Last night I was digging into my RX-320 to address the "no
antenna attached S-5 S meter reading" problem I've been
seeing. I just happen to have the schematics up when the
Jupiter vs. RX-320 question came up. What I suspected as the
cause of the S5 minimum reading was the IF gain inside the
RX-320 being too high. The RX-320 does not have a IF gain pot
like the Pegasus/Jupiter but instead has just a fixed value
resistance in a nearly identical circuit to the Peg/Jup for
3904 gain stage just after the 455 kHz IF filter.
So I unsolderd the 10 ohm emiter resistor from Q17 (the first
3904) and temporarily attached a pot. I then fired it up and
adjusted the S reading back to the S0-1 area. I then attached
my signal generator to make sure I didn't kill the MDS. I was
able to hear a -137dBm 10M signal just fine. It wasn't as
loud with the new IF gain setting, but the S/N ratio
subjectively seemed the same. The value I came up with was 80
ohms. I found a 68 ohm 0805 sized surface mount resistor and
solderd it in the place where the 10 ohm resistor was. The S
meter is now closer to S1 with no antenna attached, much
better.
So if you have a RX-320 that has a perpetually high S meter
reading without an antenna attached you may want to consider
this tweak. The resistor value may be some different from
radio to radio, so you would want to do the temporary pot to
find the ideal value. You also don't want to be too squeamish
about working with 0805 sized surface mount parts. Since it
was just one target component and I have an ample supply of
0805 chip resistors I dug in.
And as a side effect of this IF gain reduction the amount of
attenuation that I need to put in line to keep it sounding
decent on 80M is noticeably less. Overall the RX-320 simply
has way too much RF/IF gain for the lower HF bands and with a
decent antenna.
Standard disclaimer: If you attempt any such mod, you do so
at you own risk.
Duane
N9DG
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