My Telefunken Collection


TA 750



Device Type

Integrated Amplifier

Start of Sale

1979

General Description

Both in terms of features and output power, the TA 750 is probably the largest integrated amplifier Telefunken built in that era. Even the discrete main amplifiers like the STM 1 didn't match its output power of 2x90W RMS, and no other device featured controls like high/low pass filters with selectable turnover frequencies. A look into the housing shows that many modules were reused from other devices, like the input section from the TA 350 or the power amp section from the HR 3000/4000. However, the power amp module was driven to its limits, which can make this amplifier a bit...problematic. More about this later.

Features

2x90W RMS output power, bass/presence/treble control indivdually adjustable for both channels, subsonic/noise filters with selectable turnover frequency, LED VU-meters, two speaker outputs, individually switchable.

Connectors

Microphone (DIN+6.3mm), 2xPhono (1xCinch, 2xDIN), Tuner (DIN), Aux (DIN+Cinch), 2xMonitor (1xCinch, 1xDIN), Tape (DIN), output for additional amplifier (DIN), output for equalizer (DIN), speakers (both DIN and clamps), headphone (DIN+6.3mm)

Technical Data

(taken from the User's Manual)

Nominal Load: 4 Ohms
RMS Output Power: 2x90 W
Peak Output Power: 2x130 W
Distortion Factor: 0.02 % at 1 kHz (70 W / 4 Ohms)
typ. < 0.1 % at 90 W / 4 Ohm, 10-16000 Hz
Intermodulation Factor: 0.1 % at nominal power, 250 Hz/8 kHz, 4:1
Power Bandwidth: < 5 Hz ... > 50 kHz for K <= 0.7 %
Dampening Factor: 26 at 40 Hz - 12.5 kHz
Transmission Range: < 4 Hz ... > 50 kHz for +/- 1.5 dB
Phono deemphasis: according to DIN 45 547
Nominal Input Voltages/ 190 mV/470 kOhms/27 dB (Tape)
 
Input Impedances/ 190 mV/470 kOhms/27 dB (Aux)
Overdrive Resistance 230 mV/~470 kOhms/27 dB (Monitor)
(at 4 Ohms) 1.8 mV/47 kOhms/27 dB (Phono)
1.5 mV/47 kOhms/27 dB (Micro)
190 mV/470 kOhm/27 dB (Tuner)
Output Levels 1.4 mV/kOhm at Tape Rec and 40 kHz Modulation
at Nominal Output Power 18.4 V at Speaker Output at 4 Ohms
7.4 V at DIN Headphone (200 Ohms)
8.8 V at 6.3 mm Headphone (8 Ohms)
1V at 'Amplifier Out' Output
 
Signal/Noise Ratio 63 dB (Tape + Aux)
(DIN 45 500) 62 dB (Phono + Micro)
Dampening between L/R: > 55 dB at 1 kHz
> 40 dB at 10 kHz
Dampening among Inputs: > 70 dB at 1 kHz
> 60 dB at 10 kHz
Tone Controls: Bass: +17.5 to -17.5 dB at 40 Hz
Treble: +12.5 to -12.5 dB at 15 kHz
Presence: +10 to -10 dB at 2 kHz
Balance Control: +3 to -16 dB
Subsonic Filter: 20+100 Hz Frequency = 12 dB/Oct.
Noise Filter: 5+10 kHz Frequency = 18 dB/Oct.
General
Semiconductors: 19 Integrated Circuits
16 Transistors
19 Diodes
23 LEDs
Mains Voltage: 220/110 Volts at 50/60 Hz
Fuses: Mains 220 V 1 x T 2.5 A
110 V 2 x T 2.5 A
Secondary 4 x T 4 A (Power Amp)
T 1 A
T 500 mA
Dimensions: 460 x 145 x 350 mm
Weight: 11 kg

Common Failures

The TA 750 uses an LM391-based circuit concept, and it shares the same problems with other devices that use this power amp module: occasional blowups without obvious reasons. If you look closely at the IC sockets of the LM391 chips, you can see the remainders of their predecessors: The IC literally explodes since it is directly connected to the +/- 42 V main supply and a short is absolutely lethal to it. Typically, such a blowup also takes the driver and power transistors with it. Even when transistors seem to look OK, on a component tester, it is good practice to swap them either - they may have partial damages that only show up when operating under full load and cam kill a freshly repaired amplifier within seconds. That is no fun these days, since National Semiconductor has discontinued the LM391 some years ago and a replacement IC easily costs about 10 to 15 Euro...

