My Telefunken Collection


STM 1



Device Type

Main/Power Amplifier

Start of Sale

1980

General Description

THE STM 1 was the power amplifier in Telefunken's 1980 'state of the art' Hifi System. It practically contains the power amp section used in the TA 750, modified to fit into a 'slim-line' case. The power transistors in TO-3 case were replaced with types in plastic case, and the heatsink is flatter and wider and extends out ouf the housing at the rear side. The power amp board is the same, however a lot of components responsible for auxiliary supplies could be removed. The plastic frame with the main rectifier diodes and filter capacitors remained, but was 'laid flat' to make it fit.

The disadvantage of the STM 1 using the same circuitry as the TA 750 however gives it the same problems, but let's speak of that later...

like all components from this system, the STM 1 was also available as a non-rackmount version, which was named CM 20.

Features

Two speaker pairs attachable, headphone jack

Connectors

Input, Speaker Outputs, Headphone Output (all DIN)

Technical Data

(taken from the service manual)

Nominal Output Impedance:4 Ohms
Nominal Output Power: 2 x 85 W RMS
Peak Output Power: 2 x 120 W
Distortion Factor: < 0.1% @ 90 W / 1 kHz
Power Bandwidth: < 5 Hz ... > 58 kHz for K <= 0.5%
Dampening Factor: 20 @ 40 Hz / -12.5 kHz
Nominal Input Voltage: 1 V
Nominal Input Impedance: 47 kOhm
Output Level at 18.5 V at speaker output (4 Ohms)
Nominal Output Power: 7.4 V at headphone (200 Ohms)
Channel Separation: > 64 dB @ 1 kHz
> 47 dB @ 10 kHz
General
Semiconductors: 7 integrated circuits
14 transistors
12 diodes
21 LEDs
Power Supply: 220 / 110 V AC
Fuses: Mains 220V = 1 x T 4 A
110V = 1 x T 6.3 A
Secondary 4 x T 4 A (Power Amps)
2 x T 200 mA
Dimensions: 482 x 80 x 390 mm (W x H x D)
Weight: 8.8 kg

Common Failures

Unfortunately, there are. First of all, the STM 1 is a power amp based on National Semiconductor's LM 391 chip, a chip that integrates all necessary active components except for the driver and power transistors. Unfortunately, this IC has to be supplied with the full main supply of +/- 39 volts, which puts it under high stress, and even the smallest failure will make it go up in smoke. I have ICs that have literally a crater in their housing! The main problem seems that one of the IC's pins is directly connected to the speaker output, which may swing beyond the Vcc levels. National Semiconductor proposed a protective circuit with a resistor and two clamping diodes, but Telefunken did not use it. The typical result is that everything blows up, and even if a driver transistor still looks good on a component tester, it is good practice to swap it anyway.

The other problem is located in the PCB's front right corner: a linear regulator made of a zener diode and a transistor. This regulator provides the +18V supply for the LED chains, power LED, and relay control. The transistor becomes fairly hot, and may fail after a few years of operation. Replacing the heatsink with a larger one might be wise...

Furthermore, the STM 1 suffers from the same, simple failure as may other Telefunken devices in that era: a broken mains switch! Symptoms are that the device cannot be turned on or off any more, or a power switch button that doesn't move at all. The problem is the the 'ball pen' mechanics consist of a piece of metal moving in a plastic bearing, and the plastic wears out over time. Contrary to other devices, the switch is four-fold - The first contact pair switches directly, and the second pair bridges a pair of wire resistors a moment later. This way, the maximum in-rush current is limited, but it makes it hard to find a correct replacement. Seems however that a normal power switch without this current limiting trick works as well, like in this unit...

