Telex MCE325 - Manual (Page 34)

3-2 C S I - 2 0 0 U s e r I n s t r u c t i o n s
button action can be changed, via the front panel programming, so that the button controls talk
only, with listen either always on or always off.
Panel Mic/Headset Mic Selection
The PANEL MIC button selects either panel microphone (LED on) or headset microphone (LED
off).
Microphone On/Off
The MIC ON button turns the microphone on (LED on) or off (LED off).
NOTE The MCE325 can be programmed, via the front panel, so that the microphone will automatically
turn on whenever a channel is activated for talk. The MCE325 may also be internally
programmed for voice-activated microphone (VOX) as previously described in Section 2.
Speaker On/Off
The SPKR ON button turns the external speaker on (LED on) or off (LED off).
Volume Adjustment
Table 3.1 summarizes the operation of the volume controls for the various intercom line
configurations. The MCE325 is pre-programmed for stereo operation; CH 1 (VOL 1) adjusts the
level to the left headphone and CH 2 (VOL 2) adjusts the level to the right.
NOTE The MCE325 can be programmed, via the front panel, for monaural operation if desired.
The PGM VOL control adjusts the monaural mix of both program inputs to both headphones and
to the external speaker. It does not affect the program volume on the intercom lines.
Front Panel Sidetone Nulling Trimmer Adjustment
When the MCE325 is used with two-wire lines, the station operator’s voice signal is not only
transmitted onto the intercom lines, but also echoed back into the MCE325 receive circuits. This
can result in acoustic feedback when the MCE325 is used with an external speaker. The sidetone
nulling trimmers are used to null, or subtract, the station operator’s voice signal from the rest of
the received signal to prevent this feedback. (NOTE: This is not necessary in four-wire systems
since transmit and receive use separate circuits.) A trimmer is provided for each of the four
channels. The trimmers are factory-preset for maximum nulling of the microphone signal when
an ideal 200-ohm resistance terminates each intercom channel; however, the trimmers may
require slight readjustment after the MCE325 has been installed to compensate for variations
from the ideal.
To adjust the sidetone nulling trimmers:
1 Turn the speaker switch on (even if an external speaker is not connected). This will turn off the
internal, true sidetone trimmer (see note below).
2 Turn on the microphone.
3 Activate one talk button at a time, and speak into the microphone. Slowly increase the volume,
and note the level of your voice in the headphones or speaker. Adjust the corresponding
sidetone nulling trimmer to minimize your voice signal. Repeat for each channel being used,
making sure that only one channel is activated at a time.
NOTE There is also an internal, true sidetone trimmer, which allows the station operator to hear his or
her voice when using headphones. This sidetone signal is turned off when the speaker switch is