This page describes the German SEM-25 transceiver. I compiled this information to help other collectors who have (or are thinking of getting) a SEM-25, but cannot find technical information about it; manuals and schematics for this set are very rare in the U.S. at this time.

Please note that I do not have any original manuals for this set, and I do not even have a complete set of copied manuals (for example, I do not have any aligment procedures, and I would have great difficulty understanding them even if I had them due to my very poor command of the German language). I cannot provide any further information beyond what is described here at this time, and I cannot provide manual photocopies or other printed information at this time. This page truly contains everything I know about the SEM-25!

Please comment me if you discover any errors in this paper.

I hope you find this information useful!

 

---

Description

 

The SEM-25 is a West German short-range tactical vehicular VHF FM transceiver which was introduced in the 1960’s, and was apparently used until at least the 1980’s. Although surplus SEM-25 radios are not yet very common in the U.S., some have recently appeared on the U.S. surplus market from vendors such as Murphy’s Surplus Warehouse.

The SEM-25 covers 26.00 to 69.95 MHz, in 50 kHz steps. It is all solid-state, except for three tubes in the transmitter power amplifier. A SEM-25 system will typically consist of at least the following items:

• Transceiver chassis
• Control box (may be mounted on front of transceiver, or remotely)
• Mounting/power supply
• Power transient suppressor
• Antenna tuning unit
• Antenna
• Audio accessories ranging from a simple handset to several junction boxes and headsets
• Various cables

More complicated systems may consist of two SEM-25 transceivers and one EM-25 receiver, with lots of audio gear and cabling, thus providing a set with retransmission and intercom features.

So far, I have only obtained a transceiver chassis, control box and mounting, so I cannot provide much information about the antenna, antenna tuning unit, power surge protector or other accessories at this time. The audio connections on the control box and main chassis are compatible with common 1950’s U.S. audio accessories such as the H-33/PT handset and LS-166/U loudspeaker. Pictures I have seen of the correct German audio accessories (well, actually photocopied photocopied photocopies…) suggest that the correct German handset (called an H-33G) appears virtually identical to its U.S. counterpart, and the correct loudspeaker has a case like a U.S. LS-7 loudspeaker with a mounting like a U.S. LS-166. The headset looks a lot like a U.S. H-63/U with an AN/GSA-6 control box. The antenna whip elements are called MS118AK, MS117A and MS116A, and look very similar to U.S. elements with similar names, and the antenna base looks a lot like the U.S. MP-65. The knobs on the control box look just like 1950’s U.S. knobs. I’m really surprised at the similarities between this set and comparable U.S. sets, in spite of the fact that they were designed and built in different countries.

I first learned of the existence of the SEM-25 when I saw a pile of them on the shelves in the famous back room of Murphy’s shop in late 1998. Although I had never been very interested in foreign military gear before, the SEM-25 looked like a neat radio, so I talked my folks into getting me one for Christmas in 1998. My dad and I made the hour-long drive down to Murphy’s shop on Christmas Eve, and I picked out the nicest-looking radio from the pile. It was even gift-wrapped in a stylish olive drab canvas cover!

At the time, Mike was charging $350 for the SEM-25, and $250 for a smaller but similar-looking radio which he called a “SEM-20”. The smaller radio is actually an EM-25 receiver; it’s basically a receive-only version of the SEM-25. It’s easy to get confused about the features and identification of these two radios, because they both use the same control box and share many modules, so the EM-25 is prone to have stickers which say “SEM-25” (or even “SEM-35”; it shares two internal modules with the SEM-35 backpack transceiver) on and inside it, and it will have controls for transmitting functions which it does not perform.

The “SEM” in “SEM-25” is short for “Sender-Empfangsgerät”, which is German for “transmitter-receiver”. Similarly, the “EM” in “EM-25” is short for “Empfangsgerät”, which is German for “receiver”.

 

---

Specifications

General

 

Modes	Simplex, retransmit
Frequency range	26.00-69.95 MHz
Channel spacing	50 kHz
Number of channels	880
Channel selection	44 1-MHz bands, 20 50-kHz channels each
Programmable channels	10
Modulation	FM
Operation, including frequency selection	Operated from a transceiver-mounted control box, or from a relocatable control box in vehicle (max. cable length 10m)
Transmitter operation	From handset or aux. receiver From on-board intercom From telephone remote control
Remote input and output arrangement	From telephone remote control over field phone wires, up to 3 km long, max. resistance 480 ohms
Temperature range	Operating range from -45°C to +60°C
Overvoltage protector	For transients up to 65V from the battery regulator
Power supply	24V battery supply, negative ground, permissible voltage range 21V to 29V, short term overvoltage up to 32V

