Yamaha MS400 Service Manual page 16

MS400
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INSPECTIONS
• Amplifier unit
1. Preparation
1) Input terminal
LINE INPUT (XLR)
MIC INPUT (XLR)
2) Output terminal: CN203 (VH4P)
LF 1P(+) 2P(-), RL=4 ohms / Power capacity 500W
HF 4P(+) 3P(-), RL=16 ohms / Power capacity 200W
or more
3) LINE LEVEL: MAX (10)
4) MIC LEVEL: MIN (0)
6) TONE CONTROL HIGH: Center click position (0)
7) TONE CONTROL LOW: MAX (0)
8) The input signals should be in the form of sine wave.
2. Inspection
1) Power ON muting time
Check that the muting function is cancelled and the
relay turns on in 3±1 seconds after the power switch
is turned on. Also, check that the power indicator
lights up.,
2) Idling current adjustment
TE
Adjust VR201 and VR202 so that the voltage
L 13942296513
between Pins 1 and 2 of CN206 and CN207
becomes 1±0.2mV respectively.
3) Midpoint potential
Check the DC voltage of CN203 (VH4P) when no
signal is applied. It should be within ±70mV both at
LF (between Pins 1 and 2) and at HF (between Pins
3 and 4).
4) Gain / Efficiency
Input a 500Hz, -1dBu signal to the input terminal
Line INPUT (XLR) and check that the output voltage
at the output terminal LF is +28±1dBu. Also, check
that the primary side power consumption at this time
is 180±30W.
5) Full harmonic distortion
Input a 500Hz signal to the input terminal LINE
INPUT (XLR) and check that the full harmonic
distortion when the output voltage at the output
terminal LF is 300W is THD+N=1% or less.
Input a 5kHz signal to the input terminal LINE INPUT
(XLR) and check that the full harmonic distortion
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when the output voltage at the output terminal HF is
100W is THD+N=1% or less. This measurement
should be completed within 30 seconds.
.
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