Hal Communications ST-5 User manual

\.,.,
SiTIEi
FITTY
trIEMcIDTJLATcIFI
INSTRUCTION
MANUAL
HAL COMMUNICATIONS CORP.
BOX 365
URBANA, ILLINOIS 61801
QU ALITY COM M U NICATIONS EQ U I PM ENT

siTIEi
FITTY
trIEMcItrITJLATCIFI
Copyright @ 1973 by HAL Communications Corp.
i n the Un i ted States of Amer i ca. Al I r i ghts
publ ication may not be reproduced in any form
sion of the copyright owner.
, Urbana, lllinois. Printed
reserved. Contents of th i s
without the wr i tten permis-
December,1979

HAL ST-5 RTTY DEMODULATOR KIT
CONTENTS
! NTRODUCT I ON I
2
4
B
l2
SPECIF!CATIONS
oPERAT!0N 0F THE ST-5
ST-5 KIT CONSTRUCTION
TEST AND ALIGNMENT
I NTERCONNECTI ON WI TH OTHER EQUI PMENT
D I AGRAMS r5
t7
)q
IB
lg
20
2l
22
23
2\
25
26
27
2B
PARTS LI ST
Figure I
Fi gure 2
Fi gure 3
Fi gure 4
Figure 5
Figure 6
Figure 7
Fi sure B
Figure 9
Fi gure I 0
Figure I I
ST-5 Schemat i c
ST-5 Power Supply
ST-5 Power Supply
ST-5 Demodulator
ST-5 Demodu I ator
Loop Plug Preparat
I nterconnect ion to
I LLUSTRAT I ONS
Layou t
Board Layout *.
Board . . -.
ton
Other Equ i pment
Toroid \^/iring . .
Toroid Mount ing
Mechan i ca I Layout fo r CI)ZI l0A Box
P I ug Connect i ons

I NTRODUCT I ON
The model ST-5 RTTY Demodulator is a high-performance, all solid-
state demodulator for reception of radio teleprinter signals. The ST-5
features a very high gain I imiter stage capable of a 70dB dynamic range
with Iow zero- Ievel cross-over d istort ion. The d iscr iminator c i rcu its
of the ST-5 are broad-band and linear in response throughout the required
bandwidth. The Iinear discriminator allows reception of signals with
non-standard f requency shif t and assures rel iable recovery of weak signals
under noisy conditions. Discriminator bandwidth is switch-selectable to
provide enhanced reception of the two standard frequency shifts (tZO Hz
and 850 Hz). A high-gain slicer stage assures that keying pulses from
the sol id-state keyer have the fast switch ing t imes necessary for Iow-
distortion keying of a teleprinter. The internal power supply of the
ST-5 furnishes regulated plus and minus 12 volts to operate the denrodulator,
and includes a 175 volt, 50 ma loop supply. A tuning meter is also furnished
with the ST-5, thus reducing the number of external pieces of equipment
reguired to receive RTTY signals. All parts necessary to construct and
interconnect the ST-5 are furnished with the kit with the exception of a
cabinet. The ST-5 can be installed in a user-supplied cabinet or in a
Bud Minibox available separately f rom HAL Communications Corp.
This manual contains basic operation instructions, construction
procedures, and a guide for servicing the ST-5. Please read the complete
manual before beginning assembly of the ST-5.

2, SPECIFICATIONS
I nput Frequency Range : 2125 2975 Hz (Standard)
1275 - 2125 Hz ("Low-Tones")
lnput lmpedance: 500 ohms, uhbalanced
Limiter Performance: (at 2125 tlz)
Limiting Threshold - l.OmV rms input
Maximum Signal lnPut - 3.5V rms
D i scr im inator Perf ormance:
Mark Frequency, BII shifts - 2125 Hz (Standard)
1275 Hz ("Low-Tones")
Space Frequency, 170 Hz shift - 2295 Hz (Standard)
I Ir45 Hz (" Low-Tones" )
Space Frequency, 850 Hz shift - 2975 Hz (Standard)
2125 Hz ("Low-Tones")
Discriminator Filter Bandwidth 140 Hz
Minimum Usable Frequency Shift - (Standard Tones): 850 Hz 50 Hz
170 Hz - l0 Hz
Maximum Usable Frequency Shift - (Standard Tones): BSO Hz - ll00 Hz
I70 Hz - 350 Hz
Low-Pass Filter Cut-Of f Frequency: 30 Hz (60 wPffi, l+5 baud)
50 Hz (tOO wpffi, 75 baud)
(Setectable by proper choice of component values)

