B
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Old 13th Oct 2020, 10:49 am   #3
G6Tanuki
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Default Re: Grinding crystals

Here's a photo of the insides of a glass-cased HC6/U manufactured by STC: the quartz itself is roughly circular, and has blobs of metallisation, one on each side, connected by stripes of metallisation, again one on each side, to the 2 lead-outs; how the lead-outs are attached to the metallisation I can't tell.

I'm pondering whether it is _just_ the thickness of the quartz that determines the resonant frequency - or whether milling some quartz off around the circumference might also affect resonance? The photo shows that the 'disc' is not perfectly circular - could edge-milling have been part of the original calibration process ??
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Old 13th Oct 2020, 11:47 am   #4
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Default Re: Grinding crystals

It's chiefly the thickness. It's a half-lambda long acoustical fabry-perot resonator with two end correction effects.

However, crystals also have some spurious modes and also overtone modes.

To discourage unwanted modes, sometimes the surface of the crystal is profiled. Very slightly convex is common, but funny ones have had rosette like ripples. Some have patterns of dots of damping ink in the ring outside the electrode. Choice of electrode size, shape and positioning come into play.

Artful crystal manufacturers may have to switch between a few different mode control schemes in only a small range of frequency. HC6 crystals with deposited electrodes aren't grindable unless you have vacuum sputtering facilities to redo the side you ground.

All the aftermarket-reground crystals were 10X and FT243 holder types where the bit of rock went between sheet metal plates.

David
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Old 13th Oct 2020, 6:46 pm   #5
G6Tanuki
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Default Re: Grinding crystals

I'm coming to the conclusion that without some serious-tech-which-I-do-not-have-access-to, shifting a 4.9MHz HC6/U rock 400KHz HF is really not practical. My really-techie books-on-quartz-crystal-resonance-modes all date from the late-1930s!

Now, let's step forward to the 1970s and engineer a 'vintage' series-tuned-Colpitts [a.k.a. Clapp] coil-and-capacitor arrangement to plug into that crystal-socket. Some seriously-pricey mil-spec ceramic coil-formers are on order from the US of A.
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Old 14th Oct 2020, 8:06 am   #6
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Default Re: Grinding crystals

You will never get that far!
I reckon perhaps they managed 10kHz or so at 7MHz with their FT243s.

Even desoldering an HC6U and letting the air in and some flux vapours out sends it massively LF. A failed attempt on a Pye U450 TX crystal tells me this.
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