Studio Technologies IFB Plus 2005 - Manual (Page 36)

Issue 8, December 2005 Model 2 User Guide
Page 36 Studio Technologies, Inc.
Power Supply
The Model 2 Central Controller provides
+18 Vdc modulated with channel 1 audio
on pin 2 of the XLR connector. Channel 2
audio is provided on pin 3, with pin 1
supplying common. From the Model 2,
the nominal audio levels are –10 dBu. A
3-terminal adjustable integrated circuit volt-
age regulator is configured to provide +10
V, nominal, from the incoming +18 V that
is modulated with channel 1 audio. The
important characteristic about this circuit is
its constant input impedance characteristic.
The input impedance is fixed at a moderate-
ly high value, approximately 500 ohms, and
does not vary appreciably with load. This is
important so that the channel 1 audio signal
is not significantly attenuated, nor distorted
by normal fluctuations in the power draw. An
LED indicator light provides indication that
+10 V power is present. A zener diode, in
series with the LED, is used to generate a
5.6 V reference for the circuitry.
Mixer/Output Amplifier
IFB channel 1 and 2 audio is capacitor-
coupled to two audio-taper potentiometers.
Resistors in series with the pots ensures
that the audio level can never be fully turned
“off.” The output signals from the pots
connect to an NE55320-type operational
amplifier that is configured as a combining
amplifier. The faithful ’5532 is used both to
mix the audio sources, as well as to drive
the audio output. Signal enters the op-amp
via a switch that selects which of he two in-
put sources are active. Note that the switch
actually selects one source by muting the
other! The op-amp’s output is capacitor
coupled, via a series resistor, to the output
connector. The capacitor serves as a DC
blocking function, changing the output so
as to be referenced to signal common. The
series resistor helps the op-amp maintain
stability, as well as providing protection in
the event of a shorted output connecting
cable.
Troubleshooting
Troubleshooting the Model 2 and related
components is to be performed only by a
qualified technician. Operating the Model
2 with the cover removed exposes you to
hazardous voltages.
Block diagrams are located at the end of
this guide. Schematic diagrams are avail-
able upon request. These documents pro-
vide assistance during maintenance and
repair. Understanding the schematics and
block diagrams will lead to much better
understanding of the individual circuits and
how they work together (or, in case of trou-
ble, how they don’t work together!)
Model 2 Central Controller
The Model 2 consists of many relatively
simple circuits. Detailed troubleshooting
procedures are difficult to describe because
there are few specific circuits that are prone
to failure. The following paragraphs may
give a few helpful hints if problems do arise.
In all cases, you are welcome to contact
Studio Technologies for technical support.
Wiring Errors
The Model 2 provides +18 Vdc for use by
the Model 22 Access Stations. Up to four
access stations can be connected, drawing
a total maximum of 100 mA nominal from
the +18 Vdc supply. The +18 Vdc is pro-
duced by a 3-terminal regulator. Should the
+18 Vdc get shorted to common up to one
amp of current may flow through the regula-
tor. By design, the +18 Vdc regulator does
not use a heat sink. This helps the device
heat faster under excessive current flow,