0utputs:
L00P: An internal 175 volt, 50 ma loop power supply is keyed by the
demodulator and connected to a panel connector to which the
selector magnets at the teleprinter can be directly connected
FSK: A bi-polar keying voltage output is supplied that is EIA
(nS-232) compatible. This voltage is keyed by either the
demodulator itself or by interruption of the loop circuit
by external devices such as a keyboard or tape transmitter.
This output can be used to key an FSK circuit for transmitting
RTTY or to drive the HAL RVD-1005 Solid State Visual Display
System, or the HAL AK-l AFSK osci I lator.
Nominal Vol tages: Ma rk:
Space:
KEY: A telegraph key may be connected to the FSK system to provide
narrow-shift CW-lD transmission. A separate potentiometer is
provided to allow adjustment of the CW shift.
SCOPE: AC voltages proportional to the signals in the discriminator
f ilters are provided to allow use of an oscilloscope to aid in
tuning the receiver. Since a tuning meter is provided with the
ST-5, use of the'rscopeil output is optiona'l . Approximately 4V
peak-to-peak (at the center frequency of each discriminator
filter) is furnished with an internal impedance of I megohm.
Miscel laneous Data:
Fuse Protect ion:
Power Requ i rement:
0.5A fuse in AC power I ine
l2o/2\o vAC, 50- 60 -Hz, 1 lo%
(Power cords are furnished for U.S. standard
three-wire 120 VAC connections; other connectors
must be user-suppl ied i f requ i red. )
Power Consumption: I4 watts maximum
Shipping Weight: 5 lbs.

3. OPERATI ON
The ST-5 demodulator is constructed on two printed circuit boards
as is shown in the schematic diagram (Figure l). Audio input signals are
connected to jack Jl and therefore to point rrAil on the demodulator board.
Resistor Rl provides a 560 ohm termination for the receiver output.
lf at all possible, the receiver output impedance should be matched
to 500-500 ohms by either proper selection of output transformer windings
or with an external voice-coil to I ine transformer. The ST-5 will work
with audio signals f rom the 4-B ohm voice-coil output, but only with the
sacr if ice of 30 to 40db of I imiter dynamic range. Also note that res istor
Rl is rated for % watt; this should be quite adequate for most operations;
use of more receiver output power is unnecessary and will result in damage
to Rl and possibly to the I imiter, 0AI.
Diodes Dl and DZ in conjunction with R2 provide transient protection
of the limiter operational amplif ier f rom static bursts.
Operational amplif ier 0Al, a type 709 integrated circuit, serves
as the Iimiter stage of the demodulator. Resistors R5, R7, and RB provide
adjustment of the limiter switching level for minimum distortion. Proper
adjustment of R5 wi ll be discussed in Section 5. Components C4, C5, and
Rll provide extended high-frequency compensation of the 709. Components
C2, R9, C3, and Rl0 provide power supply decoupl ing at the limiter.
The output of the limiter stage (pin 10,0A-l) is connected through
a resistor balancing circuit (ntZ, Rl3, Rl4, Rl5) to the discrimirrator
fi lters. Proper adjustment of Rl3 (aiscussed in Section 5) al lows balancing
of the amplitude and bandwidths of the mark and space filters of the
discriminators. Capacitor C7 (with C7A) tunes Ll to the mark frequency
(2125 Hz) and CB (with cBA) tunes Lz to the *space f requency (2295 Hz or
2975 Hz). Diodes D5 and D6 provide full-wave, neg€tive detection of the
mark signal and, Similarly, D7 and DB provide a positive output for a
space signal. The two detector outputs are summed at pin 4 of 0A2 through
R2l and R22. lf the two filters are balanced in both amplitude and band-
width and the input to the demodulator is noise, equal but opposite currents
are produced in R2l and R22, resulting in zero volts at pin 4 of 0A2. Since
noise f Iuctuates randomly in both amplitude and f requency, pin 4 will osci I-
Iate from plus to minus but with an average value of zero. When a signal of
mark frequency (ZIZS Hz) is applied to the ST-5 input, D5 and D5 generate a
strong negative signal that forces the input of 0A2 (pin 4) negative.
Similarly, a space signal (2295 or Z97S Hz) will force pin 4 positive. Thus
the two f ilters and associated diodes provide discrimination between various
audio frequencies. Proper choice of the balancing resistors Rl2, Rl3, Rl4,
and Rl5 will result in a frequency to voltage response at the input of 0A2
that is maximum (but of different sign) "t the center frequencies of LI and
LZ and linear in response for intermediate f requencies. This linear discrim-
inator characteristic along with the wide dynamic range of the Iimiter stage
are the major factors in the superior weak-signal performance of the ST-5
over other demodu I ators.

