Alignment
Levels.
Audio levels can be adjusted with the front panel audio pots (Input and Output). Start with the Input levels
and feed the unit with normal program audio. Adjust the input until the input audio presence LED flashes
with about 50-70% duty cycle with the incoming audio; it can flash more if the audio is densely processed.
Output audio should be adjusted for desirable levels into your console or automation. If the red "Out Clip"
LED is illuminating at all, you are getting within 4dB of clipping on peaks. If this same LED is not lighting,
but your audio out is distorted, you are probably driving the input too hot. Check for a flashing "Input Clip"
LED and set the input gain jumpers for the maximum gain that does NOT cause the input clip LED to flash.
Filters and Detectors.
All adjustments to the decoder assume 25 and 35 Hz signal decodeing. The process for setting 50 and 75
Hz decoding is identical, except, of course for the frequencies. 50Hz is substituted for 25Hz and 75Hz subs
for 35Hz. The filter sections and detectors are tuned at the factory with standard reference tones and
should not need further adjustment by the user. However, if the need unexpectedly arises to tune these
circuit sections you will need a scope or analog audio level meter that has good display characteristics at
low audio frequencies, a signal generator or known accurate 25Hz and 35Hz signal source, and an accurate
frequency counter if your signal source is a continuously adjustable device (most are). The frequency
counter can be left hung across the signal source for fine tuning the frequency of the signals being fed during
the alignment process. The audio level of the tone source should be between -10 and +8 dBm, depending
upon jumper settings. If the signal is unbalanced, be sure to tie ring and sleeve on the input connector to the
unbalanced source's shield.
To Tune the filter sections set the generator or signal source for the operating frequency of the filter being
tuned and adjust the associated multiturn pot for minimum audio (the null point) at the test point for the filter
you are adjusting. The two main tone test points are square pads on the PC Board near variable resistors
r1 and r6. They are labelled 25N T.P. and 35N T.P. Adjust R20 to null 25Hz. Adjust R39 to null 35Hz
Secondary 25Hz - adjust R15 - test point is the main audio output.
The detectors should not be aligned unless the filter sections have been aligned first.
The detector alignment process is the same for both detectors. First set the signal source for the operating
frequency of the detector you will be tuning, then find the turn-on point of the detector in each direction of
rotation of the multiturn detector's alignment pot. Set the pot for half way between thee two points.
R1 is the 25Hz detector adjustment; R6 is the 35Hz detector adjustment. To find the center of the
detector's range, reduce the input gain until the detector just barely operates. Very slowly adjust the
multiturn pot about a turn in each direction noting where the center of the turn on range is. Set the pot to the
center of the range and you're done. Note that there is hysteresis in the individual detection outputs to
allow the combo tone logic a chance to operate. That means that you must adjust the detection circuit
extremely slowly in order to get an accurate idea of the center range.
