Midian TRA, Instruction Manual

Page: 9 / 20

9
To set earpiece volume, adjust R17 to a comfortable level.
Do not set this level too high as this can cause hybrid
ringing.
NOTE: The hybrid balance pot R13 , has been set at the
factory using a standard telephone. The transmit audio
level pot
R27 has been factory set to about mid-range and
so has the receive earpiece level pot R17 . R27 and R17
and should not need readjustment. You may need to
readjust the hybrid balance
R13 . The simplest method is
to turn up the receive earpiece level pot R17 , and transmit
audio level pot R27 . You will then notice that when the
hybrid balance pot R13 is adjusted, there will be a null
between a low frequency howl and a high frequency howl.
Set the hybrid balance pot in the quiet zone between the
two howls, then turn down the receive input and the ear
piece levels according to the instructions above, and reset
the TX audio level pot
R27 to the factory setting or to the
point of 3.3 kHz of deviation when pressing a 5 on the
telephone's touch tone pad. If either the transmit or
receive audio path levels are turned up too high, there
may be some noise heard during conversation that
sounds like the hybrid is on the verge of singing. If this
occurs, lower the TX and/or RX level controls. Generally,
the hybrid pot may need readjustment if you change from
one manufacturer or type of telephone to another.
5.2. Trunked Radiotelephone Setup
When the telephone handset is taken off hook, a dial tone
will be heard if the ANI was received and validated at the
repeater terminal. Enter the desired phone number and
set the modulation to 3.3 KCs using Mod Pot R58.
Then, using the "5" button on the Touch Tone phone, set
modulation to about 3.3 KCs using TX Gain Pot R27.
Peak voice should now be going out at about 4 KCs,
assuming the radio was properly set at 5 KCs maximum.
If the injection point to the radio is Low Z and the
maximum tone output level is insufficient, install jumper
JU6, which shorts out R59, lowering the output
impedance. Then repeat above steps.
Using a service monitor, with a 1 KC test tone set to 3.5
KC of deviation, inject an RF signal into the receiver
sufficient to quiet it, and set RX Gain Pot R34 so that test
point 2 is set at 1.5 vp-p (500 mV RMS).
To set earpiece volume, adjust R17 to a comfortable level.
Do not set this level too high as this can cause hybrid
ringing.
NOTE: The hybrid balance pot R13, has been set at the
factory using a standard telephone. The transmit audio
level pot R27 has been factory set to about mid-range and
so has the receive earpiece level pot R17. R27 and R17
and should not need readjustment. You may need to
readjust the hybrid balance R13. The simplest method is
to turn up the receive earpiece level pot R17, and transmit
audio level pot R27. You will then notice that when the
hybrid balance pot R13 is adjusted, there will be a null
between a low frequency howl and a high frequency howl.
Set the hybrid balance pot in the quiet zone between the
two howls, then turn back down the receive input and the
ear piece levels according to the instructions above, and
reset the TX audio level pot R27 to the factory setting or to
the point of 3.3Khz of deviation when pressing a 5 on the
telephone's touch tone pad. If either the transmit or
receive audio path levels are turned up too high, there
may be some noise heard during conversation that
sounds like the hybrid is on the verge of singing. If this
occurs, lower the TX and/or RX level controls. Generally,
the hybrid pot may need readjustment if you change from
one manufacturer or type of telephone to another.
6. T
HEORY
O
F
O
PERATION
The receiver audio applied to the Orange RX Tone Input
lead will be present at connector P1 pin 11 feeding 3 basic
paths:
- the receiver earpiece audio via RX Gain Input Pot
R34 and U5A through U4A and Earpiece Level Pot
R17;
- the tone signal for DTMF decode through R62 and
Gain Control Feedback Resistor R63;
- and the two-tone/five tone circuit U6A and U6B.
Receive earpiece audio is enabled/disabled by U4A and
controlled by pin 27 of the microprocessor. This path,
normally disabled, is enabled after the TRA sends its ANI.
If the channel is busy, the microprocessor generates a
Busy tone on pin 20 and applies it to U4 Pin 12 The
amplified voice audio is applied to hybrid transformer T2,
which drives the telephone handset. Trimpot R13 is the
trans-hybrid isolation balance control.
Audio/tones are fed to U8, DTMF decoder, via U4C Pin 4.
Gain for U8 is determined by the ratio of R62/R63, tone
acquisition time by the RC time constant, C42, R64 and
R65. With the values shown, this timing is approximately
35 msec.
U6A—along with U6B (a Schmidt trigger)—acts as a low
pass filter for processing two-tone and five-tone when
decoded by the microprocessor at Pin 41.
Relay RLY1 is used to switch the telephone handset from
the ringer transformer T1 to the balanced hybrid
transformer T2. The ringer, composed of T1, Q1, and Q2
and associated components, generates 70V RMS when
ringing. The 33 Hz ringing frequency is generated by the
micro at pin 24 and 28. These two signals are 180
degrees out of phase and are integrated by R6, R8, C6
and C7. The opto isolator U2 monitors this voltage during
each cycle. If the telephone handset is taken off hook
during ringing, the voltage drops, causing U2 Pin 5 to go
high. This causes the micro to terminate ringing.
Resistors R10, R11, and Q5 act as the off hook detector
when making or terminating a call.
The transmit audio , voice and tones, are generated in the
telephone handset and appear at hybrid transformer T2
terminal 10. Amplified by U5-B, which is then fed to U4-B
and U4-C. U4-C is turned on by the microprocessor
during dialing so that the Touch Tone decoder can see the
dialing sequence to tell the micro to regenerate its own
dialing sequence. During this time, U4-B blocks handset
Touch Tone audio from going out the low pass filter Q7