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THEORY OF OPERATION
Initial Power-up:
Upon power-up the TRC defaults to Frequency 1 when the [SEND] button is pressed.
Analog Circuit
RX Audio Input Path:
RX audio from a tone remote adaptor at the remote site travels down the phone line to the
RJ-11 line interface connector PL-1. Line 1 uses the standard red/green Telco ring tip on positions 5 and 6 of the
RJ-11 cable or pins 5 and 6 of the RJ-45 10-pin connector. This signal is fed into a transient absorber across the
line, then through an auto-resettable polyswitch fuse to another set of transorbs that can shunt a signal to ground
if the ground lead is tied to Earth. There are then 2 capacitors across the line, which also go to Earth to remove
RF. The audio signal is then fed into transformer T201 through jumpers JP202 and JP203 or Line 2 relay K201.
From the bottom winding of T201 audio is fed to jumper JP204 contacts 1 and 2 or contacts 2 and 3 if using T202
in a 4-wire duplex configuration. From JP204 audio is fed into the RX Input Gain Pot RP-306 where the output
level of the RX line amp U302 is typically set to 200 mV p-p with –20 dbm from a Line Test Set tone generator.
There is a disable transistor Q303 that can mute the incoming audio under microprocessor control. Coming out of
the RX line amp the audio is passed to the compression circuit U102B. The audio output of the compressor is
also about 165 mV p-p and will change no more than 3 db over a 30-35 db input change. The audio then goes
into the RX Bandpass amplifier U301D and the output of this Bandpass is fed into the Guard Tone detector which
turns on the busy light and depending on how the unit is programmed can activate the crossmute function when
another unit in the same room is transmitting to prevent howling and feedback. The output of the Bandpass is also
fed over to the RX notch filter U301C through a nulling pot. When the Bandpass filter has been tuned to the
appropriate frequency for the desired Guard Tone (usually 2175 Hz) its output is fed through the nulling pot and
R303 and R346 where it mixes with audio from R324 coming from the compressor. These two signals cancel at
the summing input of Notch filter U301C thus removing the continuous Guard Tone audio leaving only the
resulting voice and dialing audio to be transmitted through the volume control SR601B to the speaker amplifier.
Keypad beeps are also input on this same pot to be heard out the speaker. When the handset is taken off-hook,
the magnetic reed switch located inside of the handset opens it’s contact which signals the microprocessor to
activate transistor Q302. Activating transistor Q302 mutes the speaker amplifier. Audio from U301C is fed into
ear level RP304 which drives the earpiece driver Q303B to drive the handset earpiece. There is also an ear mute
transistor Q304 that allows the microprocessor to mute the earphone. There is also a jumper JP302 that feeds
handset mic audio back into the earpiece for sidetone. This jumper should not be used in 4-line duplex mode.
There is another jumper JP201 that feeds DTMF and paging and dialing tones into the RX audio path so that they
can be heard during the dialing time.
TX Audio Output Path:
Mic bias is developed with R100, R134 and R101. This provides mic bias to either the
internal mic, or gooseneck mic. These are all electret type mics. When using the internal mic or gooseneck mic,
Q306 switches audio into the mic amp U101A where the gain is set by RP101. Q101 on the input of the mic amp
kills mic audio during the High-Level function tones and dialing times. Coming out of the mic amp the audio level
is approximately 200 mv peak to peak where it is fed into the compressor U102A. The audio out of it is also about
200 mv peak to peak and will change no more than 3 db over a 30-35 db input change. It is then fed into notch
filter U103C and U103D. This notch filter is usually set to the standard Guard Tone of 2175 Hz and is designed to
remove 2175 Hz components from the voice so that they do not mix with the 2175 tone generator and cause
phase cancellation and thus momentary breaks in the Low-Level Guard Tone. The notch filter feeds into the audio
mixer along with the signaling and dialing tones from the digital to analog converter. The tones and voice out of
the audio mixer are presented to the TX line level pot RP202 and then to the line amplifier U201A. U201A then
feeds the top winding of T201. When the line level pot RP202 is set all the way to maximum it is sufficient to drive
a single phone line with up to 8 additional tone remotes to about +10 db on the high level guard tone.
The regulator has an input from a wall charger of around 16-20 volts which is passed through a auto-resettable
polyswitch F601 into transient absorber TZ601, then through the volume control on/off switch SR601A to an RF
bypass cap C602, across several filter caps into the input of the 13.4 volt regulator. D601 is a reverse polarity
diode that will blow the auto-resettable polyswitch if the power is reversed. Powering off for one to two seconds
will reset the polyswitch. The output of the 13.4 volt regulator is fed to all of the analog circuitry on the analog
schematic page. U303A has a voltage divider R373 and R374 that creates the analog pseudo ground reference to
all of the op amps on the analog schematic page.