Components R2l, RZL, and Cl7 form a simple R-C 'low-pass f ilter
for the discrirninated signals. lf only 60 wpm (t+5 baud) op*ration is
anticipated, the value of Cl7 should be set to.058 mfd, c.orresponding
to a -3dn response of approximately 30 Hz. This value wil I qive enhanced
iow-pass f ilter performance for 60 wpffi, but will not allow satisfactory per-
formance at 100 wpm. lf operation at 100 wpm or both 50 and 100 wpm is
anticipated, a better choice is to set C17 equal to .039 mfci, corresponding
to a -3dg response of approximately 50 Hz. The change in 60 wpm performance
under these condi tions is sl ight and general Iy not noticeable. Moreover,
equalization for 100 wpm allows compatibility with other commonly used
keying speeds (60, 66, and 75 wpm) whereas the 60 wpm equalization does not.
Operational amplifier OAZ, also a 709N, is used as a very high-gain
slicer amplifier to "square-up" the filtered keying waveform from the
discriminator. The output of the slicer (pin I0, 0A2) is a square-wave with
positive voltage corresponding to a mark condition and negative to space.
This keying signal, through R27, switches the high-voltage keying transistor
(42), keying the TTY loop. Diode D9 prevents negative breakdown of the
2N5655 base-emitter junction. CI5 and R2B are a transient supPression net-
work to protect QZ from the high-voltage transients that can be induced by
the selector magnets of the teleprinter.
A convenient tuning meter circuit is provided with the ST-5. Diodes
D3 and Dq provide positive half-wave rectification of the mark and sPace
signals respectively. The detected outputs are summed in the base of Ql
with C6 and Rl7 providing filtering of the a-c component. Transistor Ql
is an emitter-fol Iower current ampl if ier that drives the meter through Rl5
and Rl8. Since both mark and space signals provide a positive current to
the meter, it wiil always read up-scale. However, the current to the meter
will be a maximum only when a signal is matched in frequency to the discrim-
inator mark or space filters. ln fact, if both the mark and space frequencies
of the input signal match the discriminator tuning, the meter will have
maximum def Iection for both tones and wil I not f luctuate when the signal
changes from mark to space. Therefore, correct tuning is indicated on the
meter as an up-scale reading that does n6t chaqge when the signal changes
state. lf the shift of the incoming signal does not exactly match that of
the discriminator, the signal can still be tuned so that the meter does not
fluctuate, but the steady reading obtained will not be as great as before
since the signal has been "straddle-tuned" and is not peaked on either filter"
This "plus-plus" meter signal from Ql is also used to drive the ST-5AS Auto-
start opt i on.
Provision is also made in the discriminator circuit to use an
oscilloscope as a tuning indicator. The a-c signals f rom each discriminator
f ilter are connected to J5 through isolating resistors R29 and R30. lf these
two signals are then connected to the X and Y axis inputs of the oscilloscope,
a conventional "cross-type" of RTTY tuning indicator can be used and the
receiver tuned so as to maximize the signal on each axis. This technique
works very well rvith older demodulators with sharply tuned discriminator
filters and works to a limited extent with the purposely wide bandwidth
discriminator f ilters in the ST-5. To achieve the highly desirable Iinear
discriminator response, the ST-5 filters are considerably broader than those

