McIntosh C28 Preamp Service Information Early Production.pdf

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1、HMntosh C 28 PREAMPLIFIER SERVICE INFORMATION FROM SERIAL NO. 10X01 TO 91X49 f McIntosh laboratory inc. 2 chambers street binghamton, new york C 28 C 28 ELECTRICAL SPECIFICATIONS FREQUENCY RESPONSE +0 to 0.5 dB from 20 Hz to 20,000 Hz. DISTORT ION Less than 0.1% at each output at rated output level,

2、 20 Hz to 20,000 Hz. INPUT SENSITIVITY AND IMPEDANCE Phono 1 and Phono 2 2 millivolts at 47k ohms (1,000 Hz) Aux, Tuner, Tape 1 and Tape 2 0.25 volts at 250k ohms M i c rophone 2.5 millivolts at 500k ohms Tape Head 2 millivolts at 500k ohms (500 Hz) HUM AND NOISE Aux, Tuner, Tape 1 and Tape 2 90 dB

3、below rated output Phono 1, Phono 2 and Tape Head 78 dB below 10 millivolt input, equivalent to less than 1.2 microvolts at the input terminals. Mic rophone Less than 1.5 microvolts at the input terminals. OUTPUT LEVEL AND IMPEDANCE Main Output 2.5 volts with rated input, less than 100 ohms source i

4、mpedance, to operate into 47k ohm or greater load. Tape Output 0.25 volts with rated input. Less than 150 ohm source impedance, to operate into 47k ohm or greater load. Headphone/Line Output 0.75 volts into 8 ohm load or 3.0 volts into 600 ohm line. Less than 0.2 ohms source impedance. Level control

5、s provided. Center Channel 0.7 volts with rated input to both channels. Level control provided. BASS CONTROL + +20 dB at 20 Hz. TREBLE CONTROL +18 dB at 20,000 Hz. L.F. FILTER Flat or roll off below 50 Hz, active filter 12 dB/octave down 18 dB at 20 Hz. H.F. FILTER Flat or roll off above 7,000 Hz, a

6、ctive filter 12 dB/octave down 20 dB at 20,000 Hz. POWER REQUIREMENT 117 volts, 50/60 Hz, 45 watts. C 28 C 28 BLOCK DIAGRAM RED/WHT a_U/VWT EXTERNAL TAPE REC. (SHOWN IN PHONO 1 POSITION) C 28 SCHEMATIC NOTES 1. Unless otherwise specified: Resistance values are in ohms, 1/2 watt, and 10/ tolerance; c

7、apacitance values smaller than 1 are in microfarads (pF); capacitance values greater than 1 are in picofarads (pF); inductors are in microhenries (pH) . 2. Printed circuit board components are outlined on the schematics by dotted lines. The circled numbers around the dotted lines correspond to the n

8、umbers on the PC Board layouts. 3- The heavy lines on the schematics denote the primary signal path. 4. The terminal numbering of rotary switches is for reference only. 5. All voltages indicated on the schematics are measured under the following conditions: a. Use of an 11 megohm input impedance VTV

9、M. b. All voltages +10% with respect to chassis ground. c. No signal at input terminals. d. AC input at 117 volts, 50/60 Hz. e. Front panel controls at: Volume Fully CCW, but with power switch on Mode Stereo Input selector Phono 1 Loudness F1 at All other controls at normal positions. 6. In units wi

10、th Serial No.s below 23X01, S302 is SPST switch (McIntosh Part No. 148-017); R303 is 10% tolerance and R304 is 8.2k. 7. In units with Serial No.s below 17X01: R65 R219 and R63 R235 and R236 are used. 13. in units with Serial No.s below 56X57: R243 and R244 are 4.7k. HIGH LEVEL S C 28 C 28 C 28 C 28

11、GRN / WHT LEFT INPUT ( FROM LEFT OUTPUT OF HIGH LEVEL SECTION ) FILTER When customer complains of this type of interference. Parts Required: Quantity Part No. Description 12 6 2 136296 061128 061023 Resistor; lKfl, 1/4w, 10% Capacitor, disc: 470pF Capacitor, disc: lOOpF Procedure: 1. Remove top and

12、bottom covers. 2. Remove the screws holding the level set panel in place. 3. Referring to diagram 1 and with the C 28 upside down, locate the four coaxial cables that connect to push button switches, Sl-B and Sl-D. Insert the 1Kft resistors in series with each coaxial cable right at the switch termi

