McIntosh MR71 Tuner Service Manual.pdf

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1、FM STEREO TUNER WARRANTY STATION MR 71 SERVICE MANUAL Ifltlntosh MR 71 FM STEREO TUNER INTRODUCTION All McIntosh tuners are carefully aligned and tested at the factory using the finest available test equipment. All McIntosh tuners will meet their published specifications when shipped from the factor

2、y. After extensive operation, especially when tubes have been replaced, it may be desirable to realign the tuner circuits for best performance. This manual gives complete information on the circuit realignment pro cedure for the MR 71 stereo tuner. The test equipment listed below (or its equivalent)

3、 is necessary to properly align an MR 71. The accuracy of the alignment will be directly related to the accuracy and calibration of the test equipment used. FM Signal Generator (Measurements 210A or equivalent) VTVM Multiplex Generator (RCA WR-51A or equivalent) 10.7 MC Generator (Preferably crystal

4、 controlled) Oscilloscope (Hewlett-Packard 120B or equivalent) Harmonic Distortion Analyzer, desirable but not essential (Hewlett-Packard 330B or equivalent) If the necessary test equipment is not available, alignment should not be attempted. You may contact the McIntosh Customer Service Department

5、for additional information. Customer Service McIntosh Laboratory, Inc. 2 Chambers Street Binghamton, New York Our telephone number is 723-5491 The direct dial area code is 607 MR 71 FM TUNER DIAL SETTING SIGNAL GENERATOR INDICATOR FREQ COUPLING MODULATION TYPE CONNECTED TO Point of no interference o

6、r signal 10.7MC Through external .01MF cap to pin 7 of 1 2AT7 mixer cw VTVM TP #1 SAME SAME SAME SAME SAME Pin 6 of T5 SAME SAME SAME SAME SAME Junction of D4 and R42 SAME SAME SAME SAME SAME T6, Pin 6 SAME SAME SAME SAME SAME TP #2 105MC 105MC 300 ohm antenna terminals with *matching network 400 cy

7、cles 75KC deviation (100% modulation) VTVM connected to TP1 and scope connected to L or R audio output 90MC 90MC SAME SAME SAME 105MC 105MC SAME SAME SAME 90MC 90MC SAME SAME SAME Point of no interference Scope L or R output 105MC 105MC SAME 400 cycles 75KC deviation (100% modulation) attenuated to

8、2.5 microvolts output VTVM connected to TP #1 and Scope connected to L or R audio output FM SIGNAL GENERATOR ANTENNA MATCHING NETWORK * -O 100 OHMS AAA/ AAA 150 OHMS TUNER 300 OHM ANTENNA TERMINALS * IF SIGNAL GENERATOR HAS OTHER THAN 50 OHM INTERNAL IMPEDANCE, USE A RESISTOR OF 150 OHMS, LESS INTER

9、NAL GENERATOR IMPEDANCE. ALIGNMENT ADJUST TEST LIMITS REMARKS Top (Secondary) and bottom (Primary) cores of T1, T2, T3, and T4 Maximum possible negative voltage Shunt to ground the winding not being adjusted with a .01MF capacitor in series with a IK resistor. Attenuate signal generator until output

10、 voltage at TP #1 is less than 1.5 volts with one IF transformer winding shunted. IF transformers have terminal #1 marked with a green dot and are numbered clockwise. T5 Primary (Bottom core) Same as above T5 Secondary (Top core) Adjust for 0 volts T6 Primary (Bottom core) Maximum negative voltage I

11、f a distortion analyzer is available, omit this step at this time. Adjust T6 primary after step 9. At that time, use a strong signal from FM generator, modulate 100%, and use 75KC deviation. Adjust primary for minimum distortion. Should be no greater than 0.5%. T6 Secondary (Top core) Adjust for 0 v

12、olts Oscillator Trimmer Maximum negative voltage As output increases, attenuate signal generator to keep maximum output at TP # 1 to a low level. By doing so, precise alignment can be achieved. Oscillator Coil SAME Repeat steps 6 and 7 until dial calibration is accurate. Mixer trimmer, RF trimmer, a

13、nd Antenna trimmer SAME Mixer, RF, and Antenna coil Tuning slugs SAME Repeat steps 8 and 9 until output is as high as possible. Muting adj. control Turn muting switch to “in” position. Adjust muting control until background noise just disappears. IHFM sensitivity 2.5 microvolt for 3% total noise and

14、 distortion Step 11 is an overall sensitivity check, and requires a distortion analyzer and FM signal generator with attenuator. With 2.5 microvolts input at the 300 ohm antenna terminals, TP #1 voltage should be 3.0 volts or more. MR 71 MULTIPLEX TUNER SIGNAL GENERATOR INDICATOR STEPS DIAL SETTING

15、FREQ. COUPLING MODULATION TYPE CONNECTED TO 100MC 100MC 300 ohm 1 9KC pilot only DC Pin 7 of modulated by antenna VTVM 6U8 MPX generator terminals with approx. 1 000 microvolt signal (V14B) 8 SAME SAME SAME SAME SAME SAME SAME SAME SAME SAME SAME SAME SAME SAME SAME SAME Tune to a strong MONO FM sta