constructed of tuned, but unloaded, BB mhy toroids. The effect on the scope
output is that straight, crossed I ines are no Ionger formed, but rather,
the traces are ell ipses with considerable opening. These ell ipses will ,
however, with sufficient operator experience, provide as much infornration
regarding proper tuning with the ST-5 as with other demodulators. AIso,
since the ST-5 uses semiconductor devices, the scope outputs are only of
the order of 4 volts peak-to-peak, thus a stage of amplification may be
required in the oscilloscope.
The power supply of the ST-5 furnishes zener regulated +12 volts
and -12 volts for the demodulator as well as ll5 volts @ 60 ma for the TTy
Ioop. Approximately -15 volts unregulated, partially f iltered power is
available at point AD for use with the ST-5AS Autostart option. lt is not
recommended that this point be used to power any other accessories. Suffi-
cient additional low-voltage power capability is provided that both the
ST-5RS anci Al',.- I opt ions can be powered f rom the ST-5, but any f urther
accessories should obtain their power from another source.
The "l{a in I ine" f loat ing- loop supply is incorporated in the ST-5
to produce a loop-keyed voltage (FSK output) that is negative for mark
and positive for space. ln mark state, q2 is conducting and point 'rLn
is held close to qround potential. Since there is little voltage drop
in the TTY machine, point "Y", the positive terminal of the Ioop supply
is also close to ground potential. Therefore, most of the output voltage
of the loop supply (175 volts) appears across Rl07 and the loop current
is therefore control Ied by RI07. The junct ion of Rl05 and Rl07 is there-
fore at approximately -155 vo'l ts and the junct ion of R104, Rl05 and Rl05
at approximately \ that, -80 volts (t 5 volts). This is further divided
by Rl05 and R l0B in para I lel wi th any externa I Ioad across P in 3 of JZ so
that approximately -15 volts is presented to a lK Ioad in the mark state.
Conversely, if the Ioop is open (qZ not conducting or machine not conducting)
no current flows through Rl07 and point "Y" is allowed to,'float-up', to
the f ul I output voltage of the unloaded loop supply, +175 volts. The
divider Rl04-R105 and Rl05-Rl0B again reduces the voltagl at the FSK out-
put to approximately l5 volts, but this timg positive. This minus to plus
transition is very convenient to use to key cliode-FSK circuits and is
compat ible wi th the input key ing requ i rements of the AK- I AFSK osc i I 'lator.
This output is also compatible with RS-232 standards and can be used
directly to drive the HAL RVD-1005 visual Display system.
Another convenient feature of the FSK output of the ST-5 is that
it is keyed by either the incoming signal or by the TTY keyboard. This
means that if an AK-l is connected to this point, it will regenerate an
AFSK signal from the ST-5 processed incoming signal, producing a very
clean output s ignal that cculd be repeated via a vHF I ink, etc. conversely,
if the RVD-1005 is connected to the FSK output, anything printed at the
machine wi ll be displayed on the monitor screen, be it of loca'l -loop or
off-the-air origin.
Resistor Rl09 and Pin 5 of JZ provide a means to use narrow-shift
CW identification when a diode FSK keying circuit is used.

A special transformer that has both low-voltage and loop-voltage
windings as well as a 120-2\0 volt primary is furnished with the ST-5
Kit. Connections for either primary voltage are shown in Figure l.
Swi tch 53 al lows defeat of the keyer stage , Q2, when no RTTY
signals are received or when it is desired to transmit using the FSK
output. A DPDT switch is furnished so that this switch could also be
used to control the transmitter.

ASSEMBLY INSTRUCTIONS
Construction of the ST-5 will be carried out in three steps:
I . Assemble the two ci rcu i t boards.
2. Prepare a cabinet.
3. lnstal I and interconnect the boards and
connectors in the cabinet.
Before starting construction, check the parts received against
the parts list. lt will simplify assembly if you separate the parts by
type into small boxes as you check them off the list that is, resistors
in one box, semiconductors in another, etc.
iour kit was carefully checked for quality and completeness before
shipment. Should you find any parts missing or damaged, please notify the
factory in writing so that a replacement can be suppl ied.
Since the construction of the ST-5 involves many soldered connections,
it is important that you make each one carefully. lt is assumed that the
builder has some prior experience in kit construction and is acquainted with
good soldering practices. lf not, it would be well to practice on scrap
components, preferably with the help of an experienced individual, before
starting work on the kit. lt wil I also be helpful to keep the following
points in mind:
l. Since mcst of the components are small, only a moderate amount
of heat is needed to make good solder joints. Use a l5 to 25 watt iron
with a penci l tip or smal l (l /8") chisel tip.
2. When soldering components to the printed circuit boards, avoid
overheating the connections. Al though high-grade'epoxy-glass boards are
suppl ied, excessive heat can cause the printed conductors to separate
from the board and can also damage components. Be careful not to allow
the solder to bridge between closely-spaced adjacent conductors and solder
pads. Also, be careful to prevent solder splashes f rom falling on the board.
3. When soldering the lC socket terminal strips, be particularly
careful not to use too much solder or heat. Excess solder with enough heat
can be transferred through the board hole and onto the socket pins themselves.
4. lf you make an error and find it necessary to remove a component
from the board, heat each connection with the iron and use a suction de-
soldering tool or solder wicking (copper braid) to remove the excess solder.
The component can then usually be Iifted from the board without applying
additional heat. ln the case of inexpensive components, it may be better to
break the component and remove the leads one at a time than to risk damaging
the board by overheating it. Replacement parts may be obtained locally in
most cases.
4.

