Studio Technologies IFB Plus 2001 - Manual (Page 43)

Model 2 User Guide Issue 7, November 2001
Studio Technologies, Inc. Page 43
are prone to failure. The following para-
graphs 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 +18Vdc for use by
the Model 22 Access Stations. Up to four
access stations can be connected, draw-
ing a total maximum of 100mA nominal
from the +18Vdc supply. The +18Vdc
is produced by a 3-terminal regulator.
Should the +18Vdc get shorted to com-
mon up to one amp of current may flow
through the regulator. By design, the
+18Vdc regulator does not use a heat
sink. This helps the device heat faster
under excessive current flow, 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 +18Vdc be “dead shorted”
the monitor performance will also suffer.
The talent amplifier output also uses an
adjustable 3-terminal regulator to create
+18Vdc modulated with channel 1 audio.
In this case, the design required the use
of a heat sink. Like the +18Vdc supply for
the access station, a shorted talent ampli-
fier output 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 +34V
UNREG and +25V UNREG can vary
greatly from the nominal value.
There are two very common reasons why
a power “rail” can go down: defective
electrolytic capacitor and a shorted power
bus. With time, over-temperature, etc.,
an electrolytic 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, such as a
TL072, can draw enough current to cause
a regulator to go into the thermal shut-
down mode. Eliminate the short, wait a
minute or two, and the supply should
come up again.
Audio Circuitry
The Model 2 uses straightforward audio
circuitry 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.
Telephone Interfaces
Each telephone interface contains a
protection fuse. Each fuse is electrically
connected between the ring lead of the
modular jack and the transient suppressor
(varistor). The fuse will open (blow) should
the interface be exposed to extremely
large current flow. These currents can
occur because of a “mega” lightning hit,
or because of a high voltage power line
“cross.” Should a telephone interface fail
to function be certain to check the fuse
first. Perform any repairs only after the
Model 2 has been disconnected from