UHF converter

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Telefunken UHF converter UV2 with tension grid tubes EC86 and EC88.

A UHF converter is an assembly of the communications technology , the signals from the UHF frequency band linearly in another frequency band (typically channel 3 or 4 in the VHF band I ) is reacted .

UHF converters were used in the 1950s / 60s, particularly in television technology, to enable non-UHF-compatible television sets to receive television programs broadcast in the UHF band . Here, parts of the UHF frequency band have been converted into the VHF frequency band . The first devices were initially equipped with electron tubes , later with low-noise transistors .

In terms of their intended use, UHF converters are similar to today's set-top boxes , which also enable the reception of programs that cannot actually be received.

Commitment and history

Since television programs in Germany were only broadcast in the VHF frequency band until the 1960s, most of the television sets in use did not have a UHF-compatible tuner . In order to still be able to receive television programs in the then newly introduced UHF range with the existing devices, the UHF converters were developed for them. This enabled the new television stations, such as the ZDF , which started in 1963, to be received from the UHF band , later also the third television programs or, from 1969, the second program of the GDR German television station.

The first UHF converters were equipped with tubes, e.g. B. a model from Philips NT1152 with two triodes EC86.

In the beginning there were no UHF converters to buy in the GDR . In order to be able to receive West German UHF television stations, many citizens built themselves UHF converters. This required the illegal import of the required transistors from the Federal Republic of Germany. With the recordings of the first UHF transmitters in the GDR (second program of GDR television) there were detailed building instructions in later. It was a fixed frequency converter that could only convert one UHF transmitter into VHF band I. The transistors are specified as AF139 (BRD) or GF145 (DDR). Later there were commercially manufactured devices - fixed frequency converters and also converters that could be tuned by hand, such as the "UHF converter II" from VEB Elektrotechnik Eisenach, which could receive CCIR television channels 21 to 39.

There were also transistorized UHF converters in Germany such as the "transUHF" from Quelle GmbH.

In the early days of some large community antenna systems , the forerunners of cable television , special UHF converters were used in the head-end station in order to bypass the higher attenuation of the antenna cable network in the UHF range and thus to be able to transmit all TV stations cheaply in the VHF range. Older televisions in the connected cable network could also receive the desired UHF channels in the VHF band without their own converter or UHF tuner, since the UHF signal was converted to a free channel in band I. There were small versions, which technically differed only slightly from UHF converters, as well as larger versions for up to 50 participants.

In the 1980s there were still so-called cable converters for individual television sets, which converted the special channels of cable television into the UHF range, which was not used by cable television at the time, and thus made it possible to receive them without a cable tuner in the television set.

technology

Interconnection and operation

Converters fitted with tubes weighed over 2 kg and were e.g. B. equipped with the tubes EC88 and EC86. The transistor-equipped devices were much lighter. UHF converters had to be connected to a mains socket, to the existing VHF antenna and the (new) UHF antenna. The RF output cable from the converter is connected to the VHF antenna input on the TV.

For VHF reception, the converter had to be switched to VHF and the desired channel selected on the television set as usual. For UHF reception, the converter had to be set to UHF and the television set to channel 3 or 4. The desired station was tuned into the tuning button on the converter.

On some devices, the TV's power plug was plugged into a power socket on the converter. In the transistorized devices, the converter automatically supplied itself from a small transformer, the primary side of which was connected in series with the mains power supply of the television. Nevertheless, the signal had to be switched between VHF and UHF on these devices because there was no crossover . A tube-equipped device from Philips also had a mains connection looped through to the television; Here a bimetal switch ensured that the converter switched on when the television was drawing power.

electronics

UHF converters work according to the superposition principle .

The antenna signal is passed through a band pass to a selective preamplifier with a pre-circuit tuned to the receiving frequency. The tuning was done with a multiple variable capacitor , but there were also fixed frequency converters that only had trimmers and were tuned to the (often only) UHF transmitter.

The amplifier stage was a transistor (e.g. a germanium mesa transistor AF239) or a grid-based circuit with the EC86.

