Probably the most effective mod in the WB3JZO Nov. '86 article is a
capacitor change in the 40823 product detector audio out circuit area.
The board involved is the IF-AGC board - board 80785 in the Argosy, 80279 in
the 544, and 80448 in the 546.
For the 525/Argosy, change C34 from a .01 to a .1 (use 35V ceramic)
In the 544 this cap is C17, and in the 546, it's also C17. Change to the .1
ceramic.
This mod should reduce the noise floor considerably by rolling off more of
the notorious TT audio hiss in these older rigs . As an alternative to the
cap change, the article detailed a more sophisticated, more effective mod.
This involves insertion of a 2nd order low pass audio filter consisting of
two new resistors, two new caps, and a 470mh ferrite inductor (J.W. Miller
available thru Digi-Key). A SASE will get you a drawing of the circuit.
Another cap change could benefit AGC action and reduce popping in some
rigs - C12 on the same 525 IF/AF board gets changed to a 22uf 35V tantulum.
In the 544 and 546, experiment with C22 on the IF/AGC - try out the 22uf
value.
The 544 and some 525's may have an IC rx mixer. Where a discrete diode mixer
was employed, changing the diodes to closely matched Hewlett Packard
5082-2800 Schottkys should help IM and noise floor performance. Al, WA2VEZ,
and Gary, N9WR say the 1N5711 is an equivalent for the 5082-2800.
Gary has found a small quantity source for the 1N5711's, bought some, and
found four closely matched diodes by comparing forward voltage drops with a
good DMM.
Some of the fellows have reported finding cap and diode changes already made
in their rigs. Production changes or previous owner mods.
WB3JZO / KW7CD detailed more mods, and an SASE get you info I can't detail
here. Thanks to Don, N9WR, for the additional 525 info.
73, GL
Larry Kozal K8PUJ
2500 Michigan St. NE
Grand Rapids, MI 49506
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