4. I Ci rcui t Board Assembly
Before assembling any components on the circuit boards, carefully
i nspect the boards themsel ves for defects i n etch i ng and pl at i ng. Al though
all boards have been inspected before shipping, it is far easier to find
and correct such problems before assembly than after.
Assemble the power supply circuit board first. The power supply
circuit board is the simpler of the two boards and can be further identif ied
by the large unplated areas in the middle and near one end of the board.
Refer to the layout drawing (Figure 2) and the photograph (Figure 3) for
correct location of the parts on the circuit board. Note that both figures
give the component-side view of the circuit board.
0rient the board as in the f igures and instal I al I circuit-board
mounted resistors in the locations shown. Note that the loop resistor,
R107, is NOT mounted on the circuit board. Resistor RI09, the CW-Shift
potentiometer, can also be mounted at this time. Solder all connections.
!nstal I al I diodes in the locations indicated,
observe correct orientation. Solder al I connections.
heat, porticularly when soldering to the zener diodes,
two jumpers near DI07 and Dl08 can also be soldered-in
Finally, install the capacitors, being careful
polarities. Solder al I connections.
be i ng carefu I to
Avoid excessive
Dl07 and DI08. The
at this time.
to observe electrolytic
power supply board itself. 0ther
in the cabinet. lnspect all
found.
This completes the assembly of the
connections wi I I be made upon instal Iation
solder connections and correct any defects
Most of the ST-5 circuitry is mounted on the remaining demodulator
circuit board. This board is more complex than the power supply and great
care should be taken to avoid poor solder joints, solder bridges between
paths, and solder splashes. Refer to the layout drawing (Figure 4) and the
photograph (Figure 5) for proper part placement.
NOTE: lf the ST-5 is to be used with the ST-5AS Autostart Kit, refer to
page \ and Figures 4A and 4g of the ST-5AS manual for modifications
required to the ST-5 demodulator circuit board. As is shown in
Figure 4g (of the ST-5AS manual), it is necessary to cut one path
on the ST-5 demodulator board to use the autostart option. Use a
sharp knife or scribe to cut a 1/16" wide opening in the path
indicated. lf the ST-5 is not being constructed with the autostart,
do not make this modification as it can be done later if desired.
The two integrated circuits should be instal led on the circuit board
first. Suppl ied in the kit are four seven-pin socket strips. D0 NOT
separate the pins from the carrier strip before soldering them in place.
A simple technique to hold the strips in the proper position for soldering
is to first install the strips on the lC's and then insert the pins into the
circuit board. Solder al I pins to the circuit board, observing the precautions

mentioned earl ier. AFTER S0LDERING, remove the carrier strips from the tops
of the pins by f irmly gripping the strip with long-nose pliers and gently
bending the strip away from the lC. There is a pieformed indentation along
the carrier at each pin so that it wil I snap-off without too much force. Be
careful not to bend any of the terminals when removing the carriers. Be sure
that the integrated circuits are oriented as shown in the figures. tf they
have been installed backwards, pull the circuit from the socket and turn it
a round .
Additional pads are provided so that the round, To-5 packaged 7og lc
can be used if desired. Since the kit is normally supplied with l4 pin Dlp
lCrs, these extra holes are usually not used.
lnstal I al I res istors, potent iometers, and d iodes in the pos it ions
shown. Note that all the resistors and diodes are mounted in a vertical
pos it ion. Be caref ul to observe proper d iode polarity. AIso, instal I the
wi re j umpers at th is t ime. sor der a I I connect ions.
I nsta I I both trans i stors and sol der a I I connect ions.
the Ieads are correctly oriented. Be sure that
lnsta'l I all capacitors in the locations shown. Solder al I connections.
Capacitor locations C7A, CBA, and C94 are provided for additional tuning
capacitors if needed.
Finally, install the two 88 mHy toroidal coils, being careful to
connect the leads as shown. see Figures 6 and 7 for connection and mounting
details. strip enamel from the wires and solder to points indicated in Figire 4r-
This completes the assembly of the demodulator board. 0ther con-
nections will be made upon installation in the cabinet. lnspect all solder
connections and correct any defects found.
4.2 Cabinet Preparat ions
ln the interest of economy, no spepial cabinet is provided for the
sr-5 kit. The kit builder is therefore free to ehoose whatever housing best
matches his equipment and is most convenient for his station. A populir
housing for the ST-5 is the BUD CU2ll0A }linibox which is available irom
HAL Communications. Whether the BUD box or other housing is used, a few
general guidel ines are in order:
Resistor Rl07 dissipates 9.9 watts during mark condition and
should therefore be mounted away from other parts. A satis-
factory mounting is on a terminal strip, vertical ly positioned
on either the front or rear panel.
Place switches sl (srrift), s2 (power), and s3 (Receive-standby)
as well as the pilot lamp and the meter on the front panel. The
connectors and line cord should be placed on the rear panel.
It is wise to leave space for the ST-5RS and AK-l options even if
they are not used initial ly. You may or may not wish to put in extr
hol es for the connectors and switch assoc iated with these opt ions.
2.
3.
l0

