TAPR T-238+ User manual

T-238+ Assembly Manual Rev. 8 March 2006 Page 1 of 6
Tucson Amateur Packet Radio
Internet: [email protected] www.tapr.org
Non-Profit Research and Development
Corporation T-238+
Modem
T-238+ Modem Assembly Manual
This document was originally written June 2005. © 2005 Tucson Amateur Packet Radio Corp.
Reproduction or translation of any part of this work beyond that
permitted by sections 107 or 108 of the 1976 United States
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Amateur Packet Radio Corporation. Except as noted above,
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individual to reproduce any portion of this document provided
that: the reproduction is not sold for profit, the intent of the
reproduction is to further disseminate information on Amateur
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is given to Tucson Amateur Packet Radio Corporation (including
address) as the original source of information, and Tucson
Amateur Packet Radio Corporation is notified in writing of the
reproduction.
The information contained in this document has been carefully checked and is believed to be entirely reliable. However, no
responsibility is assumed for inaccuracies. Tucson Amateur Packet Radio Corporation (TAPR) reserves the right to make
changes in any products to improve reliability, function or design without obligation to purchasers of previous equipment.
TAPR does not assume any liability arising out of the application or use of any product or circuit described herein; neither
does it convey license under its patent rights or the rights of others.
Introduction......................................................................1
Acknowledgments............................................................1
Parts list...........................................................................2
Construction notes ...........................................................3
Construction.....................................................................3
Power-up and initial testing ..............................................5
Board layout.....................................................................7
Schematic diagram..........................................................8
Introduction
The T238+ modem board provides the hardware
necessary for the T238 or T238+ to transmit APRS
packets directly to a radio instead of requiring an external
TNC. The board and the software on the T238 do not
provide a full-featured TNC, just the basic functions
needed to get your APRS packets on the air. While wired
up to support data reception as well, no reception
functions exist at this time.
Support for this board requires software revision 1.15 or
later.
The modem board plugs into the 16-pin header on either
T238. All power and control signals come from the main
board. Connection to the radio is by means of a DE-9
connector.
The modem board has two LEDs to indicate the radio
status, a red LED to indicate an active transmission, and a
yellow LED to indicate incoming signal.
The modem assumes an active low transmit enable that
can be a separate signal or combined with the transmit
audio in the DE-9 connector. Audio level control is
possible using a potentiometer.
While mechanically designed to mate with the T238+ as
well as T238, this board can be used for any project
wanting to add an MX-614 based modem to a project.
Acknowledgments
The Tucson Amateur Packet Radio T238+ Modem Kit was
made possible by the efforts of (in alphabetical order):
William Beals, N0XGA -Conceptual Designer
Russ Chadwick, KB0TVJ -Conceptual Designer
John Koster, W9DDD –Project Manager
If you got this board as part of the T238+ kit, assemble the
main board and button board and complete the checkout
process for those boards first. Once verified, then build
and verify this board.

T-238+ Assembly Manual Rev. 8 March 2006 Page 2 of 6
T238+ MODEM BOARD
NOTE: Build and debug the T238+ main board before
starting this board
PARTS LIST
The parts list is organized by quantity and part type.
Verify that all parts are present by checking in the [ ] as
you locate the part in the list. You may wish to take this
opportunity to sort the parts into a compartmented
container such as an egg carton or muffin tin as you
inventory them. This will aid you in kit building.
Resistors ¼W, 5% Carbon Film:
[ ] {1} 100 ?(brown-black-brown-gold) R110
[ ] {4} 1.2K ?(brown-red-red-gold) R106,109,114,115
[ ] {2} 10K ?(brown-black-orange-gold) R100,111
[ ] (3) 100K ?(brown-black-yellow-gold) R101,102,105
Resistors, ¼W 1% Carbon Film:
[ ] {1} 1.24K W (brown-red-yellow-brown) R103
[ ] {1} 1.43K W (brown-yellow-orange-brown) R112
[ ] {1} 3.48K W (orange-yellow-gray-brown) R113
[ ] {2} 24.9K W (red-yellow-white-red) R107,R108
[ ] {1} 511K W (green-brown-brown-orange) R104
Resistor, Trimpot
[ ] {1} 10K ?Trimpot (103) V100
Capacitors
Capacitors may be marked in various ways. The typical
markings are listed but may vary. Find all that match and
the remaining ones, if any, should become apparent by
elimination.
