Studio Technologies IFB Plus 2007 - Manual (Page 28)

Issue 9, January 2006 Model 2 User Guide
Page 28 Studio Technologies, Inc.
Wiring Errors
The Model 2 provides +18 Vdc for use by
the Model 22 and Model 24 Access Stations.
Up to four access stations can be con-
nected, drawing a total maximum of 100 mA
nominal from the +18 Vdc supply. The +18
Vdc is produced by a 3-terminal regulator.
Should the +18 Vdc get shorted to common
up to one amp of current may flow through
the regulator. By design, the +18 Vdc regu-
lator does not use a heat sink. This helps the
device heat faster under excessive current
flow, allowing the internal overpower protec-
tion 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 electro-
lytic capacitor can fail. The caps used in the
Model 2 are heavily overrated for normal use,
but still could fail. Use an oscilloscope to ex-
amine the unregulated voltage points. Check
for excessive “ripple.” The 3-terminal regula-
tors 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. 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, sig-
nal generator, and audio VTVM should be all
that is required.
Model 22, Model 24, Model 32A,
and Model 33
The Model 22 and Model 24 Access Stations
and the Model 32A and the Model 33 Talent
Amplifiers all have fairly simple circuitry. A
careful review of the block diagrams, located
at the end of this guide, and schematic dia-
grams, available upon request, should allow
rapid problem solving. All share the condi-
tion 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 or Model 24 units are get-
ting +18 Vdc for operation. The Model 32A
and Model 33 use +18 Vdc modulated with
channel 1 audio. Use an oscilloscope to
check for both the DC voltage, and the su-
perimposed audio signal.