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

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

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

Available Documents

User's Manual
Service Manual

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