It's difficult to tell why the TA 750's power am section blows up so often. One reason is surely that the LM391 is driven near to its limits (National specifies it for amps up to 100 W RMS), but it's also true that Telefunken omitted a protective circuit with a resistor and clamping diodes. This way, the LM391's feedback input is directly connected to the output and is potentially faced with voltage swings beyond the supply voltages. This can lead to a breakdown in the IC and a catastrophic chain-reaction...

Spare Part Numbers

(taken from the service manual)

Cabinet Parts
501 309 796 042 cabinet, silver
502 309 796 043 cabinet, brown
503 309 833 559 front cover, silver
504 309 833 560 front cover, brown
505 309 833 561 cover for VU meter
506 309 833 890 cover for fuse
507 309 770 920 cabinet foot
508 309 802 122 lower control knob for treble, bass, and presence
509 309 802 123 upper control knob for treble, bass, and presence
510 309 802 117 lower control knob for volume
511 309 802 118 upper control knob for volume
512 309 802 115 control knob for speaker and function
513 309 802 113 control knob for level
514 309 800 129 push button knob, silver
515 309 800 128 push button knob, brown
516 309 809 951 toggle switch knob, silver
517 309 809 952 toggle switch knob, brown
518 309 833 535 cover for toggle switch knob
519 309 833 562 spacer for toggle switch knob
520 309 866 983 guide for toggle switch knob
521 309 921 915 mains switch rod with knob, brown
522 309 921 917 mains switch rod with knob, silver
Electrical Parts
Position Order No. Designation
BS 5341 349 654 943 AF control board
BS 5342 349 369 991 input selector switch board
BS 5345 349 369 986 re-record board
BS 5419 349 364 974 AF power amplifier board
BS 5508 309 653 943 mains fuse board
BS 5521 309 310 132 mains transformer with fuse board
BS 5733 349 395 023 VU meter board
BS 5735 309 369 979 relay board
BS 5741 309 369 984 level switch board
BS 5761 309 651 970 loudspeaker socket board
BS 5763 309 369 992 loudspeaker switch board
BS 5764 309 369 980 microphone socket board
BU 4 309 672 917 pawl socket
BU 4 309 672 919 microphone socket
BU 5/6 309 672 503 DIN socket, 5 pins
FU 1 309 627 916 fuse T 2.5 A
S 1 309 630 994 main switch, 2-phase
S 301/1301 309 639 011 rotary switch
525 309 695 935 power cord with plug
AF Adjust Board
Position Order No. Designation
BS 5341 349 654 943 AF adjust board
C 301/307/352/ 309 410 634 AL-electr. capacitor 4.7 µF/25 V is.
331/1301/1307/
1325/1331
C 306/316/329/ 309 412 645 AL-electr. capacitor 47 µF/25 V is.
1306/1316/1329
C 319/1319 309 411 659 AL-electr. capacitor 22 µF/25 V is.
C 340/1340 309 412 653 AL-electr. capacitor 47 µF/35 V is.
D 5 309 327 022 LED FLV 110 red
IC 301/302/303/309 368 212 IC HA 1457
1301/1302/1303
L 301/1301 309 249 083 filter coil 120 kHz
L 302/1302 309 249 082 filter coil
R 314/1314/318 309 511 064 variable resistor 2 x 100 kOhm, volume
R 322/1322 309 511 065 variable resistor 2 x 100 kOhm, presence
R 325/1325 309 511 066 variable resistor 2 x 100 kOhm, bass
R 331/1331 309 511 067 variable resistor 2 x 100 kOhm, treble
S 301 309 639 980 toggle switch 2xU
S 302/303 309 639 971 toggle switch 4xU
S 304 309 639 977 toggle switch 5xU
S 305/306 309 639 012 toggle switch 5xU
526 309 639 006 frame for 2 toggle switch
527 309 639 007 frame for 4 toggle switch
528 309 900 295 diode support
Input Selector Switch Board
Position Order No. Designation
BS 5342 349 369 991 input selector switch board
