[Hallicrafters] FT-241 crystal information

[Mike Everette]

In response to an off-list query, I have done some
research on the FT-241 crystals.  

These were manufactured in three groups for three
different operating frequency ranges:  20-27.9 mc
(black holders), marked "Channel O, 20.0 MC" through
"Channel 79, 27.9 MC;" and 27-38.9 mc (brown holders),
marked "Channel 270, 27.0 MC" through "Channel 389,
38.9 MC."  The third group was used for 70-100 Mc.  I
don't recall how they are marked, but these have red
holders.

The Channel 0-79 crystals were used in BC-604
(SCR-508) FM tank transmitters during WW2.  Channels
270-389 were used in the similar BC-684 transmitters. 
The 70-100 Mc channels were, I believe, used in the
AN/TRC-1 FM radio relay transmitters.

And, the Collins ART-13 transmitter uses a 200 Kc
crystal in an FT-241 holder -- usually red, marked
simply "200 KC" for its frequency calibration
standard.

Surplus FT-241 crystals were widely used by postwar
hams to build crystal lattice filters for receivers
(there is an article that detailed doing this for the
S-40B, in an old QST that I have seen), and in early
home brew SSB exciters (yes, this was once common!).  

I have not actually tried this, but if you have an
early Halli (SX-16/17/24/25 etc) that is missing or
otherwise needs an IF crystal, the FT-241 should work
fine as a substitute.

The pins and spacing are the same as the FT-243
holders that usually contain high frequency crystals.

HOWEVER: DO NOT try to sweat-solder larger pins over
the ones on the FT-241 holder!  You will probably
destroy the crystal.  You'd be better off changing the
5-pin socket in the receiver to an octal.  (Yeah,
yeah, I know, "It's not original;" but you can always
return to original configuration if you ever find the
OEM crystal -- in the meantime, GIT 'ER GOIN'!) Here's
why.

The FT-241 is a very stable, high precision crystal. 
60 years later, however, you may occasionally
encounter one that has an "open" because the wire lead
from a pin has broken loose from the SOLDERED joint to
the quartz element; so you should try to get a couple
or three for each frequency you are interested in.

For Channels 0-79, determine the fundamental by
dividing the channel frequency by 54.  The crystals
are, therefore, about 1.852 kc apart.  For use in a
nominal 455-465 kc IF as a filter, you'd use Channel
50 through about 60 or 61.

For Channels 270-389, determine the fundamental by
dividing the channel frequency by 72.  These crystals
are about 1.395 kc apart.  The channels of interest,
therefore, are approximately 320 through 340.

As you can see, someone homebrewing a lattice filter
can combine crystals from either set to come up with
just about any desired bandpass.

These crystals are still seen at hamfests; but the
ones in the 448-470 range are harder to find, compared
to "the rest."  Fair Radio Sales still has, I think,
complete sets of Channels 0-79 for about $55.00.  

I don't know if JAN Crystals in Florida (Fort
Myers?)is still around, but they used to sell the
FT-241s re-etched to any frequency you wanted if the
standard spacing didn't provide your frequency.  I
once used some of these to put a BC-604 on 26.11 MHz
to serve as a remote pickup broadcast transmitter.  As
I recall, the re-etched crystals weren't quite as good
as the stock ones.

Hope this information helps someone.

73

Mike
WA4DLF

The 70-100 Mc crystals have a much higher fundamental.
 I believe they are 96th harmonic.  Some of these have
fundamentals that fall in the 160-meter band.



		
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