Engl INVADER II Operator's Manual page 32

-> You may be looking at a faulty tube or another defect. In this case,
be sure to take the preamp to an authorized, professional service center.
The speaker is emitting loud humming noises:
-> Is the Ground Lift switch (42) set to Ground? If you are operating the
amp without other grounded gear (power amp, effect devices) connected,
this switch must be set to the Ground position; otherwise, it's goodbye silence,
hello humming! In this case, the amp will hum even without a guitar connected.
-> The amp and mains grounds are not connected properly or are altogether
disconnected. Have an experienced specialist check this.
-> Cords connected to the input or effect loops may not be shielded properly.
Replace them to check if this is indeed the case.
-> The amp or speaker cords may be picking up interference from powerful
magnetic fields (for example, of nearby power transformers or electrical motors).
Reposition the amp and connector cables.
-> The amp or speaker cords may be picking up radio signals, for example,
from activated mobile telephones or powerful local transmitting stations nearby.
Switch off mobile phones while troubleshooting noise problems.
-> Note that feeding the amp's signal via the Out (60) or Line Out (61) jacks to a
tuner or ancillary power amp may create a ground loop. Set the Ground Lift (41)
switch to the
Issues or problems when switching channels or the Sound (1) and High Gain (19)
functions:
-> If the Sound Wizard module is installed and enabled on the amp,
first disable the Sound Wizard and then try switching that function again.
-> If that doesn't fix the problem, try detaching the Sound Wizard module from
the amplifier by unplugging the connection between the two. To do this,
first disconnect the amp from the mains power, let it cool down if it was
powered up, and remove the rear panel by unfastening the screws and turning
the left release lever on the box header to the left and the right lever to the
right to release the IDC connector.
Replace the rear panel and then try switching that function again.
-> If this fails to rectify the problem, definitely take the amp to an authorized
service center staffed with professionals who earn their living handling
screwdrivers and soldering irons.
The electronic power amp protection circuit has tripped:
-> The given power tube is defective and must be replaced if the electronic circuit
breaker continues to trip after several attempts to reset the Tube Monitoring
System by flipping the Standby switch off and back on again after a brief pause.
-> The amp has been overloaded, perhaps by excessive volume levels,
mains over-voltage, or the wrong output impedance (the impedance setting
does not match the connected speaker's impedance).
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Ground Floated position.