Mylar or Monolithic
[ ] {2} 100pf (101) C100,C111
[ ] {9} 0.1uf (106) C101-107,109-110
Electrolytic or Tantalum
[ ] {1} 10uF C108
Transistors
[ ] {1} 2N3904 Q100
Light Emitting Diodes
[ ] {1} Amber LED D100
[ ] {1} Red LED D101
Integrated Circuits
NOTE: Do not handle the ICs at this time! Carefully
remove the black foam carrier with ICs from the bag and
verify the ICs against this list. Do not touch the ICs!
Leave them in the protective foam.
[ ] {1} MX-614 U100
[ ] {1} LM358 U101
Crystals
[ ] {1} 3.58MHz X100
IC Sockets
[ ] {1} 16-pin DIP Socket U100
[ ] {1} 8-pin DIP Socket U101
Connectors
[ ] (1) 2x8-pin female header P101
[ ] (1) DE9 Male PCB right angle mount J4
Miscellaneous
[ ] (1) Printed Circuit Board (Rev A)
[ ] (1) Assembly Manual (this document)
Construction Notes
You are now ready to begin construction of the T238+
Modem. Follow these standard construction practices
when building the unit.
Use a temperature-controlled, fine-tipped soldering iron of
relatively low wattage (25 watts maximum, 15 watts is
ideal) and a good quality 60/40 or 63/37 rosin-core solder
for construction. Keep the tip of your soldering iron bright
and clean, wiping it frequently on a wet rag or sponge.
Make solder joints carefully, but swiftly. Prolonged heat
on a PC board pad can be as disastrous as it can ruin the
PC board. Two to three seconds should be enough time
to apply heat to any joint. Due to the proximity of some of
the traces on the PC board, solder bridges are a very
distinct possibility. Following the important points above
could eliminate several hours of troubleshooting (or
worse). This is good practice when working on any kit.
You will need small flush or semi-flush cutting pliers and
small-tipped long nosed pliers. A magnifying glass may
prove helpful to identify the values of the small
components.
Pay careful attention to the directions that follow:
1) Read this entire document prior to starting construction
of your kit.
2) Identifying the pins on headers and ICs -Pin 1 is
identified by the square pad on silk-screen.
Electrostatic Protection
The Integrated Circuits are susceptible to static discharge.
Observe anti-static precautions when assembling the T-
238+ Modem. Never handle the ICs without grounding
yourself, the protective foam, and the PCB first. Always
touch and hold the foam first before touching the IC.
Always touch and hold the printed circuit board before
inserting the IC into its socket. Never leave the ICs on
anything but the foam or the PCB.

T-238+ Assembly Manual Rev. 8 March 2006 Page 3 of 6
Construction
Refer to the layout diagram for clarification of parts
placement. All references to up, down, left, and right
assume that you are looking at the PCB with the text right
side up. The DE-9 connector is at the bottom right.
Sample construction pictures are also available on the
T238 website.
Resistors
Resistors have a lead spacing of 0.4" and should lie flat on
the PC board. You may wish to use a lead former to pre-
form the resistor leads for neatest appearance. The 1%
resistor have an extra band.
Install the following 5 resistors:
[ ] R100 10K ?(brown-black-orange-gold)
[ ] R101 100K ?(brown-black-yellow-gold)
[ ] R102 100K ?(brown-black-yellow-gold)
[ ] R103 1.24K ?1% (brown-red-yellow-brown)
[ ] R104 511K ?1% (green-brown-brown-orange)
WARNING! –Be careful when clipping leads, as they
have a tendency to fly towards your eyes! Take
appropriate precautions (grasp leads and wear eye
protection).
