17
THEORY OF OPERATION
3.2 Clipping Eliminator Circuit
This circuit is controlled by the LED/LDR opto-isolator U3, located on the I/O Board. The LED
portion of this component is driven from a bridge rectifier (D4) that gets its input signal from
U100A-1 on the Amplifier Board. Under normal conditions (undistorted amplifier output) there
is almost no signal at this point. If the amplifier is driven into clipping, the signal level at
U100A-1 rises rapidly because the feedback signal no longer represents the input signal.
Once this occurs, the LED in U3 illuminates, reducing the resistance of the LDR portion which
in turn reduces the input signal. The clipping-eliminator circuit is activated by switch S2B on
the I/O Board. When the switch is off, the signal driving the bridge rectifier is shorted to ground.
3.3 DC Offset
DC offset is sensed by the comparator amplifier U2A on the I/O Board. If a DC component
should appear at the output, it is sensed through either D6 or D7, depending on its polarity.
The output of U2A-1 will switch from -14Vdc to +14Vdc, which turns on Q5 via D8. This deacti-
vates the relay, turns the READY LED red, and turns on the CLIP/PROTECT LED.
In addition, the output of U2A-1 is conveyed to the Regulator Board via D15, J100-10 to
Regulator Board J1-12 and D9. The positive voltage on U5A-2 causes the output of U5A-1 to
become less positive, shutting off the conduction current through optocoupler U1, which shuts
off the triac and primary current.
Note: The Regulator PCB is not used in the 1600VI amplifier.
3.4 Overheated Output Transistors
A thermistor (RT100) is positioned near the amplifier PCB's heatsink. As the negative coeffi-
cient thermistor heats up, the voltage on comparator U2B-6 located on the I/O PCB drops.
When it crosses the reference voltage set up by voltage divider R42/R43, U2B-7 goes positive.
This forward biases D17, turning on Q5, which deactivates relay K100. As the heatsink tem-
perature cools, the thermistor will cool until the voltage at U2B-6 once again crosses the
reference voltage at U2B-5, allowing the relay to reactivate.
3.4 Fan Speed Control
The fan operates at low speed when the amplifier is first turned on. The voltage at the
thermistor is connected to the Fan Drive circuit on the Output Board via D16 (on the I/O Board)
and J6-4/J2-4. As the heatsink temperature increases, the voltage at U1A-3 on the Output
Board decreases until it crosses the threshold set by voltage divider R4 and R6. When this
occurs, the output of U1A-1 toggles low, which turns on Q1. Q1 shorts across R1 and applies
the full VF+ voltage to the fan, kicking it into high speed.
3.5 Major Faults
The slo-blo line fuse protects the unit from further damage when a major fault such as a
shorted output transistor or a secondary power supply fault occurs. If the unit is run at or near
its rated power, the fuse will eventually blow. The rated line fuse allows the unit to be operated
without interruption for all musical applications.
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