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Novice Transmitter Xtal noise when keying...




 
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wx3k
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« on: January 12, 2009, 10:44:06 PM »

I bought a replica of a Philmore Novice transmitter. One odd thing I notice about the rig is when I key the transmitter the xtal clicks. I have used xtal controlled transmitters before but never noticed/heard an xtal actually making a clicking noise when keying a transmitter. I used the same xtal on another rig and there is no clicking sound with that transmitter. Is this an indication that the xtal is being driven too hard in the oscillator circuit ?
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Stephanie WX3K
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"Thunder is good; Thunder is impressive but it is lightning that does the work" ...Mark Twain
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« Reply #1 on: January 13, 2009, 10:46:18 AM »

Philmore = Cool

The mighty 6V6 should not put too much current through the rock. Do you have another Xtal and does it click?

Is the Xtal an old surplus FT-243?

If so it may be time for a cleaning and general inspection. When I get the old rocks at hamfesters, and I generally grab when I see something in the CW band, I always give them a check with my 6AG7 oscillator.

Some are liars and are not close to the marked frequency, some barely start up, and some sound great. After cleaning and resassembly, the soft ones all straighten out as long as the blank has not been cracked or abused with sandpaper.

And I always try a second holder, if a crystal does not work. Sometimes the plates or spring pressure make a big difference.

Mike WU2D
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k4kyv
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Don
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« Reply #2 on: January 13, 2009, 01:58:00 PM »

Usually, if you take the xtal apart and clean the xtal slab and the metal plates in the holder, it will  come back to life.

In the ol' days, the recommend solution was carbon tetrachloride.  You can't get that any more, but I have found some stuff that works almost as well, and even has a similar smell.  I bought it at Lowe's, and it comes in a spray can.  I believe the brand name is Electro-clean.  The can says it is designed to clean electrical contacts and the innards of electric motors. It is basically a degreaser.  I sometimes use it as contact cleaner, and then add a drop of WD-40 to provide a little lubrication (for contacts, not crystals).  I have fixed several sluggish xtals using the stuff.

I have also had success cleaning xtals with denatured alcohol.

Most of my xtals are anywhere from a few hundred Hz to 1 kHz off the nominal frequency, usually on the high frequency side, but occasionally I find one that is  dead on.  I'm not sure if this is due to the oscillator circuit, or just ageing of the xtals.
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wx3k
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« Reply #3 on: January 17, 2009, 10:33:04 AM »

Some of these xtals are old surplus but some have been remanufactured.

Philmore = Cool

The mighty 6V6 should not put too much current through the rock. Do you have another Xtal and does it click?

Is the Xtal an old surplus FT-243?

If so it may be time for a cleaning and general inspection. When I get the old rocks at hamfesters, and I generally grab when I see something in the CW band, I always give them a check with my 6AG7 oscillator.

Some are liars and are not close to the marked frequency, some barely start up, and some sound great. After cleaning and resassembly, the soft ones all straighten out as long as the blank has not been cracked or abused with sandpaper.

And I always try a second holder, if a crystal does not work. Sometimes the plates or spring pressure make a big difference.

Mike WU2D
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Stephanie WX3K
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"Thunder is good; Thunder is impressive but it is lightning that does the work" ...Mark Twain
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« Reply #4 on: January 20, 2009, 06:42:41 AM »

Stephanie

I downloaded the Philmore NT200 schematic from BAMA.

The 6V6 oscillator uses a design that is different from the design in most of the transmitters I have looked at (Ameco AC-1, DX-20, etc.).

Changing the grid leak resistor (as I suggested in our conversation on the air) in this case will not address the problem.

I believe the problem is that on key up almost the full B+  is across the crystal (with a small portion across the 1200 pF capacitor in series with it). I.e. on key up, there is very little voltage drop across the 15000 ohm resistor in series with the plate of the 6V6. On key down, the DC part of the voltage on the plate of the 6V6 drops to less than half of the B+. Thus, there is a large voltage change (around 200 volts) being applied to the crystal every time you close the key. This is probably what is causing the clicking sound.

To fix this, you could consider using the transmit/receive switch to ground the cathode of the 6V6, rather than keying it. You would continue to key the cathode of the 6L6 output tube.

As a result, the crystal oscillator would remain on during transmit... but the output of the transmitter would still be keyed.

Best regards
Stu
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Stewart ("Stu") Personick. Pictured: (from The New Yorker) "Season's Greetings" looks OK to me. Let's run it by the legal department
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