[ ] Solder and clip the leads (10 leads total)
[ ] R105 100K ?(brown-black-yellow-gold)
[ ] R106 1.2K ?(brown-red-red-gold)
[ ] R107 24.9K ?1% (red-yellow-white-red)
[ ] R108 24.9K ?1% (red-yellow-white-red)
[ ] R109 1.2K ?(brown-red-red-gold)
[ ] Solder and clip the leads (10 leads total)
[ ] R110 100 ?(brown-black-brown-gold)
[ ] R111 10K ?(brown-black-orange-gold)
[ ] R112 1.43K ?1% (brown-yellow-orange-brown)
[ ] R113 3.48K ?1% (orange-yellow-gray-brown)
[ ] R114 1.2K ?(brown-red-red-gold)
[ ] Solder and clip the leads (10 leads total)
[ ] R115 1.2K ?(brown-red-red-gold)
[ ] Solder and clip the leads (2 leads total)
Now check your work. All leads should be soldered. There
should be no solder bridges or cold solder connections.
[ ] OK so far.
This completes the resistor installation. You should have
no remaining resistors.
[ ] No resistors remaining.
IC Sockets
NOTE: If any socket pins are bent, carefully straighten
them with a pair of long-nose pliers before assembly.
Some types of IC sockets have crimps in the pins to hold
them in place when automatic wave soldering is
performed. These sockets may be tricky to install if you
are not familiar with them. If your kit contains these
sockets, you may want to straighten the pins before
attempting to insert them into the PC board.
When installing IC sockets double check to ensure that the
socket is seated properly against the board with the notch
matching the silk-screen. Pin 1 (nearest the socket notch)
has a square solder pad. Be sure that all IC socket pins
are showing on the solder side of the board. Next, tack-
solder two diagonally opposite corners first (such as pins 1
and 8 on a 14-pin socket).
Then solder the remaining pins of that socket before
proceeding to the next one. If you find a socket is difficult
to install, remove it and double-check for a bent pin.
CAUTION! –Take care to avoid solder bridges!
Install the following IC sockets. Align the notch in the
socket to the notch in the silkscreen outline:
[ ] U100 16-pin
[ ] U101 8-pin
Now check your work. All leads should be soldered. There
should be no solder bridges (a blob of solder that shorts
two adjacent soldered connections) or cold (gray and/or
grainy looking) solder connections.
[ ] OK so far.
This completes the IC socket installation.
Ceramic Capacitors
All capacitors should be mounted as nearly flush to the
board surface as practical without stressing the leads.
Install the following capacitors:
[ ] C100 100pf (101)
[ ] C101 0.1 uF (104)
[ ] C102 0.1 uF (104)
[ ] C103 0.1 uF (104)
[ ] C104 0.1 uF (104)
[ ] Solder and clip the leads (10 leads total)
[ ] C105 0.1 uF (104)
[ ] C106 0.1 uF (104)
[ ] C107 0.1 uF (104)
[ ] C109 0.1 uF (104) (no C108!)
[ ] C110 0.1 uF (104)
[ ] Solder and clip the leads (10 leads total)
[ ] C111 100pF (101)

T-238+ Assembly Manual Rev. 8 March 2006 Page 4 of 6
[ ] Solder and clip the leads (2 leads total)
Electrolytic and Tantalum capacitors
Electrolytic and Tantalum capacitors are polarized. The
positive lead goes in the hole on the board marked with a
"+". Be careful! Typically the negative lead is marked but
sometimes the positive lead is marked.
NOTE: The orientation of the positive “+” leads are not all
the same.
[ ] C108 10 uF (106 or 10uF)
[ ] Solder and clip the leads (2 leads total)
Now check your work. All leads should be soldered. There
should be no solder bridges or cold solder connections.
[ ] OK so far.
This completes the capacitor installation. You should have
no remaining capacitors.
[ ] No capacitors remaining.
Trimpot
Align the trimpot according to the three solder leads.
Note: the silkscreen is incorrect! It shows the small
plastic tabs on the wrong side.