Spare Part Numbers

(taken from the service manual)

Cabinet Parts
Position Order No. Designation
501 309 796 048 cabinet, brown
309 796 050 cabinet, champagner
502 309 863 990 frame chassis
503 309 833 581 front mask, brown / STM 1
309 833 500 front mask, champagner / STM 1
309 833 596 front mask, brown / CM 20
309 833 597 front mask, champagner / CM 20
504 309 833 582 cover for VU-meter
505 309 061 750 cabinet foot
506 309 921 921 power switch rod with button
507 309 809 954 push button for speaker
508 309 853 973 handle for cabinet, brown / STM 1
309 853 974 handle for cabinet, champagner / STM 1
Electrical Parts
Position Order No. Designation
BS 5409 309 369 964 relay module
BS 5425 349 364 973 AF-O/P module
BS 5523 309 310 135 mains transformer
BS 5531 309 369 978 power supply for power stage
BS 5734 349 395 025 VU meter module
BS 5747 309 369 976 loudspeaker switch module
BU 1 309 679 948 input socket
BU 2 309 671 966 headphone socket
FU 1 309 627 702 fuse T 4 A
510 309 630 024 mains switch
511 309 695 935 power cord
D 1 309 327 022 LED FLV 110 red
309 900 295 diode holder, 1-fold
Relay Module
Position Order No. Designation
BS 5409 309 369 964 relay module
C 701 309 414 768 Al-electr. cap. 220 µF / 3 V
D 701 309 325 927 diode 1 N 4148
IC 701 309 368 206 IC NE 555 N-H
T 701/702 309 001 956 trnasistor BC 237 B
AF-O/P Module
Position Order No. Designation
BS 5425 349 364 973 AF-O/P module
C 403/404 309 411 703 Al-electr. cap. 22 µF / 6.3 V
C 421/422 309 410 733 Al-electr. cap. 2.2 µF / 63 V
C 506 309 414 766 Al-electr. cap. 1000 µF / 63 V
C 509 309 412 655 Al-electr. cap. 47 µF / 50 V
C 510 309 412 679 Al-electr. cap. 47 µF / 25 V
D 401-404 309 325 927 diode 1 N 4148
D 505/506 309 325 047 diode BZY 85 C 18
D 507-510 309 325 953 diode 1 N 4003
FU 501/502 309 627 913 fuse T 500 mA
IC 401/402 309 368 174 IC LM 391 N-90
R 409/410 309 504 802 variable resistor 2.5 kOhm 1 0.15 W
R 429/430 309 504 924 variable resistor 2.5 kOhm 0 0.07 W
T 403/404 309 001 275 transistor BD 230
T 405/406 309 001 276 transistor BD 231
T 407/408 309 001 108 transistor BC 307 B
T 409 309 001 812 transistor BC 237 B
T 411/412 309 005 008 transistor 2 SB 700 B
T 413/414 309 009 923 transistor 2 SD 736 B
T 501 309 001 101 transistor BD 167
Power Supply for Power Stage
Position Order No. Designation
BS 5531 309 369 978 power supply for power stage
C 2/3 309 414 784 Al-electr. cap. 10000 µF / 50 V
D 1-4 309 327 008 diode MR 751
FU 401-404 309 627 702 fuse T 4 A
515 309 653 501 fuse holder
VU-Meter Module
Position Order No. Designation
BS 5734 349 395 025 VU meter module
C 801/803 309 410 655 Al-electr. cap. 2.2 µF / 25 V
C 802/804 309 411 673 Al-electr. cap. 22 µF / 16 V
D 801-804/ 309 325 927 diode 1 N 4148
821-824
D 805-814/ 309 327 053 LED CQY 85 red
825/834
IC 801 339 335 855 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 801 309 639 015 slide switch
516 309 900 292 diode holder, 12-fold
Loudspeaker Switch Module
Position Order No. Designation
BS 5747 309 369 976 loudspeaker switch module
517 309 671 941 loudspeaker socket, black
518 309 671 942 loudspeaker socket, green
519 309 382 025 push button assembly
520 309 981 716 reversing spring for push button assy
S 401/402 309 640 986 chamber switch, 12 contacts
309 689 904 IC socket, 16 contacts

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