5
* The voltages from the sensors are logic 'OR'd' through D1, D2, D3 and D10 into latching
comparator U1a, PIN4. This voltage also controls U2 to drive Q1a and Q1b, which supply
the cooling fans. The fan speed is proportional to temperature. U1b forms an additional
comparator to switch relays RLY2, RLY3 via CONN1 PIN4 (on PCB712) at
approximately 90°C. This reduces the output stage supply voltage to limit the available
power.
* If the temperature rises above around 100°C, U1a, PIN2 will swing low when PIN4
voltage exceeds PIN5 voltage and latch in that state because of R7, D5.
* The speaker outputs will be disconnected via D6, U1c, Q2A and Q2b and RL4 and RL5.
* D109 (D209) reduce the voltage on Q101 (Q201) drain to about 1.5V thus reducing the
LDR resistance to very low, attenuating the audio signal.
b. EXCESSIVE DRIVE
* The limiter circuits prevent ANY continuous excessive drive condition, provided that the
INPUT signal is less than +18dBu.
c. OUTPUT SHORT CIRCUIT
* In the event of a short circuit or very low impedance load being present on the amplifier
output, excessive current will pass through the sensing resistors R156 (R274), R151
(R253) on the emitters of transistors Q112 (Q216), Q107 (Q209). The voltage developed
across the resistors will cause conduction of Q114 or Q115 (Q214 or Q215), which will
pull current through the relevant OPTO coupler LED, OPT101 or OPT102 (OPTO201,
OPTO202). The output transistor of the relevant OPTO coupler will cause comparator
U1d to change state and latch (due to R12, D9). This will disconnect the speaker output
relays RLY4 and RLY5 via D7, and mute the drive signal. (See Temperature, para. a.)
d. DC SHIFT
* R24 and R25 detect any DC shift at the output terminals. If the average voltage on
either output connection differs from 0V by a significant amount (approx. 100mV) U1c
output will change state (high to low) disconnecting the speaker outputs via RL4 and RL5
and muting the drive signal via the limiter circuit. Spurious operation due to AC signals is
prevented by C18, C19.
6. STATUS LEDs – PCB703 and PCB701
* The status LEDs are mounted on PCB703 (display panel).
* The blue signal LEDs are driven from the signal to the gain controls, via DU1a (DU1b) on
PCB703.
* The remainder of the LEDs are driven via signals or voltages from PCB701 via CONN5
(ribbon cable).
* The -3dB indicators are driven from U5a and U5b. (See paragraph 3 above.)
* The limit indicators are driven from U4a and U4b.
* The protection indicator (A/P) is driven from CONN6 PIN5 on PCB701, which connects to
U1c PIN14 on PCB700.
7. SWITCHMODE POWER SUPPLY – PCB712
The mains input, via in-rush and RF filter circuits, and switching is bridge rectified by BR1 and
smoothed by C6 to C9.
* PTC1 is a protection device.
