T-238 Assembly Manual
Rev. 1 February 2001
Page 7 of 11
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.
Before applying power for the first time, please check the
following:
[ ] ICs U3 and U4 are NOT installed.
[ ] Check that oscillators U1 and U2 are oriented correctly.
Three of the corners are rounded and one is squared. The
squared corner is pin 1 and should match the square pad
on the printed circuit board.
[ ] Check the polarity of all the electrolytic or tantalum
capacitors. The capacitors negative 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 diodes D1, D2, and D3. The band
on the diode should match the extra stripe on the PCB
silkscreen.
[ ] Headers J6, J8 and J10 are not connected.
[ ] The LCD should not be plugged in.
[ ] Jumpers OJ1 and OJ2 have jumper between pins 1 and
2.
The T238 kit does not come with a power supply. An
ordinary wall transformer (“wall wart”) that provides 9 to 12
Volts DC at 300 milliamps will work fine. The T238 is fitted
with a 2.1mm power coaxial connector such that the center
conductor is positive and the outer barrel is ground.
Measure this with a voltmeter to confirm both the voltage,
but more importantly, the polarity. The Radio Shack Model
19-1120/19-1140, supplied with the HTX-202 and the HTX-
404 handheld radios, is suitable for this application.
[ ] The voltage is between 9 and 12 Volts DC and the
center conductor is positive and the barrel is ground
[ ] 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 2 of U3.
This will be the ground reference for the rest of the
measurements.
[ ] Measure the voltage on the voltage regulator U5 on the
pin furthest from the power jack. It should be between 8
and 14V. If it is 0V, then L1 is likely missing or damaged.
If it is low, the transformer may not be rated for the right
voltage or something may be overloading it.
[ ] Measure the voltage on the voltage regulator U5 on the
pin closest to the power jack. It should be between 4.90V
and 5.10V. If it is low, something could be overloading the
regulator and whatever is doing that should be hot.
WARNING! –
Don't proceed until the voltages in and out of
the voltage regulator U5 are within proper limits.
[ ] Voltages in and out of voltage regulator U5 are within
limits.
NOTE –
In the sections below, there are frequent
references to +5V, which actually means the voltage
between 4.90V and 5.10V that you measured above.
With the ground lead of your voltmeter still on U3 pin 2,
confirm all the following test points also measure +5V.
Remedies are listed if you do not see +5V.
Check
Pin
Remedy if not +5V
[ ]
U4 pin 16
Check socket.
[ ]
J2 pin 2
Check socket and L8
[ ]
J2 pin 3
Check socket, L2, and R11
[ ]
U3 pins 1, 20,
and 31
SW5 possibly installed
backwards. Check by
pressing SW5. If voltage
appears when the switch is
pressed, it is in backwards.
[ ]
U3 pin 14
Check OJ2
Next check the four switches following the table below. For
each pin on the microcontroller, verify that the voltage is
normally 5V. Pressing the switch makes the voltage go to
0V. If the voltage is always low (regardless of the switch),
then likely the resistor is not installed correctly. If the
operation is opposite (normally low and pressing the button
makes the voltage go high), then the switch has been
installed backwards.
Check
U3 Pin number
Switch
Resistor
[ ]
U3-26
SW4
R10
[ ]
U3-27
SW3
R9
[ ]
U3-28
SW2
R8
[ ]
U3-29
SW1
R7
Using a small wire bent in a U-shape, short between U3
pins 20 and 21. Do this carefully so you don’t damage the
IC socket. The LED should light. If it does not, there is a
problem with R12 or more likely, the LED is installed
backwards.
[ ] LED works
[ ] Remove power from the T238.