\.3
4. For those who wish to use the Bud box, a suggested cabinet
layout and drill ing guide are shown in Figure 6. when the
ST-5, 5T-5AS, and AK-l kits are installed as shown, the cabinet
is crowded and you may wish to consider a different Iayout
and/or a larger box. This layout requires that the edge connec-
tor fingers on the AK-l be sheared off and wire connections be
made directly to the circuit board.
lnstallation and lnterconnection in the Cabinet
These steps will, of course, vary somewhat with the cabinet Iayout
chosen, but a few more general points are worth consideration.
Terminal strips are provided to mount the loop resistor (R107) and
for connections to the AC power cord. When connecting the AC power cord,
be sure that the green wire connects directly to chassis ground and that
the black wire connects to the fuse first and then to the AC power switch
and then to the power transformer. The white wire shourld connect directly
to t!" power transformer as shown. The AC power cord should be by-passed
(CtO6 and CI07) immediately after it enters the cabinet to minimize potential
stray RF interference. Be sure to connect the dual primaries of the power
transformer as shown for your power Iine voltage.
The Ieads to the input connector
the audio output leads from the AK-1, if
shielded cable if they must be more than
and the scope connector (as well as
used) should be made with miniature
four inches long.
When interconnecting the boards and cabinet-mounted parts,
wise to leave encugh wire length that each board can be sufficient
removed from the cabinet for maintenance. Lead lengths should not
general be critical.
it is
ly
in
ll

5. TEST AND ALIGNMENT
After the ST-5 has been completely assembled, double-check al I
wiring to make sure there are no errors. Particularly make sure that
the ac power, loop supply, and low vol tage supp'l ies are correctly connected.
5. I Prel iminary Power Supply Tests
CAUTION: IN THE FOLLOWING STEPS, POTENTIALLY LETHAL VOLTAGES ARE EXPOSED
IN THE CABINET \,/IRING. BE VERY CAREFUL WHEI'I MAKING MEASUREMENTS TO AVOID
TOUCH I NG EXPOSED TERM I NALS.
Put a >z ampere fuse in the fuse holder and plug the Iine cord into
a grounded three-prong ac power outlet (or suitable power source if using
22A VAC. )
DO NOT DEFEAT THE GROUNDING PLUG FEATURE. TO DO SO COULD PRESENT A SHOCK
HAZARD, LEAD TO RF I NTERFERENCE PROBLEMS, AND \^/ I LL VO I D ANY \^/ARRANTY BY
HAL COMMUNI CATI ONS.
Turn the AC Power switch on. The pilot lamp should I ight. Make
the following vo'l tage measurements in the ST-5 using a 20,000 ohms-per-
vol t VOM:
i,il "i.:i: :xli r;:::;ii': l;;r:i;,',il:::, ffi : : 3::ffi :h i:ii i T:'
2. At point rrKrr on the demodulator board, measure +12 VDC, t 1.2 vol ts.
3. At point "Hil on the demoduIator-board, measure ' 12 VDC, t 1.2 volts.
\ :;:^,':i':!,'3"::,;:':;l:r,::::i;'1,ilo,i,j?Tiizi"jf::,ol;: 4
loop resistor, R107.
5' li}B';rln?.' ffii'"1';:, :l',:rl il..1""1"*3,"::,ffi;';::,:pp rox i mate I v
6. i;.T;i,ti^?r*il the power supply board, measure approximately
7. Turn the power switch off.
lf the previous voltage measurements check, proceed to the next step.
lf they do not check-out, a wiring error has been made and should be corrected
before cont i nu i ng.
l2

