TL Audio VTC - Manual (Page 47)

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CD1347.
Following the balanced receiver stages IC2A and IC2B, the return is taken to the balance control,
RV3, and buffered by IC4A and IC4B. The sensitivity of the returns is +4dBu by default, but may be
changed to -10dBu by fitting jumpers J1 and J2.
The stereo returns are controlled by a 60mm fader, followed by a +10dB buffer (IC3A and IC3B),
and routed to the L+R sub-mix. Additional switches on PC347 also provide AFL monitoring and
routing to the headphones.
4.8. Oscillator.
The oscillator circuit is contained on PC348. Refer to CD1348 sheet 3.
Two oscillators provide 1KHz and 10KHz waveforms. The 1KHz is generated by IC1B, with level
trimmed by RV3. Similarly, the 10KHz is generated by IC1A, with level trimmed by RV2. These
trimmers are located near the sub-front for PC348, but can be accessed and adjusted by propping
open the upper master front panel. The oscillators are routed to the groups and L+R mixes pre-fade
by the “On” switch SW5. The trimmers should be adjusted to give +10dBu at the outputs when the
faders are at their 0dB position. There is no provision for trimming the oscillator frequencies.
When the oscillator switch is in the “Off” position, the gain of the IC1A and IC1B stages is reduced
by the signals E1,E2,E3 and E4, causing the oscillators to shut down, to eliminate any risk of
crosstalk. The oscillators can therefore only be adjusted when SW5 is in the “On” position.
4.9. Talkback.
The talkback mic amplifier and routing is also located on PC348. Refer to CD1348 sheet 4.
The input from the microphone is on screened cable, directly from the socket on the lower master
module to CN1 on PC348. This connector carries a further connection from the logic controller,
PC351, which is the talkback on/off signal derived from the talkback “On” switch near the
microphone. Care must be taken when removing either master module to ensure that this cable is not
damaged.
The microphone signal is amplified by IC6, with gain controlled by RV8. The amplified signal is
only passed to the routing switches if the logic control signal is at 1 (+15V). A logic 0 (0V) will mute
the talkback signal. The talkback control logic may be either momentary or latching, depending on
jumper configuration on the logic PCB (see below).
Talkback can be routed to the groups plus L+R busses, studio and headphones. Both the routing
switches and “On” button must be enabled to route talkback through to the outputs.
4.10. Headphones.
There are two similar stereo headphone circuits, located on PC349. Refer to CD1349
.
Each headphone circuit has five sources selected by the push buttons on PC349. These sources are
Control Room, MixB, Aux 3/4, Aux 7/8 and the EXT input. The sources are additive, i.e. more than
one source may be selected and they will be mixed together. The mix points “PHONES 1L” etc. are
linked out onto the upper master module inter-connect IDC cable, to enable the headphone sends
from the stereo returns to also be added. The Aux 3/4 and 7/8 sources route the odd numbered Aux to
the headphone left output, and the even numbered Aux to the right output, to facilitate stereo