Manufacturer:	Loewe
Model	Multitel D
Year:	1992
CPU:	Intel 8051
Memory:	64kB of RAM Battery-backed NVRAM
Display:	CRT, monochrome, white
Keyboard:	Built-in
Main port:	Telephone line
Additional ports:	Centronics port for printer V.24 serial port Handset connector 12V printer power connector
	Manual Manual for Multikom L1
		Peripherals in collection:  - None Similar unit: Teleguide

My unit has been stored probably in a garden shed and then, in state of quite large humidity, turned on. While it worked before I bought it as "operating".
After drying and a quick review I turned it on again. A few seconds and loud cracking sound came from the read accompanying with sweet-like odour. The power supply shut down.
Disassembly is easy - unlock two latches in the upper side, similar are in lower one, no single screw (like in Siemens units where there was one bolt).
After inspecting it again, I found 2 filtering capacitors (Y configuration - capacitors between L or N wires and unit's ground) cracked. I decided to temporarily run the machine without input power filter (as it usually goes through filter in surge protector). Still not starting, no power, no sign of life... and no current consumption.
I found that soldered-in cylindrical fuse has been faulty. Replacing it with polymer fuse gave not-so-good results (with sparks and fuses blown in a whole building) so I decided to stuff a typical glass fuse holder there, especially that I had space for it.
Starting through the light bulb, I found that it indeed draws a small current, but it doesn't even start the converter.
I decided to go with multimeter from power cable, through keying transistor, to transformer. If everything would be OK there, then from transformer's secondary coils to regulators, then to feedbacks. In this unit power supply's secondary side smoothly blends into CRT electronics on mainboard. First, I found that not only capacitors in filter were bad - the coil was broken too, and it was broken specific way: Two coils not conducting from power cable's side, but both shorted in rectifier side. After skipping coils I decided to unsolder rectifier too.
Between coils and keying transistor, usually not many things are present. First, there is a rectifier bridge, here made of four 1N4007 diodes. Next, a large electrolytic capacitor has its place to store DC for operation. Next, we have smaller capacitors, optionally voltage-equating high-power resistors helping transistor in achieving working condition, and finally the switching transistor.
2 diodes in rectifier conducted in both directions. Replaced. High-power resistors were good. Device finally booted up.

BTX, Bildschirmtext (German: On-screen text) was a telephone service similar to French Minitel, operating by presenting text on screen of terminal. Connection was made using a simple modem and pages could be used purely for reading, as well as for ordering some services.
The BTX required special terminal which could be bought or in some cases rented from Post.
BTX was in development since mid-1970s, when British PRESTEL system has been invented. In 1980 first field tests have been performed with 2000 recipients in 3 distant cities. Finally BTX became an offered commercial medium in 1983, when contract between Germany's lands has been signed - everyone who had enough money could then buy a page in BTX. End user viewing page could pay per page viewed or per time visited on page (per minute). High-end BTX terminals had floppy drives which could store some pages for later reading.
It was predicted, based on Prestel and Minitel observations, that in 1986 number of subscribers will be one million. In fact, it was only 60000. They finally reached the million a decade later... when they connected BTX to the Internet.
After home computers and PCs became more popular, BTX modems have been sold too. At 31st of December, 2001, BTX has been turned off. A reduced version for financial institutions (which switched to BTX from telegrams) was in operation until 2007.
The BTX was also in operation in Austria, Switzerland (known as Videotex - without t at the end, VideotexT was a name for teletext).
Most of BTX pages were stored in one computer in BTX central in Ulm. The exceptions were "interactive" service pages transmitted through regional computer centers - like some banking pages or Quelle mail-order shop which was well advertised service in BTX. Notice that on this Loewe terminal there is a small Quelle logo near Esc key.
Why BTX had so small popularity? First, addressing thing. In Minitel, you just dial 3615 and page name, easy to remember. BTX has only page numbers, starting with * and ending with #. Names, could be registered, but in early times they were "phonewords". 3615 became a symbol of Minitel being part of numerous services names, while numbers-only BTX was hard to remember. More, wider access to free knowledge exchanging methods (proximity of East Germany was not only consistent threat, but also a cure for "intellectual property" mental disease) made it sufficient only for services, which, when totally switched to BTX, became outdated already.
While in Minitel setting a page or service was relatively cheap and affordable even for individuals, BTX was so expensive that usually only larger companies authored pages. Users also had to pay more and buy the terminal (in Minitel you just got it with subscription). Finally, the 0.3DM for sending a message to other user quickly became much more than price of modem-dialing to send an e-mail, even through FIDOnet chain.

Controls, pinouts

Facing screen, on the left there is a contrast knob. Right side: Knob closer to user is for speaker volume, rear knob for ring volume. To change brightness use a screwdriver, the trimmer is accessible by the hole on the bottom.
According to the manual, the AUX connector is a V.24-level serial port, by default 9600, 8-bit, 1 stop bit. Shift-F1 in some firmware allows to set parameters for AUX.
AUX connector pinout (check with voltmeter in your unit! Source: Manual)
1. DCD
2. RxD
3. TxD
4. DTR
5. SGND
6. DSR
7. RtS
8. CtS
9. +5V

Links:
https://www.loewe.de/uploads/tx_bmfaquserguides/multi_tel_d.pdf - User's manual, in German.
http://www.btxmuseum.de/Hardware/geraet.asp?id=32 - In BTX Museum