5.4.2 Auxiliary Stereo Bus Lines
If auxiliary stereo working is provided for, the left and right auxiliary stereo
bus lines which are fed from each channel of each of the multi-track Main Cassettes
are connected to the left and right inputs of an extra Main Cassette. The main
outputs of this feed the auxiliary stereo tape machine(s). The auxiliary stereo
channels of the extra Main Cassette are not connected to anything, and the auxiliary
stereo channels of this cassette are completely unused (the level controls should be
left at OFF) but they are fitted so that all Main Cassettes are interchangeable.
5.5
Metering
5.5.1 Meter Connections
The two VU Meters associated with a Main Cassette have a 7-pin plug on a
flying lead, which is inserted into socket SK3 of the cassette. On this socket the +20
and –20V supplies are available for test purposes.
5.5.2 Meter Amplifier
Each VU Meter is driven by an Amplifier U, the circuit of which is described in
12.19. The amplifier has an output impedance of 3.9k
Ω
so no build-out resistor is
required. When the standard internal signal level of –10dBV is applied to the
Amplifier U, the corresponding VU meter will give a reading of 0VU. Push buttons
PB1 and PB2 modify the feedback of both amplifiers to boost the gains by 10dB and
20dB respectively.
5.5.3 Meter Selector
The inputs of amplifiers U are connected to the slider of the meter selector
switch, which has the following positions:
1. Off
2. Record/Replay
3. Input to Main
4. Check Group
5. Check CRM
6. Check SP
7. External
8. Oscillator
The switch employed is of the shorting type. In order to prevent two
measuring points being shorted together when the switch is being operated,
alternate studs are provided with 2k
Ω
series resistors which are mounted inside the
switch. This is low, compared with the input impedance, and therefore introduces
negligible error.
In the “off” position the studs are directly connected to the 0V line. In the
“record/replay” position connection is actually made to the moving contacts of the
follow/replay key which is in the Control Room Monitor Cassette. With this key and
the record/replay key suitably set the V.U. meters can measure either record or
replay level. The record signal is supplied to the keys from the output of the