The signal then passed to a self-oscillating mixer stage (the oscillator is also a mixer), consisting of a further transistor (e.g. AF139) or a further triode. Another circuit variant used a germanium mixer diode and a separate oscillator working with a triode EC86. The oscillator's resonant circuit, which is also capacitively adjusted, had to oscillate at a frequency that is lower than the receiving frequency by the output frequency . This was the only way to ensure that the resulting difference frequency (i.e. the HF output signal in band I) had the correct position of the video sideband and the sound carrier in relation to the video carrier.

Both transistors and triodes work in the low-reaction base or grid-base circuit.

All UHF resonant circuits or band filters of the input circuit and the oscillator were line circuits or pot circles , i.e. H. they consisted of a closed chamber in which a stretched thick wire or a web served as an inductor. The coupling between the circles took place by means of coupling loops. The structure corresponded to the UHF tuners of the time.

The selection of the difference frequency in band I or the suppression of the leakage of the amplified UHF or oscillator frequency was carried out with conventional LC circuits.

Alternatives

In some cases, television receivers were prepared for retrofitting with a UHF tuner. In this case a manufacturer-specific UHF retrofit kit was available, the output of which was connected to the input of the image IF amplifier.

There were also universal UHF built-in parts (e.g. from Grundig ). These could be built into all televisions (including those not prepared for UHF), provided they used an intermediate frequency of 38.9 MHz. It was installed either on a free point on the side wall of the housing or in an unused loudspeaker cover. If there was no separate antenna socket for UHF on the television set, an additional antenna plate had to be attached to the rear wall. The IF generated by the built-in part was capacitively coupled to the anode of the VHF mixing tube via an inflation cup. The universal UHF built-in part had a switch between VHF and UHF. With VHF reception, a series resistor ensured that the UHF tubes of the built-in part were protected.

If a receiver was not prepared for the organic installation of a UHF part, UHF attachments were also possible. These had their own power supply and could be used for all makes with an IF of 38.9 MHz. They were hung on the back wall or placed near the TV. The antennas for VHF and UHF were connected to the attachment; one output was connected to the VHF antenna input of the television receiver, the other to the picture IF amplifier. The switchover between VHF and UHF took place on the front-end device, whereby the antenna was simply switched through for VHF reception.

If the three aforementioned options (retrofitting, universal built-in part or attachment) were ruled out, UHF converters were used, which could be used for all television sets, regardless of the manufacturer and the intermediate frequency used. No intervention on the device was necessary, and existing tuning aids such as magic tape or screen displays were also available for UHF reception.

Web links

literature

  • Otto Limann: TV technology without ballast . 8th edition. Franzis-Verlag, Munich 1969.
  • Werner W. Diefenbach: television service . In: Handbook of radio and television repair technology . tape 2 . Franck'sche Verlagsbuchhandlung, Stuttgart 1961.
  • Werner W. Diefenbach: Television service fault diagnosis based on test images and oscillograms . In: Handbook of radio and television repair technology . tape 3 . Franck'sche Verlagsbuchhandlung, Stuttgart 1961.

Notes and sources

  1. ↑ The best-known tuning aids with display on the screen were image direction finders at Loewe-Opta, image magnet at Philips, image compass at Blaupunkt, image conductor at Siemens, image pilot at Schaub-Lorenz, and Visiotest display at Telefunken. Their basic function was identical: a horizontal or vertical bar, wedge or strip or a circular area in the center of the image changed its size when the transmitter was tuned. The optimal coordination was achieved either with the minimum or maximum area.
  1. ↑ a b c d https://www.maximus-randd.com/tv-tuner-history-pt2.html Tube-equipped Philips KR 36187 or NT1152 UHF converter
  2. ↑ Electronic Yearbook 1971, Deutscher Militärverlag, 1970, p. 238… 243 DIY instructions for UHF converter from 1971
  3. ↑ https://www.radiomuseum.org/forum/ddr_selbstbau_uhf_converter.html Faximile GDR self-assembly instructions from RMORG, accessed on April 2, 2020
  4. ↑ https://www.radiomuseum.org/r/quelle_universum_transuhf_2.html UHF converter from Quelle with AF139 / AF239