protection system. R1 is connected via D11, D12 to the +/- 18V AC secondaries of the mains transformer. The
union of D11 and D12 will have a negative going full-wave rectified version of the secondary voltage. This is
averaged by C11 to a negative DC voltage, reverse biasing D2 and, therefore, has no effect on the protection sys-
tem. Should the power be turned off, C11 will be rapidly charged towards the +24 rail via R4 (and D1, C10 which
hold up the voltage while the supply discharges), forward biasing D2, resulting in the same action as for a DC
fault in the amplifier.
POWER SUPPLY
The amplifier operates from nominal (off-load) +/- 65V (XLS 202), +/- 80V (XLS 402) or +/- 93V (XLS 602) sup-
plies, with an aux/- 24V supply for the protection circuitry and the optional AMPSAP card. To generate the
supplies, the mains transformer has one secondary with a center tap, two 18Vac taps and the usual end taps. The
end tap outputs are rectified by BR1 and smoothed by C1,C3,C5,C7 to get +HT and C2,C4,C6,C8 to get -HT
(note: not all of C1-C8 are installed on XLS 402 and XLS 202). The +/- 18Vac taps are rectified by D13-D16 and
smoothed by C17 to get +24Vdc and C18 to get -24Vdc.
SOFT START SYSTEM
AC mains enters the board on fastons (Red, Black, Blue) and leaves for the transformer by way of
connectors Main1 (220/240) and Main2 (110/120). The soft-start components include R22, PTC1, PTC2 and the
contacts of RLY1. At switch-on, R22 limits surge current and PTC1, PTC2 protect R22 from overheating. After a
few seconds, the protection circuit energizes RLY1 coil, closing RLY1 contacts which short out R22, PTC1, PTC2,
allowing full power operation.
FAULT-FINDING HINTS
When powering-up a unit after repair, undetected faults may result in further damage when the unit is retested. To
minimize the risk of damage, please follow this procedure:
1) If the unit is set to 220V/110V, lift the 'RED' faston and connect a 100W mains lamp between the
RED wire and the 'RED' board mounted faston. If the unit is set to 240V/120V, lift the 'BLACK' faston and connect
a 100W mains lamp between the black wire and the 'BLACK' board mounted faston.
2) Power up in the normal way. The lamp will initially glow brightly, and then dim down as the internal capaci-
tances become charged. The unit may then be functionally tested with no load connected. Once the unit is operat-
ing correctly, the mains supply may be applied to the unit as normal, and the unit may be load tested.
In the event that the lamp does not dim down, this indicates that a major fault still exists, which must be
remedied before full mains may be applied.
LOCATING MAJOR FAULTS
Major faults resulting in high current draw (as indicated by the series lamp refusing to dim) can be isolated as
follows:
1) The secondaries of the transformer can be unplugged from the board. Should the fault persist, this
indicates the fault to be with the mains transformer. However, this may not be the only fault.
2) It is now necessary to unplug and remove the board.
3) A faulty channel may be isolated to only a few possibilities
•
A faulty power supply capacitor
•
A faulty output device
