Studio Technologies IFB Plus 2005 - Manual (Page 37)

Model 2 User Guide Issue 8, December 2005
Studio Technologies, Inc. Page 37
allowing the internal overpower protection
to shut down the regulator. During the short
circuit condition the power transformer may
vibrate under the strain of excessive current.
Be aware that the monitor output power
amplifier shares this transformer. Should
the +18 Vdc be “dead shorted” the monitor
performance will also suffer.
The talent amplifier output uses current-
limited temperature-sensing components to
create the +22 Vdc modulated with channel
1 audio. Like the +18 Vdc supply for the ac-
cess station, a shorted talent amplifier out-
put will result in excessive currents, followed
by thermal shutdown. Removing the short
and allowing time for the device to cool will
restore operation.
Power Supplies
Should problems occur with the Model 2,
the power supplies are the first thing to
examine. The unit contains three indepen-
dent supplies, each with transformer, bridge
rectifier, filtering, etc. Use the test points to
ensure that the correct voltages are present.
Be aware that the test points for the unregu-
lated supply points can vary greatly from
their nominal values.
There are two very common reasons why
a power “rail” can go down: defective elec-
trolytic capacitor and a shorted power bus.
With time, over-temperature, etc., an elec-
trolytic capacitor can fail. The caps used in
the Model 2 are heavily overrated for normal
use, but still could fail. Use an oscilloscope
to examine the unregulated voltage points.
Check for excessive “ripple.” The 3-terminal
regulators are quite reliable and are not a
weak link. A shorted integrated circuit can
draw enough current to cause a regulator to
go into the thermal shutdown mode. Elimi-
nate the short, wait a minute or two, and the
supply should come up again.
Audio Circuitry
The Model 2 uses straightforward audio cir-
cuitry throughout. Normal “follow the audio
flow” troubleshooting techniques should
be used. A digital multimeter, oscilloscope,
signal generator, and audio VTVM should be
all that is required.
Model 22, Model 32, &
Model 33
The Model 22 Access Station, the Model
32 Talent Amplifier, and the Model 33 Talent
Amplifier all have fairly simple circuitry. A
careful review of the block diagrams, locat-
ed at the end of this guide, and schematic
diagrams, available upon request, should
allow rapid problem solving. All share the
condition that all signals, including power,
audio, and logic, begin and/or end with the
Model 2 Central Controller.
The first thing to determine is that the wiring
linking the units together is correct. Ensure
that the Model 22 is getting +18 Vdc for
operation. The Model 32 and Model 33 use
+18 Vdc modulated with channel 1 audio.
Use an oscilloscope to check for both the
DC voltage, and the superimposed audio
signal.