13、nal. 4. Referring to diagram 1 and with the C 28 upside down, replace each of the two wires (Blue and Blue/white) that connect the front panel Tape-Output jack with push button switch Sl-B, with 1Kfl resistor. Provide sleeving on both resistor leads. 5. Referring to diagram 2 and with the C 28 right

14、 side up, locate Section 3 - wafer (front and rear) of the input selector switch. Locate the coaxial cables connected to lugs 9-rear and 3-front. Insert a 1Kft resistor in series with each cable right at the switch lugs. 6. Referring to diagram 3 and with the C 28 right side up, locate the volume co

15、ntrol underneath the level set panel. Locate the coaxial cables connected to the center lugs of the volume control. Insert a 1kQ resistor in series with each cable right at the control lugs. Use sleeving to prevent shorting to panel. 7. Referring to diagram 4 and 5 and with the C 28 upside down loca

16、te the 044309 high and low level PC board. Connect a lOOpF and a 470pF capacitor for each channel as indicated on the PC board diagram 4 and schematic diagram 5. Capacitor leads should not exceed 1/4 inch, (OVER) CO co - McIntosh Laboratory Incorporated 2 Chambers Street Binghamton, New York 13903 P

17、hone: Area Code 607-723-3512 No. 8. Referring to diagram 6 and with the C 28 upside down, locate the filter and headphone PC board- 044509. Insert a 1KQ resistor in series with the blue wire connected to pin 24 and the blue/white wire connected to pin 13. Both resistors to be inserted right at the p

18、ins. 9. Referring to diagram 6 ans with the C 28 upside down, locate the filter and headphone PC board- 044509. Connect 2 each 470pF for each channel as indicated on the diagrams. Capacitor leads should not exceed 1/4 inch. 10. Referring to diagram 7 and with the C 28 right side up, locate the blue/

19、white and green/white coaxial cables that connects to the right and left output level controls on the level set panel. Disconnect the blue shield - wire of the blue/white coaxial cable from the terminal strip and reconnect it to an adjacent lug of the same terminal strip. This lug is already occupie

20、d by a yellow wire. Disconnect the green shield - wire of the green/white coaxial cable from the terminal strip and reconnect it to an adjacent lug of the same terminal strip, already occupied by a yellow wire. The net effect of this would be as shown on diagram 7. 11. Replace level set panel and to

21、p and bottom covers. Test for proper function of all inputs and outputs. Check specifications. 039-079 SH. 1 of 3 DIAG. 3 HIGH 8 LOW LEVEL PC 80AR0 044-309 DIAG. 4 039-079 SH. 2 of 3 o* DIAG. 5 FILTER a HEADPHONE PRINTED CIRCUIT BOARD 044-509 DIAG. 6 DIAG. 7 039-079 SH. 3 of 3 Tone control (treble)

22、130-017 Tone control (bass) 130-018 Tone control (bass) 130-019 LAMPS #1866 (Front Panel) 058-01 A #1847 (Pushbutton Lamps) 058-008 1 ndicator Lamp | 058-041 FRONT PANEL & TRIM Front panel 044-067 Front panel end caps 018-120 Volume contrpl knob 044-372 Mode selector knob 044-372 Input selector knob

23、 044-372 left bass knob 044-371 Right bass knob 044-37) Left treble knob 044-371 Right treb1e knob 044-371 Ba1ance knob 090-100 Comp, knob 044-375 Pushbutton O 1 ro CO Bass trim knob 090-010 Phono level knob 090-010 Headphone level knob 090-010 Output level knob 090-010 Center channel knob 090-010 M

24、OUNTING SYSTEM Shelf Bracket (right) 043-622 Shelf Bracket (1 eft) 043-623 Mounting Template #100 038-179 Hardware Package 043-792 MISCELLANEOUS ITEMS Line Cord 170-021 Fuseholder 178-001 Sh i pping carton 044-082 Plastic feet 017-041 Shorting p1ug 127-001 Audio cable (61) 170-015 Owners manual 038-

25、628 C 28 SCHEMATIC PART NO. 038-755 REPLACEMENT PARTS FOR SCR 1 K1 ,2 Relay 3PST 087-011 Terminal Block 074-003 Interconnecting Cable 170-062 REPLACEMENT PARTS FOR SCR 2 K1 ,2 Relay 3PST 087-01 1 K3 Relay SPST 087-009 Terminal B1oc k 074-003 AC Power Cord 170-068 Interconnecting Cable 170-067 1 OC102 5S7-M3755