16、tion SAME 1KC 100% modulation left or right only, pilot on Audio VTVM Pin 1 or 2 of 38KC transformer (T8) L or R output jack SAME Turn off 1 KC audio modulation SAME SAME MPX stereo indicator light on tuner SAME SAME SAME DECODER ALIGNMENT ADJUST TEST LIMITS On the top of the chassis is an opening l

17、abeled MPX Light Adjust. Insert a screw driver into this opening and turn the control completely counterclockwise. 1 9KC phase coil Adjust for and 19KC maximum DC transformer (T7) voltage Adjust MPX Light Adjuster” control completely clockwise. 38KC transformer bottom core 38KC transformer top core

18、Adjust for maximum voltage Adjust for stable scope display 1. Turn off 19KC pilot on MPX generator. 2. Adjust top core of 38KC transformer to obtain a stable and uniform 1 KC signal scope display. This adjustment may be critical, so turn core very slowly. 3. Turn 19KC pilot back on. 1 9KC phase coil

19、 30db separation Modulate left channel and measure right channel output. Adjust 19KC or more phase coil for minimum right channel output (maximum separation). Remove all test leads from TP #2 for separation checks. SAME Modulate right channel and measure left channel output. Separation in steps 5 an

20、d 6 should be at least 30db. MPX light adj. controlR3 This step checks the rejection of 19KC and 38KC frequencies. Residual output should be at least 40db below modulated output. Turn control until light comes on. Then back off just enough to cause the light to go off. Then back off about 14 of a tu

21、rn more. Light should operate ONLY on an MPX signal. 5 ITEM NO. USE MR 71 PARTS LIST TUBES IDENTIFICATION NUMBER VI RF Amplifier 1 6DS4 V2 RF Amplifier 2; Mixer 12AT7 V3 Oscillator 6BN4A V4 IF Amplifier 1 6AU6 V5 IF Amplifier 2 6AU6 V6 IF Amplifier 3 6AU6 V7 IF Amplifier 4; Limiter 1 6AU6 V8 IF Ampl

22、ifier 5, Limiter 2; Muting 6CS6 V9 Multipath Indicator 6HU6/EM87 V10 Muting Amplifier; Muting Detector; A negligible with AFC in operation. Image Rejection Better than 80db at 90MC; better than 70db at 105MC. Hum Better than 70db below 100% modulation. Output Approximately 2.5 volts; low impedance.

23、Antenna Inputs 300 ohms balanced; 75 ohms unbalanced. RF Amplifier Cascode with 6DS4 Nuvistor in first stage. IF Stages Five, with 200KC bandwidth Limiters Two. Radiation Substantially below FCC requirements. Multiplex Channel Separation Better than 30db at 1000 cycles. Multiplex Filter Greater than

24、 48db suppression of 19KC pilot and 38KC carrier. Multiplex Indicator Front panel multiplex stereo light activated by 19KC carrier-only. Multiplex Type Peak-detecting, self-matrixing detector. SCA Filter 50db down at 67KC to 74KC 275db per octave slope. 12 U.5.A. 9642MSM HMntosb LABORATORY INC. 2 Ch

25、ambers St., Binghamton, N. Y. Made in U.S.A. PhoneArea Code 607-723-6491 Design subject to change without notice. MODEL NUMBER: MR 71 MODIFICATION: This modification increases stereo separation on MR 71 tuners to better than 35 db separation. This modification is in units with serial numbers above 4

26、5B00. DESCRIPTION: The stereo multiplex detector in the MR 71 tuner is a self matrixing detector. It automatically recombines the L+R and LR stereo information to produce L and R output. The SCA filter causes a slight loss in the LR portion of the composite multiplex signal. This limits stereo separ

27、ation to about 30 db. Sepa ration is improved by increasing the difference signal gain in the tuner left and right audio amplifiers while leaving the common signal gain unchanged. This is done by connecting a RC network between the cathodes of the R and L in put audio amplifier stages. PROCEDURE: 1.

28、 Connect a 27K 5% Vj watt resistor and a .0015 MF 10% capacitor in parallel. Connect this network between pin #8 on one 6BL8 audio tube to pin #8 on the other 6BL8 audio tube. 2. Feed a multiplex signal modulated with 1KC audio left or right channel only into the tuner antenna input. Tune the tuner

29、to the MPX signal. Align the 19KC phase adjust (coil L5) for maximum stereo separation. It is pos sible to obtain greater than 35 db separation between channels. NOTE: Before making this alignment be sure the MPX generator is accurately set up. View the composite stereo signal at the generator with an oscilloscope. Do not attempt to estimate stereo separation by viewing the tuner discriminator output at TP 2. The signal at TP 2 should NOT look like an ideal multiplex signal. MATERIAL NEEDED:!, ea. resistor, 27K 5% Yzw 1. ea. capacitor, .0015 MFD 10%