Ampetronic MLD5 Installation Handbook
Page 9
• Check that nothing is blocking the free flow of air through the vents at the front and back of the
unit.
• Check the loops are not open circuit, short circuit, or connected to earth (see instability or high
frequency noise section of Troubleshooting)
LOOP LED
(
s
)
illuminated
Indicates the loop cables connected to the unit are outside the loop resistance test limits (see
technical specifications). If these values are exceeded the LOOP LED(s) will illuminate and the
amplifier output(s) will be disabled.
Important: Once the cause of any loop error has been established the unit will need to be restarted.
Turn the power OFF - wait 5 seconds, and switch ON.
• Check that the loop cables are connected, terminated correctly and inserted into the LOOP
OUTPUT sockets. Restart the amplifier as above.
• Ensure the DC resistance is between the upper and lower loop resistance test limits (see
technical specifications).
Note: Meter leads can easily measure 0.2Ω, and should be factored out.
• If either loop is open circuit - it may have been cut somewhere and will require investigation
and potentially re-laying.
• If either loop is high resistance >5Ω - check all connections and re-tighten, there may be
something loose.
• Check for short circuits in the loop connectors.
• If either loop has very low resistance, use an appropriate length of cable to ensure the loop
resistance is greater than the lower loop resistance test limit (see technical specifications).
Note: High or low resistance loops can be encountered in a venue with a pre-installed loop, which
was designed for use with an old loop driver which might be being replaced. Before using this loop
ensure that the existing loop will allow the system to perform to an acceptable level by checking it’s
layout details in the ‘Designing Induction loops’ handbook.
Low magnetic field strength
Due to insufficient CURRENT or excessive metal loss.
May require a special loop design to achieve acceptable performance, contact Ampetronic for
advice.
Instability or high frequency noise
1. It is possible for the loop cables to be grounded under fault conditions, resulting in instability
which may sound like high frequency noise, buzz or whistling. The results of this type of fault
are unpredictable and may appear as any combination of the front panel indication errors
2. This fault is easy to determine. Simply unplug the loop cables from the amplifier and test
with a resistance meter between either of the loop wires and a good earth point such as a
metal radiator. There should be an infinitely high reading i.e. no connection at all. Any reading
indicates a failure of the loop cable insulation and you will need to either repair or replace the
loop cable.
3. Instability can be caused by using poor quality signal cables, long unbalanced (2-wire)
signal runs to the inputs, or by running input cables in close proximity with the loop wire over
an appreciable distance. Loop amplifiers are capable of delivering high currents at audio
frequencies. If the loop cables are run close to sensitive signal cables it may be possible to
induce a signal back into the input of the amplifier causing feedback. Cable runs and loop
