Studio Technologies AN-2 - Manual (Page 10)

Issue 3 AN-2 User Guide
Page 12 Studio Technologies, Inc.
The first step in troubleshooting the AN-2 is
to remove the top cover (two Phillips-head
screws on each side panel), and check the
DC voltages at the test points listed on the
schematic. If this technique fails to pinpoint
the trouble, connect an oscillator to the
input and trace the signal (again, using the
schematic) until the source of the problem
is found. The following paragraphs will aid
in aligning the AN-2 circuits, which will
restore proper operation. It is important to
perform the alignment procedures when-
ever you have replaced a component in the
unit.
CAUTION: DO NOT attempt to adjust any
of the trim pots inside the unit before you
understand the entire alignment procedure.
Improper or out-of-order adjustment may
destroy some of the integrated circuits.
U9
To achieve the best common-mode rejec-
tion, feed a 1kHz signal at +4dBm into the
input connector: high side to pins 2 and 3
(short 2 and 3 together), and low side to pin
1. Check the output at TP9, and set R89 for
a minimum signal.
U3 and U14
Set R110 to 1.34Vdc at TP8 with no input
signal. Feed a 1kHz input signal at +4dBm,
set gain control to read –10dBm at TP9,
and read the AC signal at TP7. Drop the
input signal exactly 20dB. The signal at TP7
should drop 15dB. If not, reset R110. Re-
peat this procedure several times, as R110
also affects the reference level.
U1
The clock frequency of this VCO should
vary between approximately 100kHz and
375kHz, depending on the setting of P1
(stereo width) when measured at TP1
(clock set). It should produce approximately
12V, measured peak-to-peak, at TP1.
Connect frequent counter between TP2
(clock set) and common. Set P1 (stereo
width) fully clockwise. Set SW2 (modulation
ON/OFF) to the OFF (out) position. Adjust
R97 so as to read 62kHz on the frequency
counter. If it is ever necessary to replace
U1, use only the part manufactured by
National Semiconductor.
U3 and U11
Set R5 to give 1.34Vdc at TP5 with no input
signal. Feed a 1kHz input signal at +4dBm,
with gain control set as before, and read the
AC signal at TP6. Drop the input signal
exactly 20dB. The signal at TP6 should
drop 20dB. If not, reset R5. Repeat this
procedure several times.
U6
Set R46 for minimum THD with P1 (stereo
width) set to midpoint. The input for this
procedure should be 5kHz at +4dBm.
U11
Using the tone burst generator, feed an
input signal to the unit of 1kHz at + 4dBm,
and set the gate duration for 2 cycles open
and 64 cycles closed. Set R82 for a flat
base band. If a tone burst generator is not
available, set R82 to midpoint.