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TransSystem 3733 mods



I'm posting this for Don, W4VQA, who did the mods and provided the
narrative. My 3733 was one of two that "went under the knife". I'll sweep it
with a sig generator/spectrum analyzer on Monday and report more definitive
measurements. Also, the filter to which Don refers is the Toko 4DFA-2450-10P
dielectric filter (Digikey). The pinout arrangement of this filter is
different than the Murata, so you'll have to adjust accordingly. In short,
the mod was straightforward and seemed to produce excellent results. Don's
message follows.

73,
Art N3OY

Don, W4VQA writes:

This modification on the TranSystem downconverter is based on using a DEM
downconverter as the reference source using an home brew 2mtr converter into
a Collins 75A-3 receiver with external FM demod. Signal source is DEM 2401
with fixed 120 dbm pads. Recovered audio was measured on B&W 410 Distortion
meter (OK they are old but acceptable). Base line for tests were fixed
signal into downconverter(s) to determine reference level without any signal
(-20dbm equals "0" dbm). Signal with DEM source using pads into DEM
downconverter was baseline at -3 dbm. All readings were relative to that for
TranSystem testing. In all tests we are looking at improvement in "quieting"
, not s/n as an exact measurement.

Note: TranSystem units are all at 144Mhz IF with 3hour warm-up.


Step one:
·        Remove the first filter completely. Save the copper finger stock
for reuse. Top filter cover and all screws can be discarded. Save the Teflon
that was on the upper side of the filter.
·        Cut the copper finger stock into two pieces starting with the
antenna end so you have one with the "tab" attached, and one from the next
section without "tab". Cut should be clean on the axis toward mounting
holes. You should end up with two strips about ¾" long & 1/16" wide.
·        Remove the "tab" from first section.
·        Solder this first section to the antenna connector & keep it
horizontal to bottom of case. Should be under 1/16" from case.
·        Take second piece of finger stock and overlay it from the input of
first RF amp (one with loop)/(pad) and cut off excess to provide 1/16"
overlap on first piece.
·        Solder the two together, and solder far end to RF amp pad (one with
loop).
·        Carefully push the assembly toward the case and slide the Teflon
piece under where you soldered the two pieces together going cross ways
rather then lengthwise..

This modification provides about 5db improvement over original TranSystem
unit.

Step two:
·        Remove second filter section components (watch power lead near side
of case) and discard top cover & Phillips head screws. Retain hex head
screws.
·        Remove copper "fingers" and trim both outer fingers to about 1/8"
from mounting holes. Using cutters trim the top of the remaining two inner
fingers to 45 degrees pointing inward. (save pieces)
·        Reposition "fingers" so mounting holes are on hex head side of
case.
·        Bend case tabs on filter to wrap around the two outer (1/8") tabs
and solder.
·        Solder the filter middle tab (ground) to the two center fingers.
·        Using remaining finger stock pieces solder tabs to original pad
locations for input/output when viewed from top, and replace hex head screws
to secure new assembly.

This modification provides another dB or so improvement.
 With both mods the TranSystems unit is about 4db better than stock DEM
downconverter.

Misc:

·        Xtal mods using the International rock (catalog # 540288) work best
with the SMD trimmer removed, and replaced with about ¾" of #22 solid jumper
with one turn twist adjusted as needed to bring on freq.. On two units
tested final freq was within 5Khz at 144 IF without major effort. The
suggested 5pf across Xtal only moved things higher. Not a good option.
·        DC input should be 13.5vdc to let the internal 7812 do it's thing,
but 12vdc seems to work just fine.

Don, W4VQA


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