16
U.420 4-Channel Line Mixer
U.
4
20 4-Channel Line Mix
er
Appendix A: Service Information
• If you are playing a guitar into channel 2, 3, or 4,
you need to use a DI box for the correct impedance
match and high frequency fidelity.
• Try the same source signal in another channel, set
up exactly like the suspect channel.
• Only phono-level sources (turntables) should be
connected to the RCA phono inputs [3]. The phono
section requires your cartridge to be a moving-mag-
net type. It may be too low to amplify the low levels
of a moving-coil type.
• If there is no phono output, check that there is
nothing plugged into channel 4’s line inputs. The
phono inputs are internally disconnected if any-
thing is plugged into channel 4’s line inputs.
• Only line-level sources should be connected to the
line-level inputs [2] of channels 2, 3, and 4.
• If possible, listen to the signal with headphones
plugged into the input source device. If it sounds
bad there, it’s not the mixer causing the problem.
Bad Output
• Is the associated level control (if any) turned up?
• If a left output is presumed dead, switch the left
and right cords at the mixer end. If the problem
stays on the left side, it’s not the mixer. but maybe
your left speaker, left speaker cables, or left ampli-
fier channel.
Noise/Hum
• Check that your turntable’s audio ground wire is
connected to the ground terminals [4].
• Turn down each channel, one by one. If the noise
disappears, it’s coming from whatever is plugged
into that channel. Check your whatever.
• Check the signal cables between the input sources
and the mixer. Disconnect them one by one. When
the noise goes away, you’ll know which input source
is causing the problem.
• It helps to plug all the audio equipment into the
same AC circuit so they share a common ground.
• If the annoying humming continues, ask the drum-
mer to leave the room, or at least have him learn
the words.
If you think your U.420 has a problem, please check
out the following troubleshooting tips and do your best
to confirm the problem. Visit the Support section of our
website (www.mackie.com/support) where you will find
lots of useful information such as FAQs, documentation,
and user forums. You may find the answer to the prob-
lem without having to send your U.420 away.
Troubleshooting
No Power
• Make sure the external power supply is securely
seated in the DC input [8] and plugged all the way
into the AC outlet.
• Make sure the AC outlet is live (check with a tester
or lamp).
• Is the power LED [18] illuminated? If not, make
sure the AC outlet is live.
• Are all the lights out in your town? If so, contact
your local power company to get power restored.
• If no LEDs are illuminated, and you are certain
that the external power supply is working, it will be
necessary to have your mixer serviced.
There are no
user serviceable parts inside.
Refer to “Repair” on the
next page to find out how to proceed.
Bad Channel
• Check the connections from your source are made
securely, and that the cords are in good condition.
• Check the channel’s program gain controls [12] are
set correctly.
• Are that channel’s EQ controls all turned down to
the kill position perchance?
• Is the signal source turned up? Make sure the
signal level from the selected input source is high
enough to light the level set LEDs [10] when ad-
justed with the gain controls.
• If you are playing a guitar or other instrument level
source into channel 1, make sure the instrument
switch [11] is engaged.
