Here are some experiments to try on your SW40+
receiver to help determine noise sources in
the rig. Just how much audio noise is internally
generated, and how much is due to RF noise coming
in the antenna?
Experiment #1. Pull out U3(SA612) with the power
off. With the rig powered back up, you'll hear
low-level audio hiss. Take a mental note of its
level, or measure its amplitude on your scope.
This hiss is entirely due to noise being
generated within the high-gain audio op-amp
stages of U4. It isn't very loud.
Power off, replace U3 (careful to place
pin 1 correctly) and power up. Compare the "hiss"
level: significantly louder. Even though the
bulk of this rig's gain is in audio stages, most
noise you hear is RF noise. Good.
You could do this experiment as well
by carefully jumpering pin 4 to pin 5 of U3. I
did this with power on, holding a short insulated
wire. Be careful not to short adjacent to pins.
Experiment 2: Take an electrolytic capacitor
(I had a 47uf/16v handy) and bypass U3 pin 4
to ground. GET THE CAP'S POLARITY RIGHT!
Try bypassing pin 5 to ground too.
Since half the audio signal comes from
pin 4, and the other half from pin 5, you'd
expect noise level to drop by 3dB.
But I hear noise level go up! Careful
listening revealed that noise BANDWIDTH went up
too. What's going on?
This is tricky (hope I've got it right). If you
listen carefully, you'll hear the narrowband
noise (400Hz wide) drop in level, as expected.
But wider bandwidth noise is introduced that
brings the apparent overall noise level up.
Where did this noise come from? I believe that
it comes from U2, the 78L08 regulator. It
sends wideband noise into U3's power supply
pin 8. This noise shoots thru a 1500-ohm
resistor (inside the chip) into the audio amp.
When you DON'T short one of U3's outputs to
ground, this noise appears IDENTICALLY at BOTH
outputs of U3. Since U4a is a differential
amplifier, it rejects U2's noise as a
common-mode signal. U4a only amplifies
difference-signals between pin 4 and pin 5 of
Experiment #3. Listen to the noise level while
you unplug your antenna. Noise should drop. This
is a quick-n-dirty test to determine if a
receiver has enough "sensitivity". With no
antenna, you're listening to noise generated
entirely within the receiver.
Here are some other "safe" points in the
receiver that you could try shorting to ground
with a short insulated jumper:
-link winding of T1 (J1 pin 3)
-either end of RFC1
-Junction of Y3(4MHz) and C15(150pf)
Glen VE3DNL email@example.com