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Drake 2880



Good Morning to All,

I have completed my latest round of tests on the Drake converters.  To
recap - I stripped all components except coupling and bypass caps and all
tuned circuits except the interdigital filter and installed a MAR-3 MMIC
in one unit.  In the other unit I installed a NE42484A, changed the drain
resistor from 120 ohms to 220 ohms and put a short stub (approx 1mm x
2mm) on the input line before the hairpin and removed the short across
the bottom of the hairpin.  I removed the coils and SM caps from the IF
amp and extended the elements of the interdigital filters by about 2 mm
in both units.  I also started with a virgin unmodified unit.  Gain
comparisons for these units were:

The NE42484A was +15 db compared to stock Drake.
The MAR-3 was +9 db compared to stock Drake.

I then removed the IF coils and caps from the new unit,  extended the
elements of the interdigital filter and scratched out the short across
the input hairpin.  I compared the gains again with the following
results:

NE42484A was +4 db compared to modified Drake.
MAR-3 was -4 db compared to modified Drake.

I also attempted some weak signal measurements by putting all my
available microwave attenuators on the input to the converters and
comparing the outputs.  The Modified was (maybe??) slightly better than
the MAR-3 unit.  The NE42484A unit was (probably??) better than either
one of the other units.  It is difficult to quantify the differences here
since I was using a radiated signal form my weak signal source but I
could see a little difference with the NE42484A unit.  Incidentally I
used the waterfall display in DIGIPAN (PSK-31 software) to assist my ears
in this test.

Bottom line is that I do not think the difference in any of these units
is enough to prevent hearing AO-40.  The unmodified unit is pretty bad
without some modification although people have used it as is with a
preamp and copied the beacon from DOVE and UO-11.  If you use a preamp
then the MAR-3 device should work fine (although it requires major
surgery to install).  I plan to modify the MAR unit with the G0MRF mods
but I am going to use it for some more antenna tests before I do that so
it will be a little while before I get that comparison.

Incidentally, the "loose" pieces inside the converter go on top of the
RF/IF cover (which is aluminum coated plastic).  My guess is that the "U"
shaped shield was supposed to insure contact between the shield and the
cover.  The small piece of aluminum goes underneath the "U" shaped piece
and I assume that is to assure contact with the cover.  The hole?  Maybe
to make some sort of measurement in production.

Good luck to all with the new units.

73 De Jess - W4MVB

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