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 Post subject: Crystal Cleaning: when, why, and how?
PostPosted: Nov Sun 22, 2020 6:09 pm 
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Joined: May Wed 20, 2020 1:33 am
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Location: Rockford, IL
I own a small fleet of Hallicrafters sets and three of them have crystals. In particular my SX-25 seems to work great and at certain crystal phase settings generates a low hum and rumble (like a zero beat that isn't). It might be that my finding the crystals frequency is not totally spot on but the others have no such issues with my aligning to the crystal frequency in the alignment procedures.

What are the signs of dirty crystals and is it normal to have to disassemble them and clean them. If so, what is the best way as I do not even want to be in the new crystal market (no idea where to get them).

And if you do is an alignment then in order? - I think yes but I do not know what the nature of improve is for a clean crystal set as I am not sure what a dirty crystal does.


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 Post subject: Re: Crystal Cleaning: when, why, and how?
PostPosted: Nov Sun 22, 2020 6:19 pm 
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Joined: Jun Sun 19, 2011 2:31 pm
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Crystals are often contaminated over the years from stuff that was either in the holder initially or has gotten in through less than perfect gasket seals over the years. With care, the vintage holders are easy to open and clean. Early National HRO manuals have simple instructions in them for cleaning and you can find those manuals for free download on edebris. I use highly concentrated isopropyl alcohol (may be difficult to find the higher percentage in grocery stores now due to pandemic shortage) and paper towels for cleaning/drying. Hold the crystal blank only by the edges and don't touch even the edges or the pressure plates with your bare fingers after cleaning and drying or you may make it worse than it was originally.

Don't worry about the crystal being slightly off of its marked frequency, they do age. And the oscillation frequency of the blank is affected by the pressure applied to it so you will find a slight frequency shift depending upon hardware tightness with many holders. One of the classic crystal makers had a holder available with a knob on the outside to slightly shift the frequency via varying pressure.

I do a before/after check with crystals by connecting the crystal in series between the output lead of my signal generator (high output level, unmodulated) and the input of my scope; tie ground clips of SG output and scope probe together. Slowly sweeping the SG through the crystal frequency should show a sharp peak at resonance.

The small crystal holders used in many later Hallicrafters and National sets often have contamination inside. You will also find that the ceramic "gasket" between the case halves is cracked. This can be carefully put back together with "super glue" but let it dry completely (24 hours) before reassembly because you don't want the fumes from the curing glue to interact with the crystal and pressure plates.

Rodger WQ9E


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 Post subject: Re: Crystal Cleaning: when, why, and how?
PostPosted: Nov Sun 22, 2020 7:12 pm 
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Joined: Mar Wed 16, 2011 10:44 pm
Posts: 1652
Location: Peekskill, NY
Unless you find a problem with the crystal, the default value is probably 'leave it alone.'

Hallicrafters crystal, and the network analyzer trace from it. It might be problematic to use a scope to find the dip, this was pretty narrow, and it was in a fairly well used, and poorly stored set:


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 Post subject: Re: Crystal Cleaning: when, why, and how?
PostPosted: Nov Mon 23, 2020 12:22 am 
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Location: Liberty, Missouri
From the National HRO-60 manual.

Image

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 Post subject: Re: Crystal Cleaning: when, why, and how?
PostPosted: Nov Mon 23, 2020 1:59 am 
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The peak is very easy to find using a signal generator and scope. It isn't necessary but I like to do a before/after test.

In my experience, most of the very old FT-243 and earlier crystal units are usually OK but the later small type used in Hallicrafters sets like the SX-42 and later and many of the National receivers work MUCH better after cleaning. Many of these old sets were carelessly sprayed with TV tuner cleaner or similar stuff popular in the 1970s and the residue from it works its way, along with dust, into the housing.

As long as you are reasonably careful, you aren't going to damage anything. I have disassembled, cleaned, and reassembled many of these crystal units without any casualties.

Rodger WQ9E


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