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TTS 268
Section 5
SECTION 5
MAINTENANCE
5 . 1 GENERAL.
volts peak to peak.
A slight amount of clipping
on the peaks of this waveform need not cause
5. 2 Apart f r o m the n o r m a l periodic cleaning of
the jack contacts, the TTS 26B does not r e q u i r e
any routine maintenance.
5 . 3 If any abnormal operations o c c u r s , i t can
easily be traced to one of five groups of c i r c u i t s .
These c i r c u i t s a r e a s follows:
1. Oscillator c i r c u i t .
2 .
Buffer-amplifier circuit.
3 .
Drive r e l a y , send relay, and
,
associated bias c i r c u i t r y .
a l a r m , a s this is normal.
5 . 7 If t h e r e i s no voltage between point A and
ground (with R15 i n maximum clockwise position)
check the filament of the tungsten lamp, R89, for
continuity.
Replace l a m p
i f
faulty. If the lamp
is good, substitute t r a n s i s t o r s , starting with Q1
and ending with
Q 3 .
If trouble s t i l l prevails,
check f o r p r o p e r DC voltages a s indicated on the
schematic d i a g r a m s ,
of
the circuit.
These vol-
tages should b e within 210% as indicated.
An
open condition of filter capacitor C8 can also
c a u s e faulty operation and should be checked
through substitution.
4 .
External dial and telephone s e t circuits.
5. 8 Should the preceding steps fail to produce
an output of the oscillator at point A, check
5 .
P u l s e speed measurnig c i r c u i t .
other component p a r t s contained within the
The general location of the maior component p a r t s
of the TTS 26B i s shown in figures 6-1 and
6-2.
NOTE
Unless otherwise specified, voltage o r
power readings apply between the point
r e f e r r e d to and ground.
5 . 4 When the DRIVE r e l a y fails to o p e r a t e , the
trouble is most likely to be traced to one of two
causes:
1 . Station battery polarity r e v e r s e d .
2 .
Oscillator-buffer amplifier c i r c u i t r y
inoper
~ t i v e .
5.
5 After the battery polarity has b e e n checked,
position the AD3
?&
BREAK control a t i t s mid-
position and conduct trouble shooting of the
oscillator-buffer amplifier c i r c u i t r y a s follows:
5.6
Referring to fig. 5- 1 ,
-
connect an oscilloscope
between point A and ground.
Point A is shown in
fig. 5-2 which is the drawing of the printed c i r -
cuit board assembly.
The sine wave voltage
appearing between these two points should be
approximately 2. 5 volts peak to peak.
If not,
adjust potentiometer R15 to achieve this level.
Turning R15 in a clockwise direction i n c r e a s e s
the positive feedback, and therefore i n c r e a s e s
the output level a t point A.
There should b e no
fluctuation of the signal level at this point once
the level has been adjusted to approximately 2. 5
oscillator c i r c u i t , such a s r e s i s t o r s , capacitors,
and the pulse frequency switching network. It
is
important after the correction of the trouble to
r e a d j u s t potemtiometer R15 to produce a 2.
5
peak to peak voltage indication at point A of the
oscillator board.
5. 9 With the oscillator operating properly,
check the peak to peak boltage appearing at pin
3 of the
DRIVE
r e l a y .
This voltage should be in
the o r d e r of 7 volts peak to peak.
The exact
voltage depends on the setting of the gain control
R19.
The
n o r m a l setting for this control is
approximately i n i t s mid-position.
If normal
.
signal voltage is not realized at this point, check
the buffer-amplifier portion of the circuit.
This
c i r c u i t c o n s i s t s of t r a n s i s t o r s
Q4, Q 5 ,
Q6
and
their associated c i r c u i t r y .
5. 10
Lf
a n o r m a l driving voltage a p p e a r s at
pin 2 of the
DRIVE
r e l a y and trouble s t i l l p r e -
vails, the trouble will be found in either the
DRIVE r e l a y , the SEND relay, o r their bias
networks.
When the SEND BIAS control i s in
i t s mid-position, the DC voltage between pins
1
and
8
of the DRIVE r e l a y shuold be very close
to 0.
5 .
11 Should trouble be traced to e i t h e r the
DRIVE o r SEND r e l a y , i t is recommended that
these r e l a y s be removed and tested with proper
t e s t equipment s u c h a s the Sigma 4500 S e r i e s t e s t
s e t o r equivalent.
Should these r e l a y s require
readjustment, i t is recommended that i t be done
by qualified personnel familiar with the adjust-
ment of this type of polar r e l a y .
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