 

---

Transmitter

 

Output power	High: 15 W Low: approx. 1 W
Calling frequency	1600 Hz
Audio input	0.25 V from 150 ohm carbon microphone 0 dB from 600 ohm balanced line
Warm-up time	approx. 30 seconds

 

---

Receiver

 

Sensitivity	<= 0.5 µV for 20 dB s/n with 1000 Hz modulation and 10.5 kHz deviation
Bandwidth	>= 30 kHz at 6 dB points
Selectivity	+/- 50 kHz at 80 dB points
Audio output	0.6 W into 5 ohm loudspeaker 50 mW into 600 ohm headset 1 mW into 600 ohm balanced line

 

---

Power Supply

 

Voltage	24V battery (21V to 29V), negative ground, permissible short term overvoltage up to 32V
Power requirements, RX	approx. 10 W
Power requirements, TX Low	approx. 50 W while transmitting
Power requirements, TX High	approx. 80 W while transmitting
Power requirements, TX Low or TX High	approx. 29 W when not transmitting

 

---

Sizes and Weights

 

Item	Height (mm)	Width (mm)	Depth (mm)	Weight (kg)
Control box	174	228	75.5	2.54
Transceiver	222	268	255	14.59
Transceiver mount	120	320	300	9.43
Receiver	222	196	255	11.08
Receiver mount	120	320	230	6.68
Antenna tuning unit	122	290	110	3.70
Overvoltage protector	93	250	126	2.62
Spares case	48	140	140	0.73

 

---

Controls

This section decribes the functions of the controls on the SEM-25 control box.

 

Ref.	Description
A	Channel programming drum (behind cover)
B	Call button: Keys transmitter, sends 1600 Hz tone
C	Squelch
D	Volume
E	Manual tuning, 1 MHz steps
F	Manual tuning, 50 kHz steps
G	Power
H	Channel selector (H = manual tuning)
I	Power-on light (turn bezel to dim)

 

 

 

---

Power Switch Settings (Ref. G)

 

AUS:	Off
EMPF:	Receive-only
SENDEN KLEIN:	Transmit/receive (low power)
SENDEN GROSS:	Transmit/receive (high power)

 

---

Squelch Switch Settings (Ref. C)

Note: Squelch is opened by carrier level, equivalent to “old” mode in U.S. radios.

 

RAUSCHSP. AUS:	Squelch off
RAUSCHSP. EIN:	Squelch on
RELAIS:	Relay (retransmit)

 

---

Connectors

This section describes the functions of the external connectors on the SEM-25.

 

Ref.	Description
J	From transceiver #1 in multi-radio sets
K	Antenna tuner control
L	Antenna RF
M	Power input
N	Mounting/transceiver interconnect
O	Audio
P	Control box/mounting interconnect
Q	Receiver fuse (2A)
R	Transmitter fuse (6.3A)
S	To transceiver #2 or aux. receiver in multi-radio sets

 

 

 

---

Modules

This section identifies the SEM-25’s major internal modules. Note that two of the modules (as noted below) are also used in the SEM-35 backpack/vehicular transceiver.

 

Ref.	Description
N	Mounting/transceiver interconnect (external)
O	Audio connector (external)
P	Control box/mounting interconnect (external)
Q	Receiver fuse (2A) (external)
R	Transmitter fuse (6.3A) (external)
T	Transmitter RF
U	Receiver RF
V	Modulator amplifier, 1600 Hz tone generator, and 11.5 MHz discriminator for automatic frequency control (AFC) of transmitter
W	Audio amplifier
X	Receiver power supply and control amplifier
Y	Frequency synthesizer (also used in SEM-35)
Z	IF (Intermediate Frequency) module (also used in SEM-35)
AA	Plugs for test box
BB	10-frequency crystal oscillator
CC	Receiver servo
DD	Transmitter servo
EE	Transmitter power supply
FF	Motor drive power supply for external antenna tuner
GG	Relays for interconnect of multiple transceivers
HH	600 ohm balanced audio connections

 

 

 

---

Internal Adjustments

While we have the case open, here are some of the major internal adjustments:

 

Control	Label	Location	Function
R36	Rauschperre	Audio amp. (Ref. W)	Squelch level
R5	NF-Pegel	Audio amp. (Ref. W)	Audio level
R13	Eing.	Mod. amp. (Ref. V)	Mod. amp. input gain
R26	Ausg.	Mod. amp. (Ref. V)	Mod. amp. output gain
R38	Tacho	Rx power supply (Ref. X)	Stepping speed trim (?)

 

---

Schematics

The diagrams in this section were scanned at 150 pixels per inch, and you will probably need to use an external graphics-editing program to print them (simply clicking your browser’s “Print” button probably won’t work well).