Install the trimpot:
[ ] V100 10K ohm
[ ] Solder and clip the leads (3 leads total)
Light Emitting Diode (LED)
LEDs are polarized components. The flat side on the
base and the shorter lead identifies the cathode lead.
Insert the LED according to the silk-screen outline, the flat
side or short lead to the flat side of the silkscreen
NOTE –Mounting the LEDs on this board assumes you do
not want to see the LEDs once the project is put in a
chassis. If you want to see them after the project is in a
chassis, save the LEDs and mount them to the chassis
instead.
[ ] D100 Amber LED
[ ] D101 Red LED
[ ] Solder and clip the leads (4 leads total)
Transistor
Transistors have a flat side on the package. Insert the
transistor in the board such that the flat side matches the
silkscreen.
[ ] Q100 2N3904 Transistor
[ ] Solder and clip the leads (3 leads total)
Crystal
This crystal is a three-terminal device. Fortunately,
orientation does not matter. Calling it a crystal is not quite
correct, it is a ceramic resonator. It has most of the
properties of a crystal except it is less accurate (but
accurate enough), cheaper, and easier to use.
[ ] X100 3.58MHz Crystal
[ ] Solder and clip the leads (3 leads total)
Female Header
NOTE! The female header will be installed on the bottom
side of the board! This is the header that plugs into the
T238+, so it needs to be on the bottom of the modem
board. Removing this part (if you put it on the wrong side)
will be almost impossible without destroying the connector.
WARNING! –Do not hold these parts with your fingers
while soldering. The pins get very hot.
[ ] P102 2x8 female header ON THE BOTTOM
NOTE: Only location P102 is stuffed at this time. P101 is
in case there may ever be a “granddaughter” board.
Connectors
Install the DE-9 connector. Solder in the tabs that hold the
connector in place first, then solder the pins.
[ ] J4 DE-9 female
[ ] Solder the leads
Almost done with the main board
At this point, these should be the only items left in the
main board kit:
NOTE: Keep U100 and U101 in the antistatic foam
[ ] U100 MX-614
[ ] U101 LM358

T-238+ Assembly Manual Rev. 8 March 2006 Page 5 of 6
Power Up And Initial Testing:
In this section you will slowly power-up the board and
check for assembly errors. It is important that you follow
these steps carefully. You will systematically apply power
to sections of the board while checking voltages to find
and eliminate errors. Errors take two forms, the kind that
damage components and the kind that don’t. Neither is
desirable, but the kinds that damage components will
require you to find a replacement part.
NOTE: -Before you power up the modem board, your
T238+ main board should be constructed and fully
checked out.
Pre-Power Check
Before applying power for the first time, please check the
following:
[ ] ICs U100 and U101 are NOT installed.
[ ] Check the polarity of all the electrolytic or tantalum
capacitors. The capacitorsnegative side is usually
identified with a white stripe and a big minus sign on it.
The positive side is identified on the PCB. Not all the
capacitors have the same orientation; double-check them
with reference to PCB silkscreen.
[ ] Check the polarity of the transistor, does the flat side
on the part match the silkscreen on the board?
[ ] Headers P100 and P101 are not connected.
[ ] Header P102 is installed on the BOTTOM of the board.
[ ] Plug the modem into the T238+. Take care to make
sure there are no components from the T238+ touching
the bottom of the board. Use the screws and spacers to
ensure a good fit for the board.
[ ] Apply power to the T238+. Observe anything unusual
such as components heating up, smoke or smell.
Remove power immediately if anything unusual appears.
Place the ground lead of your voltmeter on pin 4 of U101.
This will be the ground reference for the rest of the
measurements.
NOTE –In the sections below, there are frequent
references to +5V, which actually means the voltage
between 4.90V and 5.10V.
With the ground lead of your voltmeter still on U101 pin 4,
confirm all the following test points also measure +5V.
Remedies are listed if you do not see +5V.
Check Pin Remedy if not +5V
[ ] U100 pin 16 Check socket.
[ ] U101 pin 8 Check socket.
[ ] P100 pin 9 Check connector
[ ] P100 pin 6 R109, D101, Q100.
