Poor Man's Tcxo; Upgrading Stock Ceramic If Filter To A Crystal Filter; Can I Increase My Rf Power Output Beyond 100W - Icom IC-746 Faq

Poor Man's TCXO

N0UK (Chris) came up with a "poor man's" TCXO that substantially reduces frequency drift,
particularly for 2m. He discovered that the cooling fan, which runs during transmit, draws
air right across the local oscillator. The resulting temperature drop causes the LO to drift.
He stuffed the 60 Mhz LO compartment with a few "cotton wool balls" as insulation. It
doesn't eliminate drift entirely, but the improvement is dramatic.
The original e-mail message on this is at:
http://groups.yahoo.com/group/IC746/message/1959
A WWW site describing the mod (complete with pictures) is at:
http://www.chris.org/Modifications/IC-746-almost-free-tcxo.html

Upgrading Stock Ceramic IF Filter to a Crystal Filter

The stock 455 Khz IF filter is a 2.4 Khz wide ceramic. The PC board is drilled and traced for
a full-fledged crystal filter (either from Icom or Inrad). In theory, replacing the cheapo
ceramic filter (Murata CFJ455K) with a superior crystal filter should improve the rig's
performance. Three obvious choices have been suggested:
• Inrad
• Icom
• Kokusai
One source of info claims that to do this mod with crystal filters requires changing 2 resistors
(R701 and R704) from 10K to 3.9K to change the impedance seen by the filter from 2K to
about 1.2-1.5K. Most do not bother.
N9IRD talks about this type of mod at:
http://www.qsl.net/icom/n9ird.html
7.3

Can I increase my RF power output beyond 100w?

Yes, but you probably shouldn't.
Technically, this is more of an adjustment than a "mod". NI4L (and others) have outlined the
alignment procedure for increasing the RF power output above 100 watts. The basis for
these procedures can be found on page 5-4 of the Service Manual.
The procedure can be found on-line at:
http://www.mods.dk/
http://www.qrz.com/download/mods-i-k/ic746pwr.txt
After examining the circuitry in the schematics and the specifications for the transistors
involved,
http://www.qsl.net/kb9sss/Parts/2SC2694.pdf
IC-746 FAQ
109 crystal filter
FL-44A crystal filter
MF-455-11GZ mechanical filter
Page 85 of 113
2003-02-08