The diagrams were all scanned from photocopied manuals, and the originals are just as blurry as the scans, so don’t bother asking for photocopies of the schematics! :-)

 

---

Block Diagrams

Description	File	Size
Transceiver	blk01.gif	1100×1338, 33k
Frequency selection	blk02.gif	1100×1540, 44k
Receiver	blk03.gif	1172×946, 20k
Transmitter	blk04.gif	1082×888, 12k
Antenna tuner (external)	blk05.gif	948×1240, 15k

 

---

Schematic Diagrams

The “Ref.” column shows the corresponding reference letters from the module identification pictures (where applicable), to help you figure out which part of the radio is described by each schematic.

Description	Ref.	File	Size
Wiring (main chassis)	—	sch01.gif	1744×1232, 90k
Transmitter RF	T	sch02.gif	1744×1232, 70k
Receiver RF	U	sch03.gif	1744×1232, 70k
Modulator amplifier module	V	sch04.gif	1744×1232, 62k
Audio amplifier module	W	sch05.gif	1744×1232, 55k
Receiver power supply module	X	sch06.gif	1744×1232, 66k
Frequency synthesizer module	Y	sch07.gif	2381×1231, 117k
IF module	Z	sch08.gif	2539×1232, 84k
10-freq. crystal oscillator	BB	sch09.gif	1193×768, 17k
Receiver servo	CC	sch10.gif	1196×778, 38k
Transmitter servo	DD	sch11.gif	1172×809, 19k
Control box	—	sch12.gif	3042×1226, 104k
Wiring (mounting)	—	sch13.gif	1744×1232, 94k
Transmitter power supply	EE	sch14.gif	778×1159, 36k
Motor drive power supply	FF	sch15.gif	1187×772, 26k
Interconnect relays	GG	sch16.gif	1197×794, 36k
Transient protector (external)	—	sch17.gif	1201×790, 34k
Antenna tuner (external)	—	sch18.gif	1744×1232, 68k

 

---

English/German Translations

This section lists rough translations of some German words, phrases and abbreviations as used in the SEM-25’s schematics, module labels, etc. My command of German is very poor, so there may be mistakes here… please comment if you can correct any of my translations!

Abstimmgerät

Radio tuner (tune + apparatus)

Abstimmteil

Radio tuner section (tune + part)

Additionsstufe

Addition stage

Anschluß

Connection

Antennen

Antennas

Antrieb

Drive, driver, motor

Ausgang

Output

Bausteinträger

Component carrier

Bauteile

Components

Bedien

Control head

Begrenzer

Limiter

Blockschaltbild

Block diagram

Bordbatterie

On-board battery

Bordverstärker

Intercom (on-board + amplifier)

Diskriminator

Discriminator

Eingang

Input

Einsatz

Part, component

Empf.

Receive or receiver

Empfänger

Receiver

Endstufe

Output stage

erdfrei

Balanced (i.e. “balanced line”; literally, “ground-free”)

Frequenzaufbereitung

Frequency synthesizer

Funk

Radio

geschlossen

Closed

gezeichnet

Drawn

gross

Large

Grundplatte

Mounting (base + plate)

HF

Radio frequency (RF)

Hinweis

Note

im

In

Kanalwahl

Channel selection

klein

Small

Leitung

Line, wire

Ltg.

Line, wire

Masse

Common, ground

Mischstufe

Mixer

Netzgerät

Mains power adapter

NF

Audio frequency

Oberton

Overtone

offen

Open

Oszillator

Oscillator

Prüfgerät

Test meter

quarz

Quartz

rauschperre

Squelch (noise + stop)

relais

Relay, retransmit

rufen

Call

ruhezustand

Off-state

senden

Transmit

Sender

Transmitter

Servoverstärker

Servo amplifier

Steuerverstärker

Servo amplifier

Stromversorgung

Power supply

Teil

Part

Transientschutz

Transient protector

Treibertufe

Driver stage

Trennstufe

Divider stage

Umwandler

Converter

Verstärker

Amplifier

Vorstufe

Input stage

ZF

Intermediate frequency (IF)

 

---

Credits

This work would not have been possible without the generous assistance of the following people:

• Wammes Witkop <wwitkop@cable.a2000.nl>
• Menno Putman <MennoPutman@hetnet.nl>

Pictures were taken with a Sony MVC-FD91 digital camera. Reference letters were added with Visio 4 on a Windows NT box; all other image editing was performed with xv and gimp on a Redhat Linux system.

Scanned documents were scanned with a Umax Astra 1220S scanner and the SANE software under Redhat Linux, and then edited with xv and gimp.