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1) Any amplier channel pulls current through the SOA_PROTECT line for long enough to charge
up capacitors C5871 and then C5872 so that Q5874 turns on.
2) Any amplier channel pulls current through the OVERLOAD line for long enough to charge up
C5882 and turn on Q5882 and thus Q5884.
3) Any amplier channel has a large long term (DC) oset (typically greater than +/-3V) sμFcient
to charge up C5861 enough to turn on either Q5862 (positive oset) or Q 5864 (negative oset).
These then turn on Q5863. N.B. This circuit is also used to detect imbalances in both the Vcc/Vee
and +/-15V power supplies (the latter is generated on the Power Supply board).
4) When the PTC thermistor TH585 mounted at the top centre of the heatsink gets sμFciently hot
(around 100C) and thus high resistance enough to cause Q5855 to turn on via the +12V supply.
Intermediate temperatures will not activate PROTECT but will provide signals to the level detec-
tors associated with the FAN_1 and FAN_2 lines, to run the cooling fans at high or low speeds
respectively.
Note that the fans’ 12V supply is gated via Q5909 and Q5911. This means the fans will not run when
no signal is present on the FL, C or SR channels, so that during quiet passages no fan noise should be
audible.
AVR400 Main Power Supplies Circuit Description
The main DC power supplies are located on the Power Supply Board - a double sided PTH PCB adjacent
to the Main Board. Note that the mains input switching, mains fuses, standby power transformer with
its associated unregulated DC supply and the relay for enabling the power amplier supply rails are
located on the Main Board. The two boards communicate via CON501.
Considering the Main Board rst, the mains voltage switching uses a double pole double throw slide
switch accessible from the back panel. One pole addresses the standby transformer and the other the
toroidal power transformer. Ensure the switch setting matches the supply voltage before switching on the
AVR400. Nominal settings are 115 and 230V +/- 15%. The 20mm 115V fuse in line with the toroidal trans-
former is rated at 15A T (Time delay) and the 20mm 230V fuse is rated at 8A T. Always replace these fuses
with the same type and value. The standby transformer T5941 is not fused but is designed to go open
circuit in case of overheating (e.g. if left connected to a 230V supply for longer than a few minutes when
the mains voltage selector is set to 115V).
The standby transformer generates approximately +9V DC via the bridge rectier diodes D5495/6/7/8
and the 1,000μF reservoir capacitor C5947.This is sent via pin 7 of CN501 to the Power Supply PCB
(confusingly marked as 5V). The rail voltage is also routed to the system microprocessor (μP) via D5965/
R5965 and pin 6 of CN103 as POWER_MUTE.
The 5V relay RY594 is normally open. When the mains switch is closed then the SUB_POWER rail (approx
+4.3V) is activated from the Power Supply PCB via the standby transformer. When the system has
booted correctly, without any shutdown signals, then the POWER_RELAY signal from the system μP also
goes high, pulling down Q5947 hard. Only then does the relay close and switch on the main toroidal
power transformer, enabling the rest of the system to boot up.
Now consider the Power Supply Board, found on page 14 of the schematic. This generates all the main
DC supplies for the AVR400 except +Vcc and -Vee for the power ampliers and the non-logic part of the
VFD display requirements of the Front Panel Board. Note that additional local regulation also takes place
on the other PCBs, e.g. for large digital ICs.