My Argonaut 509 seems to have a serious receiver sensitivity problem.
The other night when 80M was fairly noisy, I was getting noise at S6-S8
on an FT817, hooked the same antenna to the 509, an even with the RF
gain all the way up, the S-meter baorly moved, if at all. RF noise is
generally not enough to move the S-meter, and I've made several easily
understandable QSOs with no motion of the S-meter. The S-meter is
working, with very strong signals it does occasionally get up to S-9. I
originally thought the problem was the first stage dual-gate MOSFET, but
I'm pretty sure that's not the problem at this point. When
troubleshooting the first stage MOSFET, when I hooked a DMM lead to gate
1 I get a stronger signal (not just noise, but signal as well) from the
test lead than I do from the antenna connected to the unit. On the RF
front-end board, the antenna signal goes through an input cap, a
transformer/cap combination that's a 9MHz trap (VFO), through a tuned
transformer which is part of the pre-selector, through a coupling cap,
and to gate1 on the MOSFET. Connecting the DMM lead directly to gate1
causes an increase in signal & noise, connecting it anywhere before the
coupling cap results in no change in signal level. I changed the
coupling cap, but there was no change in performance.
Anyone familiar enough with the front-end on these rigs to have an idea
as to what I can check next?
Thanks, AJ4LL
--
"The tree of liberty must be refreshed from time to time with the blood of patriots
and tyrants. It is it's natural manure."
Thomas Jefferson, 1787.
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