BU 301/302 309 679 503 cinch socket
BU 303/304/305 309 672 801 DIN socket, 5 pins
C 301/312/ 309 410 728 AL-electr. capacitor 4.7 µF/40 V
1301/1306
C 306/1306 309 412 645 AL-electr. capacitor 47 µF/25 V
C 308/1308 309 414 794 AL-electr. capacitor 100 µF/35 V
C 316 309 410 634 AL-electr. capacitor 4.7 µF/35 V
IC 301/1301 309 368 212 IC HA 1457
L 301/1301 309 249 053 RF choke
S 301 309 639 005 rotary switch
Re-Record Board
Position Order No. Designation
BS 5345 349 369 986 re-record board
BU 301-304/ 309 679 503 cinch socket
308/309
BU 305/306/310 309 672 801 DIN socket, 5 pins
S 301 309 635 911 pressure switch
AF Power Amplifier Board
Position Order No. Designation
BS 5419 349 364 974 AF power amplifier board
C 2/3 309 414 784 AL-electr. capacitor 10000 µF/50 V
C 403/404 309 411 703 AL-electr. capacitor 22 µF/6.3 V
C 421/422 309 410 733 AL-electr. capacitor 2.2 µF/6.3 V
C 501 309 412 679 AL-electr. capacitor 47 µF/25 V
C 505 309 414 737 AL-electr. capacitor 2200 µF/40 V
C 506 309 414 766 AL-electr. capacitor 1000 µF/63 V
D 1-4 309 327 008 Diode MR 751
D 401/402/ 309 325 027 Diode 1 N 4148
403/404
D 501 309 325 105 BZX 55 C 16
D 503-510 309 325 953 Diode 1 N 4003
FU 501 309 627 919 fuse T 1 A
FU 502 309 627 919 fuse T 500 mA
IC 401/402 309 368 174 IC LM 391 N-90
IC 501/502 309 368 130 IC UA 78 GU 1 C
R 1-4 309 556 224 wire resistor 0.22 Ohm/1 W
R 411/412 309 504 802 variable resistor 2.5 kOhm/0.15 W
R 429/430 309 504 924 variable resistor 2.5 kOhm/0.07 W
S 1 309 639 978 thermo switch
T 401/402 309 639 978 transistor BF 254
T 403/404 309 001 223 transistor BD 237
T 405/406 309 001 234 transistor BD 238
T 407/408 309 001 108 transistor BC 307 B
T 409 309 001 956 transistor BC 237 B
VU Meter
Position Order No. Designation
BS 5733 349 395 023 VU meter board
C 801/803 309 410 643 AL-electr. capacitor 2.2 µF/25 V
C 802/804 309 411 673 AL-electr. capacitor 22 µF/16 V
C 805 309 410 688 AL-electr. capacitor 1 µF/25 V
C 806 309 410 641 AL-electr. capacitor 4.7 µF/25 V
D 801-804/ 309 325 925 Diode 1 N 4148
821-824/835/
836/838/841
D 805-814/ 309 327 053 LED CQY 85 red
825-834/839/
840
D 837 309 325 047 Diode BZY 85 C 18
IC 801/806 339 335 855 IC MC 1458 CP 1
IC 802/804 309 368 238 IC U 247 B 5 LED
IC 803/805 309 368 239 IC U 237 B 5 LED
S 801 309 632 946 sliding switch, 3 positions
530 309 900 292 diode support, 12-fold
531 309 651 987 sockets bar, 8 contacs
Speaker Switch Board
Position Order No. Designation
BS 5735 309 369 979 relay board
BS 5761 309 651 970 loudspeaker socket board
BS 5763 309 369 992 loudspeaker switch board
BU 3 309 672 917 pawl socket
C 701 309 414 799 AL-electr. capacitor 220 µF/6.3 V
D 701/702 309 325 027 Diode 1 N 4148
IC 701 309 368 206 IC NE 555 N-H
R 403/1403 309 556 985 wire resistor 27 Ohm/4 W
RS 701 309 636 933 relay
S 1 309 639 008 rotary switch
T 701/702 309 001 956 transistor BC 237 B
Mechanical Parts
Position Order No. Designation
535 309 863 988 frame chassis
536 309 831 715 ground foil for frame chassis
537 309 866 984 guide tape-copy
538 309 866 985 guide for power switch, round
539 309 951 955 protection cap for power switch
540 309 931 966 support sheet for range rotary switch
541 309 943 017 shaft
542 309 928 904 coupling
543 309 921 918 switch rod for tape-tape copy
544 309 900 295 diode support
545 309 900 296 electrolytic capacitor support
546 309 653 501 fuse holder
547 309 689 904 IC socket

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©2002 Alfred Arnold, alfred@ccac.rwth-aachen.de