T25-35SA Alignment Service Information T25-35SA Subaudible Tone Decoder Alignment Service Information

T25-35SA SETUP & ALIGNMENT INSTRUCTIONS

This unit uses underdamped bandpass filters to achieve quick response. There are two such filters in series monitoring for each tone, the output of the last one being rectified to drive a schmitt trigger. When the tone level increases to the trip point, the trigger provides an output to both U8 (a 4528 timer) and U7 (a 4 to 16 decoder). When the timer expires (about 250 milliseconds), it lights the OUTPUT PULSE LED (I1, front panel) and latches the tone detect signal into U7, which then provides an output to the relays depending on which tone state is latched. Further changes in detector state are ignored. The other half of U8 provides a 250 millisecond output pulse. The unit will wait until there is a "no tone" condition before it resets. The delay is provided to allow both tones to detect if they are sent together, plus to give a measure of false triggering protection. If a tone detector trips, it must remain tripped for at least this long to cause an output pulse. If the detector LED goes out before the output pulse is issued, the Output LED will light, but no pulse is issued. Most sources of false trips last only for a few tens of milliseconds.

The Mueller Broadcast Design T25-35 subaudible tone decoder has no user adjustments. All setup adjustments are on the PC board and have been preadjusted to provide proper operation in most situations. The tone decoders have been set to trip at -10 dB or higher with a nominal program input level of 0 dBm. If your input level requirements are not 0 dBm, a simple internal adjustment will allow the unit to function properly.

See the Installation Instructions for connection information.

The input level adjustment (R59) sets the gain of the unit. It is properly adjusted when the level check LED (I6) glows green most of the time with program input. If it only flickers between green and off, increase the gain with R59. If it is red all of the time or is mostly red with green flickers, reduce the gain with R59. Properly adjusted, the output of the unit will be 0 dBm nominal. The remainder of the adjustments on the board deal with the frequency and output tone rejection of the detectors, as well as the trip points of I6. Do not adjust these controls unless you have the following equipment available:

  • Audio Signal Generator.
  • Audio (AC) voltmeter accurate from 10 Hz through 1 KHz.
  • Oscilloscope.

The T25-35SA will both detect and notch out 25 and 35 Hz tones. The rejection of the tones at the output is at least 20 dB, with 50 dB possible if the tones are exactly on frequency. Since the passband of each detector is +/- 3 Hz, you should not have to adjust them. If it becomes necessary, here is how to do it.

Connect the audio generator to the input, the AC voltmeter to the output.

1) Feed a 1 KHz tone to the input at 0 dBm. Adjust Input Level (R59) for 0 dBm (600 Ω) at the output. (Note:  The minimum input impedance of this unit is about 9.2 KΩ.  If your audio generator has a 600 Ω load, use it.)

2) Change frequency to 25 Hz and reduce the input level to -15 dBm.

3) Adjust 25 Hz Tune (R11) for minimum 25 Hz output read on the AC voltmeter.

4) Adjust 25 Hz Null (R9) for minimum 25 Hz output read on the AC voltmeter.

5) Repeat step 3 and 4 until at least a 30 dB null is obtained.

6) Connect the oscilloscope to the anode (opposite of the stripe) of D1 (marked 25 test on the parts locator). Adjust 25 Hz Peak (R25) for a maximum indication on the scope. If the sinewave clips, reduce 25 Hz Sense (R27) until the clipping disappears.

7) Adjust 25 Hz Sense until the front-panel 25 Hz Detect LED (I3) illuminates, and then one turn more.

8) Change frequency to 35 Hz.

9) Adjust 35 Hz Tune (R10) for minimum 35 Hz output read on the AC voltmeter.

10) Adjust 35 Hz Null (R24) for minimum 35 Hz output read on the AC voltmeter.

11) Repeat step 9 and 10 until at least a 30 dB null is obtained.

12) Connect the oscilloscope to the anode (opposite of the stripe) of D2 (marked 35 test on the parts locator). Adjust 35 Hz Peak (R26) for a maximum indication on the scope. If the sinewave clips, reduce 35 Hz Sense (R28) until the clipping disappears.

13) Adjust 35 Hz Sense until the front-panel 35 Hz Detect LED (I2) illuminates, and then one turn more.

14) Recheck the 25 Hz output. It should still be 30 dB or more down.

15) Change frequency to 1 KHz and feed your nominal input level to the unit.

16) Adjust the Input Level (R59) control so that it is halfway between where the Level Check LED (I6) turns green and then red. There is a 6-8 dB "window".

DO NOT ADJUST R60 or R61!

[If you did adjust R60 or R61, after aligning the unit as above, feed a 1 KHz tone to the input and adjust the INPUT LEVEL control until you get a 0 dBm output from the unit. Reduce the input level 6 dB. Adjust R61 (Min. Adjust) until the LEVEL CHECK LED just goes out. Increase the input level by 12 dB (+6 dBm output level). Adjust R60 (Max. Adjust) until the LEVEL CHECK LED just changes from green to red. These controls only affect the trip points of the LEVEL CHECK LED and should never have to be moved.]

If you experience difficulty detecting network tones, and the Level Check LED is indicating green most of the time, the tones are at a level below -10 dB from program level. Adjust R27 (25 Hz Sense) or R28 (35 Hz Sense) clockwise (1 dB per turn) to compensate for the lower level.

If you have difficulty with this unit, please contact us. Our hours are 11 AM-7 PM, central time Monday through Friday.

Phone: (708) 352-2166. Fax: (708) 352-2170.