5.2 Al ignmentjf the ST-5
Short the audio input to the ST-5 (jack Jl), turn-on the power,
and measure the DC voltage at pin l0 of lC 0A-1. Adjust potentiometer
R5 to obtain as close as possible to zero volts at pin 10. Since 0A-l
is connected for very high gain, it may not be possible to adjust R5
for exactly zero volts.
Remove the short from Jl and connect an audio oscillator to Jl.
The oscil lator should be capable of approximately I.0 volt rms output
in the 1.0 to 3.0 kHz frequency range. Adjust the oscillator frequency
and note the readings on the tuning meter. With the Shift Switch (Sl)
set for BSO Hz, the meter reading should peak at approximately 2.1 and
3.0 kHz. Set the oscillator frequency to the meter peak near 2.1 kHz.
Adjust potentiometer Rl8 to give a meter reading of 0.7 ma. Change the
oscillator frequency to get a meter peak reading near 3 kHz. Adjust
potentiometer RI3 to again get a meter reading of 0.7 ma.
Connect a 0 to I00 ma meter (or VOM on the 100 ma scale) between
pins 4 and I of a 6 pin connector with the positive lead to pin I and
the negative lead to pin 4. Turn the ST-5 off and wait 30 seconds to assure
that the filter capacitors have discharged. lnsert the plug into J2.
Turn the ST-5 back on. t^i ith the signal generator frequency set below
approximately 2400 Hz, the meter should indicate a loop current of 60 t I0 ma.
When the Eenerator frequency is above 2600 Hz, the Ioop current should be
zero.
The discriminator tuned circuits can now be adjusted. Connect a
frequency counter or other accurate frequency measuring device to the
audio oscil lator. Since the discriminator tuned circuits have a relatively
wide bandwidth, measurement of the frequency corresponding to a peak tuning
meter reading is difficult. A much more accurate technique is to measure
frequencies on either side of the peak and calculate the mid-point. For
example, to measure the center frequency of the mark circuit, first make
sure that the peak meter reading is approximateLy 0.7 ma. l',lext, adjust
the oscillator frequency up in frequency until a meter reading of 0.6 ma
is obtained. Record this frequency. Adjust the osci I lator down in
frequency, past the peak, until the lower frequency corresponding to a
meter reading of 0.6 ma is obtained. Record this frequency. The center
frequency can now be determined by adding the two frequencies and dividing
by two. Use this technique to measure al I center frequencies.
With the shift switch in the 850 Hz position (switch Sl), measure
the mark frequency. The measured frequency will generally be below the
desired 2125 Hz mark frequency. lf so, adjust the tuning by removing turns
from LI at the approximate rate of 3 Hz per turn removed. D0 NOT attempt to
make large frequency adjustments in one step. Rather, remove almost enough
turns and then remeasure the center frequency; it is far easier to remove
additional turns than to put some back on the toroid. lf more than I0 turns
must be removed from a toroid, remove an equal amount from each half to
maintain the detector balance. If the original center frequency is higher
than 2125 Hz, lower the resonant frequency by adding capacitance at location
r3

