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FM-100 Page 39
K. POWER SUPPLY
Oh yes! the final section of our
project and then on to testing and
playing! Don’t rush now, this
section has many parts that are
very critical in orientation,
especially the large electrolytic
capacitors. If they are installed in
reverse, the electrolyte in the
capacitor begins to boil creating a
grand amount of pressure inside a
metal can part. Even though they
have stress relief on the tops, they
still tend to explode like a
firecracker. After all this work we
certainly don’t want that to happen!
1K. Install D21, a 1N4002 type
rectifier diode (black body with
white stripe on one end or larger orange body with thick leads and black
stripe). Make sure the end with the stripe is installed in the same
orientation as shown.
2K. Install D22, another 1N4002 type rectifier diode. Again make sure the
striped end is installed correctly.
3K. Install D23, a 1N4002 type rectifier diode (orientation!).
4K. Install D24, another 1N4002 type rectifier diode (line end installed
correctly?) The four diodes we installed form a bridge rectifier providing us
with a DC voltage from an AC source.
5K. Install C48, a .01uF ceramic capacitor (marked .01, 10n or 103).
6K. Install C49, a .01uF ceramic capacitor (marked .01, 10n or 103).
7K. Install C53, a .01uF ceramic capacitor (marked .01, 10n or 103).
8K. Install C54, another .01uF ceramic capacitor (marked .01, 10n or
103). These four diodes that we just installed prevent 60Hz hum in a high
noise environment like a transmitter. When the diodes are busy switching
on and off during an AC cycle, the RF impedance of the power supply
changes slightly, causing amplitude modulation in the RF on the power
supply (there is always going to be some present in varying degrees). This
RF modulation is detected in some circuits like opamps and the BA1404
causing 60Hz hum and other noises. The capacitors reduce the problem
dramatically when placed across each diode.