[ ] Check the positive pin of C108. It should be between
2.45 and 2.55V. If not, check R114, R115, C108
(polarity), C109, R112, R113.
Using a small wire bent in a U-shape, short between U100
pins 14 and 16. Do this carefully so you don’t damage the
IC socket. The amber “BUSY” LED should light. If it does
not, there is a problem with R106 or more likely, the LED
is installed backwards.
[ ] Amber LED works
Using a small wire bent in a U-shape, short between U100
pins 3 and 16. Do this carefully so you don’t damage the
IC socket. The red “XMIT” LED should light. If it does
not, there is a problem with R111, R109, Q100, or the LED
is installed backwards.
[ ] Red LED works
[ ] Remove power from the T238+.
Next you will insert U100 and U101. Make sure you
match the pin-1 notch of the device to the notch on the
silkscreen. It can be difficult to line the pins up with the
socket. Your best bet is to place the chip sideways on a
flat static-safe surface so that one row of the pins all lie
flat on the surface. Carefully rotate the body of the IC
while pushing down on the pins. Rotating while pushing
down should bend all the pins together and bend them
where they protrude from the body. Bend the pins a little
at a time, then check to see if they are perpendicular to
the body of the IC. Once perpendicular, the pins of the IC
will be much easier to insert as they line up with the
socket.
Insert U100 and U101 slowly. Observe that all the pins
slide into the socket. Watch for pins that bend under the
IC or out of the IC socket.
[ ] Insert U100, MX-614.
[ ] Insert U101, LM358.
[ ] The notch on the chip is aligned with the silkscreen
notch on the PCB.
[ ] Apply power to the T238+. The red LED on the main
board or button board should still flash three times quickly,
and after approximately 10 seconds it should start flashing
once per second.
If the LED does not flash at all, remove the modem board
to confirm the LED works OK without the modem board
installed. If it does, check the modem board for short
circuits, especially between the connector and MX-614.

T-238+ Assembly Manual Rev. 8 March 2006 Page 6 of 6
[ ] LED flashing approximately once per second
With a the ground lead of your voltmeter on pin 4 of U101,
check the following voltages:
Check
Pin Voltage Remedy
[ ] U101 Pin 8 +5V Check IC socket
[ ] U100 Pin 16 +5V Check IC socket
[ ] C8, + pin 2.5V R114, R115, C108,
C109, R112, R113.
[ ] Remove power from the T238+.
CREATING YOUR RADIO
INTERFACE CABLE
Four signals are provided on the DE-9 connector, a digital
active-low PTT signal, Receive Audio, an AC-coupled
Transmit audio, and finally a “just in case” +5V. Five pins
(#1-5) are all ground.
Refer to your radio’s manual to determine the signaling
required to control it from the T238+. While fairly
consistent, there are two major ways radios expect control
signals for enabling PTT (Push To Talk). One class has
separate wires for audio and a “digital” PTT signal. The
PTT is invariably active low. The second class combine
these two signals onto one wire by having the audio AC-
coupled and combined with a DC offset to signal if PTT is
active or not.
If your radio is the type that has separate signals, both are
provided on separate pins of the DE-9 connector. If your
radio requires combined signals, you will need to tie the
PTT and TX_AUDIO together with a resistor. The value
should be recommended by your radios operator manual
and is not provided with this kit. The signals can be
combined inside the header or on the main board,
whichever is more convenient. The audio is AC-coupled
which works with both types of radios.
When building your interface cable, remember that this
will be carrying very low level audio signals and will likely
be in a high RFI environment. Keep the cable as short as
possible, use shielded cable if at all possible and make
sure ground connections are solid.
Testing of the modem functions requires the T238+ be
fully programmed for modem use and is covered in the
T238+ operations Manual.
Congratulations!
You have completed the power-up and initial testing
phase! To learn how to setup and operate your T238+
APRS™ 1-Wire™ Weather Station, please refer to the
T238+ Operations Manual. It is available on the T-238
web site at
http://www.beals5.com/wx
or
http://www.tapr.org/taprf/html/Ft238.html