C/A at the rate of 2 Hz per 100 pf added. After the mark circuit has been
set to a center frequency of 2125 Hz, recheck the adjustment of RIB and re-
adjust i f necessary.
Next, change the signal generator frequency to give a peak meter
indication near 3 kHz. Readjust Rl3, if necessary, to give a meter reading
of 0,7 ma. As before, compute the space circuit center frequency by
measuring the frequencies corresponding to a 0.6 ma meter reading. This
circuit will generally be tuned higher than the desired 2975 Hz center
frequency. lf so, Iower the resonant frequency by adding capacitance at
location CBA at the rate of 5 Hz per 100 pf added. lf the frequency must
be increased, remove turns from LZ at the rate of 5 Hz per turn removed,
again being careful to avoid removing too many turns. When the center
frequency has been adjusted, the 850 Hz shift mode is now aligned.
To aliqn the 170 Hz shitt mode, switch SI to 170 Hz and tune the
oscillator to the mark f requency, 2125 Hz. Since the sarne tuned circuit
is used for both 'l 70 and BSO Hz shif t, the mark circuit shoulC already be
aligned. Note the tuning meter reading at the peak for the mark frequency.
It may or rnay not be equa'! to 0.7. lf not, adjust RIB to give a 0.7 ma
meter indication. Next, locate the center frequency of the space ci rcuit
and adjust Rl3 for a meter reading of 0.7 ma. As before, compute the
center frequency and adjust ONLY the capacitor to get a center frequency
of 2295 Hz. Additional capacitors, if needed, can be placed at location
C9A. lf it is necessary to adjust L2, the alignment of the 850 Hz space
circuit must be repeated since this toroid is common to both 170 and BSO Hz
space filters. Adjustment factors are the same as for 2125 Hz,2 Hz per
100 pf or 3 Hz per turn.
Note: Because the same tuned circuit is used for both the I70 Hz
and BSO Hz space filters, potentiometers Rl3 and RIB can only be set exactly
for one or the other shifts. lt is suggested that they be adjusted for the
more critical operations at 170 Hz shift as a final alignment step. .As
discussed before, set RIB first for 0.7 ma meter reading at 2125 Hz (mark)
and then Rl3 for 0.7 ma at 2295 Hz (space).-
Al ternate Low-tone AI ignment
Measure frequencies and adjust as described above, except the
frequencies should be 1275 Hz for mark and 1445 Hz for I70 Hz shift space
and 2125 Hz for 850 Hz space. Be sure to use the alternate values for C7,
C8, and C9 as well as for RI2, RI4, RI5, and R2? described in note 3 in
Figure l.
The alignment and adjustment of the ST-5 is now complete.
l4

6. INTERCoNNECTI0N 0F THE sT-5 T0 0THER EQUIPMENT
The ST-5 is now ready to be connected to other equipment in the
station. Mating plugs are provided for al I external connections. Use
shielded cable where possible to avoid potential stray-RF interference
problems.
6.1 Printer Connection
Refer to Figure 7 and connect the printer to pins 4 and I of a
5-pin plug as shown. Note that the ground contact (pin 5) of the plug
is NOT part of the ioop circuit, although it can be used for a shield
connection to ground if shielded cable is used. The selector magnets should
be wired in series with the keyboard contacts AT THE MACHINE to permit
local-loop operation using the ST-5 internal Ioop supply. Make sure that
none of the Ioop connections to the teleprinter are connected to the machine
frame or any other ground source. Also, be sure that all internal loop
sllpp I ies in the mach ine are d isconnected.
6.2 Aud io lnput Connect ion
As shown in Figure 7, connect a shielded cable between the receiver
audio output and a phono connector. lt is highly recommended that the
500 to 600 ohm output of the receiver be used if avai'l able. lf this out-
put is not available, the ST-5 wil I work when driven f rom a 4 to B ohm
voice coil output, but at the sacrif ice of 30 to 40 dB of I imiter dynamic
range. A step-up transformer can be used to transform the low impedance
output up to 500 ohms.
6.3 Osci I loscope Connection
lf desired, an oscilloscope can il connected for use as a tuning
indicator. Make the connection with a short (less than 3 ft.) length of
two-conductor, shielded cable with the plug wired as shown in Figure 7.
As discussed earlier in Chapter 3, the voltage available for the oscilloscope
is only of the order of 4 volts peak-to-peak and amplification in the
osci I Ioscope may be necessary to produce a satisfactory pattern.
6.\ 0ther Connect i ons
The FSK Keying output of the ST-5 can be used for several purposes
such as driving a diode FSK circuit, providing data output to the HAL RVD-1005
Visual Display Unit, or keying the HAL AK-l AFSK Oscillator. Always use
shielded cable when using this connection.
The CW lD Key jack is only useful when the FSK jack is used to drive
a diode FSK circuit, although when the ST-5 and AK-l are combined, this
jack is also connected for the CW lD Key in the AFSK mode. See the AK-l
manual for further information.
l5

For connections to or from the AK-l and/or the ST-5AS options,
the pertinent manual should be consulted for proper connections.
A typical RTTY station with the ST-5 and other equipment is
shown in Figure 8.
l6

7. SCHEMATIC AND WIRING DIAGRAMS
The schematic, circuit board Iayouts and photographs, and wiring
diagrams are shown on the following pages.
t7