I am looking for FT243 xtal 1610khz or 1530>1600 as
close to center freq. as possible. The digital radios have a
hard time tuning in on some of the far right or left freqs. I currently have one at 1646.8 and this is a problem.
Or could I modify the osc. section to pull down the 1646 down
to say 1640? This is a peewee tube type xmiter. Thanks in advance for help.
I am looking for FT243 xtal 1610khz or 1530>1600 as
close to center freq. as possible. The digital radios have a
hard time tuning in on some of the far right or left freqs. I currently have one at 1646.8 and this is a problem.
Or could I modify the osc. section to pull down the 1646 down
to say 1640? This is a peewee tube type xmiter. Thanks in advance for help.
Hi,
Circuit modifications will not pull the frequency much. You might get it down about 6 kHz. by adding a parallel capacitance and that is the first thing I would try.
We old hams know how to change the frequency of a FT243. Essentially, the frequency is determined by two things, first, the thickness of the crystal, and second, by the tension in the FT243 holder.
You can RAISE the frequency by removing the crystal (about a postage stamp sized element) and gently with sandpaper, take a little off the thickness. This is a trial and error process and you will need go sand a little and assemble the unit and check, sand some more, check, etc.
The final trim can be done by tightening the screw in the center of the holder. If it doesn't have a screw, the best you can do is sand, measure, sand, etc.
I have done this but crystals are not that $$$ so I think I would just buy one in a HC-6 holder cut to frequency and be done with it.
Neil
Are you saying put the cap across the xtal? If so could I use
a small rf trimmer? Or would I need to use ceramic or mylar
ones. What is your suggestion to cut down on drift since its
stable at this time.
Try a ceramic trimmer across the crystal. If you can, get one with about a 3 to 35 pF range. Anything larger will probably stop the oscillator. If you don't have a trimmer handy, try a small ceramic cap (10 to 30 pF or so). Look for a NPO type if possible though I don't think drift will be a problem unless the temperature changes quite a bit. Find out if you can pull the frequency adequately before you worry about drift.
A cap in parallel should lower the frequency, and a cap in series should raise the frequency.
If you do this, I would like to know how it worked out for you.
Neil