7
VSP 12-Way Batteryless Telephone System
Technical Manual
A100K10873
2.3
Amplifier
Only the Main Station VSP-211-L has a built-in microphone amplifier.
This amplifier is used by all stations in the system.
● The capacitor C7 will be charged and act as the battery power for the
transistorized amplifier.
-
The voltage across C7 is limited to 39 VDC by the Zener diode Z1.
-
The voltage will slowly decrease to less than 20 V during a period
of 15 to 20 minutes. Despite the change in voltage level, full
amplification will be obtained during this period.
● The amplifier quiescent current can be measured by replacing the
strap P6 with a uA-meter.
-
The quiescent current is set to Iq = 20 uA at U = 20-30 V when
delivered from the factory and can be adjusted by RV1.
2.4 Call Signal
2.4.1 Stations for Use in Safe Areas
● The oscillator will start when the hand crank is activated and the DC
voltage exceeds Zener diode Z2 limitation (30 V).
-
A call tone will be heard in the handset microphone both at the
calling station and the station being called. (In this case, the
dynamic microphone acts as a ‘loudspeaker’).
-
The calling tone frequency is 2500 Hz.
It is not recommended to change the call tone frequency because it is
adjusted to the mechanical resonance of the handset.
The call tone has a fluttering frequency at about 15 Hz because of the
crank speed.
Depending on environmental noise, it is possible to choose a high or low
level for the call tone by strapping P7 on the station PCB.
2.4.2 Stations for Use in Hazardous Areas
● No call signal is supplied in EX approved stations (VSP-512) used in
hazardous areas.
-
Calls to a station in a hazardous area will be indicated by use of EX
approved audible or visible signal units.
-
When VSP-512 is called, a relay in the buffer unit will be activated
and necessary power for the signal devices will be supplied.
2.5 Handset
The microphones in all stations are connected to the same line when
their handset button is pressed. This line is connected to the input
transformer of the system amplifier located in VSP-211-L.
The microphone line is marked 1 and 2 on the screw terminals in all
VSP units.
The speakers in all station handsets are connected to the same line
when the handset button is pressed. This line is connected to the output
transformer of the system amplifier located in VSP-211-L.
The speaker line is marked 3 and 4 on the screw terminals in all VSP
units.
