904a Voltage Controlled Lowpass Filter Test Procedure

l.Set wirewound trimpots R17 and R25 to mid-position.

2. Set FIXED CONTROL VOLTAGE to "6", FREQUENCY RANGE to "2" and REGENERATION to "0".

3. Using a dc voltmeter, check the voltages at R35 and R39. Both levels will be approximately 1.0 volts. Adjust R40 (zero trimpot on filter board) lor approximately the same voltage at R35 and R39.

4. Connect DC VOLTMETER across R23 (470 ohms) and adjust 2.0 volt LEVEL trimpot (R24) for 2.0 volts. Disconnect DC VOLTMETER after adjusting.

5. Apply a 20kHz Odb sinewave to the SIGNAL INPUT jack.

7. Set input frequency to 312Hz, Odb and set FIXED CONTROL VOLTAGE to "0".

8. Adjust SCALi". trimpot (R8) for-8db.

9. Remove input signal, set REGENERATION to "8", RANGE to "2" and FIXED CONTROL VOLTAGE to

10. Connect a Decade Resistance Box across R11 and determine what shunt resistance is required to establish the threshold of regeneration.

Permanently install the proper shunt resistor (approximately 2.2K) and set REGENERATION control between 7 and 8.

11. With a zero db sinewave at the SIGNAL INPUT, REGENERATION at "0M, no external input control voltage and the FIXED CONTROL VOLTAGE at "0", check the FREQUENCY RANGE switch for compliance with following:

RANGE -3db Point (approximately)

2 260 to 340 Hz

12. Check to see that the cutoff frequency decreases one octave for each one volt decrease in control voltage. Use RANGE "2" and FIXED CONTROL VOLTAGE of "0". Adjust generator frequency so that output is at -3db (260 to 340Hz). Set FIXED CONTROL VOLTAGE at -5.5 volts and apply +5 volts to one of the CONTROL INPUTS. The output should be -3±2db. Set FIXED CONTROL VOLTAGE at +5.5 volts and apply -6.0 volts to one of the CONTROL INPUTS. The output should be -3-2db.

NOTES:

1. ALL NPN TRANSISTORS: 2N3392

2. ALL PNP TRANSISTORS: 2N4058

4. RANGE CAPACITOR SIZES

+1 0

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