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TTS
26B
Section 5
5.12
EXTERNAL DIAL
and TELEPHONE SET
CLRCUITS.
5. 1 3 Should the external dial fail to produce
pulses when switch S4 i s in the appropriate posi-
tion for the external dial circuit used, S5 in the
DIAL
position, and the
ADJ O/o
BREAK control in
i t s mid-position, trouble shooting should proceed
as follows;
5. 1 4 The various components parts associated
with this circuit a r e shown i n figs. 6-2 and 6-3.
Visual inspection of the
DIAL
relay will d e t e r -
mine whether o r not this relay is following the
d i a l pulses a s dialed. If this relay is not following
the pulses, examine the external dial mechanism
and the circuitry through the coil of this dial r e l a y .
If satisfactory operation of the dial relay has
been observed, then trouble shooting should be
centered around the low pass filter and i t s assoc-
iated circuitry. While dialing, battery and
ground should appear alternately on the a r m a t u r e
of the DRIVE relay.
If not, inspect r e s i s t o r s R3,
R 4 , R 5 , R 6 , R 7 and capacitors C3 and C4.
These
alternate battery and ground pulses can be traced
to the filter, over contacts 25 and 26 of S5, to
pin 3 of the
DRIVE
relay.
The filter consists of
inductors G4X, C6X and capacitors C5 and C6.
The purpose of this filter is to convert the square
wave a s received for the proper operation of the
DRIVE relay and i t s associated bias- networks.
Without the sloping waveform i t would not be
possible to control the
O/o
break
transmitted by
the s e t .
The output waveform of this filter
should approximate that of a sine wave.
If
capacitors C5 and C6 a r e suspected of being
defective, these can most easily be checked
through substitution.
The total DC resistance
reading between the a r m a t u r e of the DRIVE relay
and
the
filter output a s measured a t terminal. 25
of switch S5 should be i n the o r d e r of 17000 when
S5 is
in the oscillator position.
If this reading is
not observed, locate and replace the defective
component.
Contacts 25 and
26
of S5 should also
be inspected and cleaned if necessary.
F o r dial
pulse frequencies of 4 , 6 , 8 , 1 0 , 1 2 and 14 cps,
capacitor C5 i s switched into the filter circuitry,
and capacitor C6 i s switched into the filter c i r -
c u i t r y when dial speeds of 20 cps a r e used.
This
switching is accomplished over the
SEND PPS
switch,
5 . 1 5 TALK CIRCUITS.
5.16 When the EXT DIAL CIRCUIT switch, S4,
i s i n the TALK RT position and S5 i s in the DIAL
position, the external dial circuit i s c a r r i e d
directly through to the T&R of SEND LOOP
jacks 1 and 2.
This i s accomplished over
contacts of S4 and S5. Should the TALK circuitry
fail to operate in this position, the trouble will
be traced to either the switching circuitry o r .
the external talk mechanism.
5.17 When the EXT DIAL CIRCUIT switch i s in
the TALK E&M position, battery and ground a r e
applied to the TALK. circuit 'over r e s i s t o r s R1
and R2.
The voltage generated
by
the TALK c i r -
cuit i s coupled to the T&R of SEND
LOOP
jacks
1 and 2 through DC blocking capacitors C1 and
C2. Should the TALK circuit f a i l to function.
inspect r e s i s t o r s R1 ,R2, capacitors C1, C2,
the associated switch contacts of S4 and S5, and
the external talk mechanism.
5. 18 Pulse Speed Measuring Circuit.
5. 19 Should the pulse speed measuring circuit
fail to operate proceed a s follows: Connect the
receive circuitry of the
TTS
26B to an external
pulsing source of a known frequency, e . g l0pps.
Calibrate the meter c i r c u i t a s outlined in
Section 5 of this manual. Return the METER
CIRCUIT switch,
S8,
to i t s 0-25
PPS
position
Relay K4 should now be following the pulses as
received f r o m the external pulsing source
When K 4 is released, the voltage appearing on
.
the a r m of the calibration potentiometer, R29C,
should also appear on the a r m a t u r e of K3.
This
voltage should be in the o r d e r of approximately
30 to 35 volts. If this voltage reading cannot be
obtained, then inspect the r e l a y contacts and
R29C.
Resistor R37, which is also p a r t of the
meter calibration circuit, should also be checked
When relay K 3 is operated, the voltage stored
in capacitor C12 is then t r a n s f e r r e d to the meter
and storage circuit through c u r r e n t limiting
r e s i s t o r , R36. If no m e t e r indication i s observed
with the preceding circuitry operating properly,
inspect capacitors C12 and C13.
The trouble,
if not located in the preceding operations, will
then be most likely located i n e i t h e r the switching
associated with the circuit o r the m e t e r circuit
itself.
5.20 When the pulse speed c i r c u i t functions but
does not indicate properly, the c i r c u i t should
be recalibrated a s follows: r e t u r n the MXTER
CIRCUIT switch,
S8,
to i t s
70
BREAK
DIRECT
position and perform calibration a s outlined i n
Section 5 of this manual.
After this calibration
has been completed, position the METER CIR-
CUIT switch at i t s 0-25
PPS
position. The meter
should now read 0% break. If the m e t e r did not
read 070 break, adjust potentiometer R39 to
achieve this reading.
Move the FUNCTION
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